Glossary

Chiari malformation glossary

Every term in the Chiari malformation glossary — 265 plain-language definitions across 11 sections, each with its own page.

Core Terms & Classification

Chiari malformation: Chiari malformation is a group of structural conditions in which part of the back of the brain extends down through the opening at the base of the skull toward the spinal canal.
Arnold-Chiari malformation: Arnold-Chiari malformation is another name for Chiari malformation, used especially for type II and sometimes for the group as a whole.
Hindbrain herniation: Hindbrain herniation is the downward displacement of structures at the back of the brain through the skull's base opening, the central feature of Chiari malformation.
Tonsillar herniation (tonsillar ectopia): Tonsillar herniation is the downward extension of the cerebellar tonsils below the skull's base opening, the hallmark of Chiari malformation type I.
Cerebellar tonsils: The cerebellar tonsils are the lower, rounded portions of the cerebellum that descend below the skull's base opening in Chiari malformation type I.
Chiari malformation type I (CM-I): Chiari malformation type I is the most common form, in which only the cerebellar tonsils descend below the skull's base opening, often without symptoms.
Chiari malformation type II (CM-II): Chiari malformation type II is a more severe form, present at birth, in which the cerebellum and brainstem descend, almost always associated with spina bifida.
Chiari malformation type III (CM-III): Chiari malformation type III is a rare, severe form in which hindbrain structures extend into a sac at the back of the skull or upper neck.
Chiari malformation type IV (CM-IV): Chiari malformation type IV is a rare, severe condition involving an underdeveloped cerebellum, now considered by many an outdated or separate term.
Chiari 0 malformation: Chiari 0 malformation is a variant in which a person has syringomyelia and CSF flow problems at the skull-spine junction despite little or no tonsillar descent.
Chiari 1.5 malformation: Chiari 1.5 malformation is a variant between types I and II, with tonsillar descent plus downward displacement of the brainstem.
Complex Chiari malformation: Complex Chiari malformation refers to Chiari with additional craniocervical junction abnormalities, often requiring surgery beyond standard decompression.
Foramen magnum: The foramen magnum is the large opening at the base of the skull through which the brainstem and spinal cord connect, the site of herniation in Chiari malformation.
Craniocervical junction (craniovertebral junction): The craniocervical junction is the region where the skull meets the upper spine, the area involved in Chiari malformation and related conditions.
Posterior fossa: The posterior fossa is the compartment at the back and base of the skull that holds the cerebellum and brainstem, often small or crowded in Chiari malformation.
Congenital Chiari malformation: Congenital Chiari malformation is Chiari that is present from birth, arising from how the skull and brain developed.
Acquired Chiari malformation: Acquired Chiari malformation is a less common form that develops after birth due to another condition affecting pressure or space around the brain.
Symptomatic Chiari malformation: Symptomatic Chiari malformation is Chiari that is causing symptoms, as opposed to being an incidental, symptom-free finding.
Asymptomatic (incidental) Chiari malformation: Asymptomatic Chiari malformation is Chiari that causes no symptoms, often discovered incidentally on imaging done for another reason.
Tonsillar descent (measurement): Tonsillar descent is how far the cerebellar tonsils extend below the skull's base opening, measured on MRI to help define Chiari malformation type I.
Pointed (peg-like) cerebellar tonsils: Pointed cerebellar tonsils are tonsils with a peg-like rather than rounded shape, a feature that can support a diagnosis of Chiari malformation type I.
Cerebellar ptosis: Cerebellar ptosis is a downward sagging of the cerebellum, a term used both as a feature and as a possible complication after Chiari surgery.
Borderline Chiari (low-lying cerebellar tonsils): Borderline Chiari refers to cerebellar tonsils that sit slightly low but not clearly past the threshold for Chiari malformation type I.
Primary Chiari (small posterior fossa): Primary Chiari refers to Chiari arising from a small or underdeveloped posterior fossa, the most common underlying basis for type I.
Secondary Chiari: Secondary Chiari refers to tonsillar descent caused by another condition rather than a developmentally small posterior fossa.
Hindbrain (rhombencephalon): The hindbrain is the part of the brain comprising the cerebellum and brainstem structures involved in Chiari malformation.

Anatomy: Posterior Fossa, Hindbrain & Craniocervical Junction

Cerebellum: The cerebellum is the part of the brain that coordinates movement and balance, and the structure whose lower part descends in Chiari malformation.
Cerebellar vermis: The cerebellar vermis is the midline part of the cerebellum, which can be displaced downward in more severe Chiari malformations.
Cerebellar hemispheres: The cerebellar hemispheres are the two large lobes of the cerebellum that coordinate movement of the limbs.
Brainstem: The brainstem is the lower part of the brain that controls vital functions and connects the brain to the spinal cord, displaced downward in more severe Chiari types.
Medulla oblongata: The medulla oblongata is the lowest part of the brainstem, controlling breathing and heart rate, and can be displaced in severe Chiari malformations.
Pons: The pons is the middle part of the brainstem, relaying signals and contributing to functions that can be affected in severe Chiari malformations.
Midbrain: The midbrain is the upper part of the brainstem, involved in eye movement and other functions, generally less directly affected in Chiari malformation.
Fourth ventricle: The fourth ventricle is a fluid-filled space in the brainstem region that can be displaced or distorted in severe Chiari malformations.
Obex: The obex is a landmark at the lower end of the fourth ventricle whose downward position helps define the Chiari 1.5 variant.
Cervicomedullary junction: The cervicomedullary junction is where the lower brainstem meets the spinal cord, a region that can be crowded or compressed in Chiari malformation.
Spinal cord: The spinal cord is the bundle of nerve tissue extending from the brainstem down the spine, which can be affected in Chiari malformation through syringomyelia.
Cervical spinal cord: The cervical spinal cord is the neck portion of the spinal cord, the most common site of a syrinx in Chiari malformation.
Foramen of Magendie: The foramen of Magendie is the midline outlet of the fourth ventricle through which CSF exits, relevant to CSF flow in Chiari malformation.
Foramina of Luschka: The foramina of Luschka are the two lateral outlets of the fourth ventricle through which CSF exits, relevant to CSF flow in Chiari malformation.
Cisterna magna: The cisterna magna is a CSF-filled space behind the lower brainstem and below the cerebellum, often reduced or effaced in Chiari malformation.
Subarachnoid space: The subarachnoid space is the CSF-filled space around the brain and spinal cord, where flow is disrupted at the craniocervical junction in Chiari malformation.
Occipital bone: The occipital bone is the bone at the back and base of the skull, part of which is removed during Chiari decompression surgery.
Clivus: The clivus is a sloped bone at the base of the skull whose shape and angle are relevant to Chiari malformation and craniocervical measurements.
Skull base: The skull base is the floor of the skull, whose development and shape influence the posterior fossa size relevant to Chiari malformation.
Atlas (C1 vertebra): The atlas is the first cervical vertebra, part of which is often removed during Chiari decompression surgery.
Axis (C2 vertebra): The axis is the second cervical vertebra, bearing the odontoid process, and is relevant to craniocervical alignment and stability in Chiari-related conditions.
Odontoid process (dens): The odontoid process is the tooth-like projection of the second vertebra whose position is important in Chiari-related craniocervical conditions.
Basion: The basion is a bony landmark at the front edge of the foramen magnum used to measure tonsillar descent and craniocervical alignment in Chiari malformation.
Opisthion: The opisthion is a bony landmark at the back edge of the foramen magnum, paired with the basion to measure tonsillar descent in Chiari malformation.
Tentorium cerebelli: The tentorium cerebelli is a tough membrane separating the cerebellum from the cerebrum, whose position relates to posterior fossa size in Chiari malformation.
Dura mater (posterior fossa): The dura mater is the tough outer membrane covering the brain and spinal cord, opened and patched during Chiari decompression with duraplasty.

CSF Dynamics & Pathophysiology

Cerebrospinal fluid (CSF): Cerebrospinal fluid is the clear fluid that surrounds and cushions the brain and spinal cord, whose disrupted flow underlies many Chiari malformation symptoms.
CSF circulation: CSF circulation is the movement of cerebrospinal fluid around the brain and spinal cord, obstructed at the craniocervical junction in Chiari malformation.
CSF flow at the craniocervical junction: CSF flow at the craniocervical junction is the back-and-forth movement of fluid through the foramen magnum, impaired in Chiari malformation.
CSF flow obstruction (at the foramen magnum): CSF flow obstruction is the blockage of cerebrospinal fluid movement at the foramen magnum by the descending tonsils, the core problem in Chiari malformation.
Posterior fossa volume (small posterior fossa): A small posterior fossa is a reduced-size bony compartment at the back of the skull, a leading factor in the development of Chiari malformation type I.
Posterior fossa overcrowding: Posterior fossa overcrowding is the cramming of the cerebellum and brainstem into too small a space, the proposed mechanism behind Chiari malformation type I.
Underdeveloped occiput (mesodermal theory): The mesodermal theory proposes that Chiari malformation type I arises from underdevelopment of the occipital bone, creating a small posterior fossa.
Piston effect (water-hammer effect): The piston effect is a proposed mechanism in which the descending tonsils act like a piston on CSF, driving fluid into the spinal cord to form a syrinx.
Craniospinal pressure dissociation: Craniospinal pressure dissociation is a difference in pressure between the head and spinal compartments at the foramen magnum, implicated in Chiari symptoms and syrinx formation.
Systolic tonsillar descent (dynamic motion): Systolic tonsillar descent is the slight downward movement of the cerebellar tonsils with each heartbeat, exaggerated in Chiari malformation.
Valsalva effect on CSF: The Valsalva effect on CSF is how straining maneuvers transiently change CSF pressure, producing the characteristic headaches of Chiari malformation.
CSF pulsatility: CSF pulsatility is the rhythmic, heartbeat-driven back-and-forth motion of cerebrospinal fluid, altered at the craniocervical junction in Chiari malformation.
Intracranial pressure (ICP) and Chiari: Intracranial pressure is the pressure inside the skull, which can interact with Chiari malformation in important ways relevant to diagnosis and treatment.
Intracranial hypertension (and Chiari): Intracranial hypertension is raised pressure inside the skull, which can mimic or coexist with Chiari malformation and must be considered before surgery.
Intracranial hypotension (and acquired Chiari): Intracranial hypotension is low pressure inside the skull, which can pull the cerebellar tonsils downward and cause an acquired, sometimes reversible Chiari-like picture.
Syrinx formation theories (overview): Syrinx formation theories are the proposed explanations for how a fluid cavity develops in the spinal cord in Chiari malformation, all centered on disrupted CSF flow.
Gardner's hydrodynamic theory: Gardner's hydrodynamic theory proposes that pulse waves of CSF are driven into the spinal cord's central canal, forming a syrinx in Chiari malformation.
Williams' theory (craniospinal pressure dissociation): Williams' theory proposes that pressure differences between the head and spine, especially during straining, drive fluid into the spinal cord to form a syrinx.
Oldfield's theory (piston/pressure wave): Oldfield's theory proposes that the descending tonsils act as a piston with each heartbeat, driving pressure waves that force fluid into the spinal cord.
Greitz / intramedullary pulse pressure theory: The Greitz theory proposes that pressure effects around and within the spinal cord, related to fast CSF flow, drive fluid accumulation that forms a syrinx.
Monro-Kellie doctrine (relevance to Chiari): The Monro-Kellie doctrine is the principle that the skull's contents must stay in balance, relevant to how pressure and CSF behave in Chiari malformation.
Foramen magnum crowding: Foramen magnum crowding is the packing of tissue into the skull's base opening by the descending tonsils, obstructing CSF flow in Chiari malformation.
Compliance at the craniocervical junction: Compliance at the craniocervical junction is the ability of this region to accommodate pressure changes, reduced in Chiari malformation.

Causes, Risk Factors & Associated Conditions

Congenital (developmental) cause: A congenital cause means the Chiari malformation arose during development before birth, the usual basis for types I and II.
Small or underdeveloped posterior fossa: A small or underdeveloped posterior fossa is the leading structural basis for Chiari malformation type I, crowding the cerebellum and pushing the tonsils down.
Genetic factors (heritability): Genetic factors are inherited influences that can contribute to Chiari malformation, suggested by its tendency to run in some families.
Familial Chiari malformation: Familial Chiari malformation refers to cases occurring in more than one family member, supporting a genetic contribution to the condition.
Ehlers-Danlos syndrome (EDS): Ehlers-Danlos syndrome is a group of connective-tissue disorders that can be associated with Chiari malformation and craniocervical instability.
Hypermobile EDS (hEDS): Hypermobile Ehlers-Danlos syndrome is the most common form of EDS, frequently the type associated with Chiari and craniocervical instability.
Heritable connective-tissue disorders: Heritable connective-tissue disorders are inherited conditions affecting the body's connective tissue, some of which are associated with Chiari malformation.
Marfan syndrome: Marfan syndrome is an inherited connective-tissue disorder that can be associated with Chiari malformation among its many features.
Klippel-Feil syndrome: Klippel-Feil syndrome is a condition with fused neck vertebrae that can be associated with Chiari malformation and other craniocervical abnormalities.
Crouzon syndrome: Crouzon syndrome is a genetic condition affecting skull development that can be associated with Chiari malformation.
Pfeiffer syndrome: Pfeiffer syndrome is a genetic condition affecting skull and limb development that can be associated with Chiari malformation.
Achondroplasia: Achondroplasia is a genetic bone-growth condition that can be associated with a small foramen magnum and Chiari-like crowding.
Gorham-Stout disease: Gorham-Stout disease is a rare condition involving bone loss that has been associated with Chiari malformation in some cases.
Overgrowth syndromes: Overgrowth syndromes are conditions causing excessive growth of body tissues, among the genetic syndromes associated with Chiari malformation.
Craniosynostosis (and Chiari): Craniosynostosis is premature fusion of the skull bones that can crowd the posterior fossa and be associated with Chiari malformation.
Hydrocephalus (as cause or association): Hydrocephalus is a buildup of cerebrospinal fluid that can be associated with Chiari malformation, particularly type II, and influences its management.
Spina bifida (myelomeningocele): Spina bifida is a birth defect of the spine that, in its most significant form, is almost always associated with Chiari malformation type II.
Neural tube defect: A neural tube defect is a birth defect of the brain or spine, the category that includes the spina bifida associated with Chiari malformation type II.
Tethered cord syndrome: Tethered cord syndrome is abnormal fixation of the spinal cord that can coexist with Chiari malformation and may need to be addressed.
Scoliosis (association with Chiari): Scoliosis is a sideways curvature of the spine that is commonly associated with Chiari malformation, especially when a syrinx is present.
Acquired causes (overview): Acquired causes are conditions developing after birth that can lead to tonsillar descent, producing a Chiari-like picture distinct from the usual congenital form.
Intracranial mass or tumor: An intracranial mass or tumor can push brain structures downward, causing acquired tonsillar descent and a Chiari-like picture.
Arachnoid cyst (posterior fossa): An arachnoid cyst in the posterior fossa is a fluid-filled sac that can crowd the region and contribute to a Chiari-like picture.
Lumboperitoneal shunt / CSF overdrainage: A lumboperitoneal shunt or other CSF overdrainage can lower pressure and pull the cerebellar tonsils downward, causing an acquired Chiari-like picture.
Trauma (and Chiari/instability): Trauma can contribute to craniocervical instability and, in some cases, relate to Chiari, particularly in people with connective-tissue conditions.
Basilar invagination (as associated bony anomaly): Basilar invagination is upward migration of the top of the spine into the skull base, a bony anomaly that can accompany Chiari and crowd the brainstem.
Idiopathic intracranial hypertension (association): Idiopathic intracranial hypertension is raised CSF pressure without an obvious cause, which can overlap with or mimic Chiari malformation.

Syringomyelia & Spinal Cord Effects

Syringomyelia: Syringomyelia is the development of a fluid-filled cavity within the spinal cord, commonly associated with Chiari malformation.
Syrinx: A syrinx is the fluid-filled cavity within the spinal cord that defines syringomyelia and is a key finding in Chiari malformation.
Hydromyelia: Hydromyelia is dilation of the spinal cord's central canal with cerebrospinal fluid, a term closely related to syringomyelia.
Syringohydromyelia: Syringohydromyelia is a combined term for a fluid cavity and central canal dilation in the spinal cord, frequently used for the syrinx in Chiari malformation.
Central canal (of the spinal cord): The central canal is the small channel running through the center of the spinal cord, which can dilate or be involved in syrinx formation in Chiari malformation.
Communicating syringomyelia: Communicating syringomyelia is a syrinx that connects with the cerebrospinal fluid spaces, a form associated with conditions affecting CSF flow.
Non-communicating syringomyelia: Non-communicating syringomyelia is a syrinx that does not connect with the cerebrospinal fluid spaces, the form typically associated with Chiari malformation.
Syringobulbia: Syringobulbia is the extension of a syrinx into the brainstem (the medulla), a less common but more serious development.
Presyrinx state: A presyrinx state is early swelling or fluid change in the spinal cord that may precede a true syrinx, sometimes seen in Chiari malformation.
Cervical syrinx: A cervical syrinx is a syrinx located in the neck region of the spinal cord, the most common location in Chiari malformation.
Holocord syrinx: A holocord syrinx is an extensive syrinx spanning most or all of the spinal cord, a more severe form requiring careful management.
Syrinx expansion (progression): Syrinx expansion is the enlargement of a syrinx over time, which can progressively damage the spinal cord and prompt treatment.
Spinal cord (intramedullary) pressure: Intramedullary pressure is the pressure within the spinal cord itself, which a syrinx can increase, contributing to cord damage in Chiari malformation.
Dissociated sensory loss (cape distribution): Dissociated sensory loss is a pattern of losing pain and temperature sensation while preserving touch, classically over the shoulders and arms in syringomyelia.
Central cord syndrome features: Central cord features are neurological signs from damage to the central part of the spinal cord, which a syrinx can produce in syringomyelia.
Neurogenic scoliosis (from a syrinx): Neurogenic scoliosis is spinal curvature caused by a syrinx affecting the nerves and muscles controlling the spine, common in children with Chiari.
Spinal cord atrophy (myelopathy): Spinal cord atrophy is thinning or damage of the spinal cord, which a long-standing or large syrinx can cause, producing lasting symptoms.
Charcot joint (neuropathic arthropathy): A Charcot joint is joint damage from loss of protective sensation, which can occur in syringomyelia affecting the arms.
Neuropathic pain (from a syrinx): Neuropathic pain is pain arising from damage to nerve tissue, which a syrinx can cause, sometimes persisting and requiring specific management.
Muscle wasting (hand intrinsics): Muscle wasting is the thinning of muscles, often the small hand muscles, which a cervical syrinx can cause in syringomyelia.
Spasticity (upper motor neuron signs): Spasticity is increased muscle tone and stiffness from damage to certain spinal cord pathways, which a syrinx can cause, often affecting the legs.
Bladder and bowel dysfunction (from a syrinx): Bladder and bowel dysfunction is impaired control of urination or bowel function, which an extensive or advanced syrinx can cause.
Syrinx resolution (after treatment): Syrinx resolution is the shrinking or disappearance of a syrinx after treatment, a common and favorable result of relieving the CSF obstruction.
Persistent or recurrent syrinx: A persistent or recurrent syrinx is a syrinx that does not resolve or returns after treatment, which may require further evaluation or surgery.

Signs, Symptoms & Clinical Features

Occipital headache (Chiari headache): The Chiari headache is a headache at the back of the head, the most common symptom of Chiari malformation type I.
Tussive (Valsalva) headache: A tussive headache is a headache triggered by coughing or other Valsalva maneuvers, the characteristic headache pattern of Chiari malformation.
Neck pain: Neck pain is a common symptom of Chiari malformation, often accompanying the characteristic occipital headache.
Cervicomedullary syndrome: Cervicomedullary syndrome is a cluster of symptoms from dysfunction where the lower brainstem meets the spinal cord, seen in some Chiari malformations.
Dizziness / vertigo: Dizziness or vertigo is a sensation of unsteadiness or spinning that can occur in Chiari malformation, related to effects on balance pathways.
Imbalance / gait disturbance: Imbalance or gait disturbance is difficulty with steadiness or walking that can occur in Chiari malformation from effects on the cerebellum and cord.
Ataxia: Ataxia is a lack of coordination of movements, a cerebellar sign that can occur in Chiari malformation.
Dysmetria: Dysmetria is impaired judgment of distance in movements, a cerebellar sign that can occur in Chiari malformation.
Nystagmus (downbeat): Downbeat nystagmus is an involuntary downward jerking of the eyes, a sign that can occur in Chiari malformation due to effects at the craniocervical junction.
Diplopia / visual disturbances: Diplopia and visual disturbances are double or impaired vision that can occur in Chiari malformation from effects on eye-movement and visual pathways.
Tinnitus: Tinnitus is the perception of ringing or noise in the ears, which can occur in Chiari malformation, though it has many other causes.
Hearing changes: Hearing changes are alterations in hearing that can occur in Chiari malformation, related to effects on the brainstem auditory pathways.
Dysphagia (swallowing difficulty): Dysphagia is difficulty swallowing, which can occur in Chiari malformation from effects on the lower brainstem, and can be serious in some cases.
Dysarthria / speech changes: Dysarthria is slurred or impaired speech that can occur in Chiari malformation from effects on the brainstem and cerebellum.
Hoarseness / vocal cord dysfunction: Hoarseness or vocal cord dysfunction is a voice change from effects on the nerves controlling the voice box, which can occur in Chiari malformation.
Sleep apnea (central): Central sleep apnea is pauses in breathing during sleep from brainstem dysfunction, which can occur in Chiari malformation and can be serious.
Dysautonomia: Dysautonomia is dysfunction of the automatic nervous system that can occur in Chiari malformation and associated conditions, causing varied symptoms.
Numbness / paresthesias (hands): Numbness or paresthesias are altered sensations like tingling, often in the hands, which can occur in Chiari malformation, especially with a syrinx.
Weakness (extremities): Weakness in the arms or legs can occur in Chiari malformation from a syrinx or compression affecting motor pathways, warranting evaluation.
Clumsiness / fine motor difficulty: Clumsiness or fine motor difficulty is reduced dexterity that can occur in Chiari malformation from effects on the cerebellum or hand nerves.
Loss of temperature/pain sensation: Loss of temperature and pain sensation is a characteristic sensory change of a syrinx in Chiari malformation, often with preserved touch.
Chronic fatigue: Chronic fatigue is persistent tiredness that some people with Chiari malformation experience, often multifactorial and warranting broad evaluation.
Cognitive / "brain fog" complaints: "Brain fog" refers to subjective difficulties with concentration or mental clarity that some people with Chiari report, often multifactorial.
Pediatric presentation (infants and young children): The pediatric presentation of Chiari can differ from that in adults, with young children showing less specific signs that require careful recognition.
Failure to thrive / feeding difficulties (infants): Failure to thrive and feeding difficulties are signs in infants that can indicate Chiari malformation affecting the brainstem, warranting prompt evaluation.
Drop attacks / syncope: Drop attacks and syncope are sudden falls or fainting that can rarely occur in Chiari malformation, warranting careful evaluation.

Diagnosis & Imaging

Magnetic resonance imaging (MRI): MRI is the main imaging test for diagnosing Chiari malformation, providing detailed images of the brain, spinal cord, and craniocervical junction.
Cervical spine MRI / whole-spine MRI: Cervical or whole-spine MRI images the spinal cord to detect a syrinx and assess its extent in Chiari malformation.
Cine (phase-contrast) MRI — CSF flow study: A cine MRI is a specialized scan that visualizes cerebrospinal fluid flow at the craniocervical junction, helping assess the obstruction in Chiari malformation.
Computed tomography (CT): CT is an imaging test using X-rays that shows bony detail, useful in Chiari malformation for evaluating the skull base and craniocervical bones.
Upright / dynamic (flexion-extension) imaging: Upright or dynamic imaging examines the craniocervical junction in different positions to assess for instability in Chiari-related conditions.
McRae's line (basion-opisthion line): McRae's line is a reference line across the foramen magnum used to measure how far the cerebellar tonsils descend in Chiari malformation.
Tonsillar herniation measurement (5 mm / 3 mm): The tonsillar herniation measurement is how far the tonsils descend below the foramen magnum, with common thresholds of 5 mm in adults and 3 mm in children.
Chamberlain's line: Chamberlain's line is a craniometric reference line used to help assess for basilar invagination in Chiari-related craniocervical evaluation.
McGregor's line: McGregor's line is a craniometric reference line, similar to Chamberlain's, used to help assess for basilar invagination in Chiari-related evaluation.
Grabb-Oakes line (pB-C2): The Grabb-Oakes (pB-C2) measurement assesses for ventral brainstem compression at the craniocervical junction, relevant in complex Chiari.
Clivo-axial angle (CXA): The clivo-axial angle is the angle between the skull base and the upper spine, used to assess for abnormal craniocervical alignment and brainstem compression.
Basion-dens interval (BDI): The basion-dens interval is a measurement of the distance between the skull base and the top of the odontoid, used to assess craniocervical alignment and stability.
Basion-axial interval (BAI): The basion-axial interval is a measurement assessing the horizontal alignment of the skull base over the upper spine, used to evaluate craniocervical stability.
Platybasia: Platybasia is a flattening of the skull base, a bony feature that can be associated with Chiari malformation and a small posterior fossa.
Basilar invagination (measurement/diagnosis): Basilar invagination is the diagnosis of the odontoid migrating upward into the skull base, assessed with craniometric measurements in Chiari-related evaluation.
Atlanto-occipital assimilation (occipitalization): Atlanto-occipital assimilation is congenital fusion of the first vertebra to the skull, a bony anomaly that can accompany Chiari malformation.
Retroflexed odontoid: A retroflexed odontoid is a backward-tilted odontoid process that can compress the front of the brainstem, a feature of complex Chiari malformation.
Medullary kinking (ventral brainstem compression): Medullary kinking is a bend or distortion of the lower brainstem, often from front-side compression, seen in complex Chiari malformation.
Craniocervical instability (CCI) workup: A craniocervical instability workup evaluates whether the connection between the skull and upper spine is abnormally loose, relevant in some Chiari patients.
Atlantoaxial instability (AAI): Atlantoaxial instability is abnormal movement between the first two vertebrae, which can accompany Chiari and require evaluation and sometimes fusion.
Neurological examination: A neurological examination is the clinical assessment of nerve function used to evaluate how Chiari malformation is affecting the patient.
Sleep study (polysomnography): A sleep study is an overnight test that detects sleep-related breathing problems, used in Chiari malformation to evaluate for sleep apnea.
Somatosensory evoked potentials (SSEP) / neurophysiology: Somatosensory evoked potentials and related neurophysiology tests measure nerve signal transmission, sometimes used to assess cord involvement in Chiari.
Fetal/prenatal imaging (for Chiari II): Fetal or prenatal imaging can detect Chiari malformation type II before birth, typically as part of evaluating spina bifida.
Incidental finding (on imaging): An incidental finding is a Chiari malformation discovered by chance on imaging done for another reason, common with type I and often asymptomatic.

Treatment: Surgical Decompression

Conservative management / observation: Conservative management is monitoring a person with Chiari malformation without surgery, the usual approach for asymptomatic or mild cases.
Surveillance MRI / watchful waiting: Surveillance MRI is periodic imaging to monitor Chiari malformation over time, used in watchful waiting to detect any changes.
Indications for surgery: Indications for surgery are the specific reasons treatment is recommended in Chiari malformation, such as progressive deficits, a syrinx, or severe symptoms.
Posterior fossa decompression (PFD): Posterior fossa decompression is the main surgery for Chiari malformation, creating more space at the back of the skull to relieve crowding and restore CSF flow.
Suboccipital craniectomy: A suboccipital craniectomy is the removal of a portion of the occipital bone at the back of the skull, the core bony step of Chiari decompression.
C1 laminectomy (removal of posterior arch of the atlas): A C1 laminectomy is removal of the back arch of the first vertebra, often done with suboccipital craniectomy to create more room in Chiari decompression.
Posterior fossa decompression with duraplasty (PFDD): PFDD is posterior fossa decompression that also opens the dura and adds a patch, more effective but with a higher complication rate than bony-only decompression.
Duraplasty (dural patch graft): A duraplasty is the placement of a patch to expand the dura during Chiari decompression, enlarging the space for the tonsils and CSF flow.
Dural graft materials (autograft / allograft / synthetic): Dural graft materials are the tissues or substitutes used to patch the dura in a duraplasty, including the patient's own tissue, donor or animal tissue, or synthetic patches.
Arachnoid preservation / arachnoid-sparing technique: An arachnoid-sparing technique leaves the delicate inner membrane intact during decompression, reducing certain complications.
Intradural exploration / arachnoid dissection: Intradural exploration is opening into the CSF space to address the tonsils, adhesions, or a syrinx directly during Chiari decompression.
Tonsillar reduction (coagulation/shrinkage): Tonsillar reduction is shrinking the cerebellar tonsils during surgery to open the space and improve CSF flow at the craniocervical junction.
Tonsillar resection (PFDRT): Tonsillar resection (PFDRT) is removing part of the cerebellar tonsils during decompression, a more aggressive approach used in certain cases.
Bony-only (extradural) decompression: Bony-only decompression removes bone without opening the dura, a less invasive Chiari surgery with fewer complications but somewhat less effectiveness.
Dura-splitting technique: The dura-splitting technique opens only the outer layer of the dura, a middle-ground approach between bony-only decompression and full duraplasty.
Restoring CSF flow at the CVJ (surgical goal): Restoring CSF flow at the craniovertebral junction is the central goal of Chiari decompression surgery, relieving the obstruction behind the symptoms and syrinx.
Intraoperative ultrasound / intraoperative MRI: Intraoperative ultrasound or MRI lets surgeons assess CSF flow and decompression during the operation, helping confirm the surgical goal is met.
Intraoperative neuromonitoring: Intraoperative neuromonitoring tracks nerve and spinal cord function during Chiari surgery, adding a layer of safety.
Watertight dural closure: A watertight dural closure is sealing the dura securely after duraplasty to prevent cerebrospinal fluid leak, an important step in Chiari surgery.
Fourth ventricle stent / outlet opening (Chiari II): A fourth ventricle stent or outlet opening is a procedure to maintain CSF flow from the fourth ventricle, sometimes used in severe Chiari II.
Cervical decompression (for Chiari II): Cervical decompression is removing bone over the upper neck to relieve compression in Chiari II, where structures descend lower than in type I.
Extent of decompression (craniectomy size): The extent of decompression is how much bone is removed in Chiari surgery, balanced to relieve crowding without removing too much.
Reoperation / revision decompression: A reoperation is a repeat surgery for Chiari when symptoms or a syrinx persist or recur after the first decompression.

Treatment: Other Procedures & Management

Occipitocervical (craniocervical) fusion: Occipitocervical fusion is surgery to join the skull to the upper spine, used in Chiari when there is craniocervical instability.
Transoral odontoidectomy: Transoral odontoidectomy is removal of the odontoid through the mouth to relieve front-side brainstem compression in certain complex Chiari cases.
Endoscopic endonasal odontoidectomy: Endoscopic endonasal odontoidectomy removes the odontoid through the nose, a less invasive alternative route to relieve front-side brainstem compression.
Ventral decompression (for basilar invagination): Ventral decompression is relieving front-side compression of the brainstem, used in complex Chiari with basilar invagination or a retroflexed odontoid.
Syringosubarachnoid shunt: A syringosubarachnoid shunt is a tube draining a syrinx into the spinal fluid space, used when a syrinx persists despite decompression.
Syringopleural shunt: A syringopleural shunt drains a syrinx into the chest (pleural) cavity, an alternative shunt used for persistent syrinxes in certain cases.
Syringoperitoneal shunt: A syringoperitoneal shunt drains a syrinx into the abdominal cavity, another alternative shunt option for persistent syrinxes.
Syrinx drainage / myelotomy: Syrinx drainage with myelotomy is making a small opening into the spinal cord to drain a syrinx, used in selected cases as part of syrinx treatment.
CSF diversion (shunt) for hydrocephalus: CSF diversion with a shunt drains excess cerebrospinal fluid to treat hydrocephalus, often needed when hydrocephalus accompanies Chiari.
Endoscopic third ventriculostomy (ETV): Endoscopic third ventriculostomy is a procedure creating a new CSF pathway to treat certain hydrocephalus, an alternative to a shunt in some Chiari-related cases.
Tethered cord release (detethering): Tethered cord release is surgery to free an abnormally anchored spinal cord, sometimes done when a tethered cord accompanies Chiari.
Scoliosis management (bracing/surgery): Scoliosis management includes bracing or surgery to address spinal curvature, sometimes needed in children with Chiari-associated scoliosis.
Pain management (medical): Medical pain management uses medications and other approaches to address pain in Chiari malformation, especially when surgery is not indicated or pain persists.
Physical therapy (post-op / symptom): Physical therapy uses guided exercises to support recovery and manage symptoms in Chiari malformation, after surgery or for ongoing symptoms.
Activity modification / restrictions: Activity modification involves adjusting or limiting certain activities in Chiari malformation, particularly around surgery or with specific risks.
Headache medical management: Headache medical management addresses the headaches of Chiari malformation with medications and strategies, important since headache is the most common symptom.
Management of dysautonomia: Management of dysautonomia addresses autonomic symptoms that can occur in Chiari and associated conditions, using targeted, often multidisciplinary approaches.
Multidisciplinary management: Multidisciplinary management coordinates care across multiple specialties for Chiari malformation, important given its many possible associations and effects.
Pregnancy and delivery considerations: Pregnancy and delivery considerations involve planning care for women with Chiari malformation, addressing questions about symptoms, anesthesia, and delivery.
Anesthesia considerations: Anesthesia considerations involve planning safe anesthesia for people with Chiari malformation, accounting for the craniocervical anatomy and any instability.
Staged surgery (complex cases): Staged surgery is performing treatment in planned separate steps, sometimes used in complex Chiari requiring decompression plus fusion or other procedures.
Shared decision-making: Shared decision-making is the collaborative process between patient (or family) and care team to make treatment choices in Chiari malformation.

Complications & Outcomes

CSF leak (post-operative): A CSF leak is the escape of cerebrospinal fluid after surgery, one of the more common complications of Chiari decompression with duraplasty.
Pseudomeningocele: A pseudomeningocele is a collection of cerebrospinal fluid under the skin after surgery, a complication that can follow Chiari decompression with duraplasty.
Aseptic (chemical) meningitis: Aseptic meningitis is inflammation of the membranes around the brain without infection, which can occur after Chiari decompression with duraplasty.
Bacterial meningitis / wound infection: Bacterial meningitis or wound infection is an infection after surgery, an uncommon but serious complication of Chiari decompression requiring prompt treatment.
Cerebellar ptosis / slump (post-decompression): Cerebellar ptosis is downward sagging of the cerebellum after decompression, a complication that can occur if too much bone is removed.
Hydrocephalus (post-operative): Post-operative hydrocephalus is a buildup of cerebrospinal fluid that can develop after Chiari surgery, sometimes requiring treatment.
Hemorrhage / hematoma (post-op): Hemorrhage or hematoma is bleeding or a blood collection after surgery, an uncommon complication of Chiari decompression that may need attention.
Persistent symptoms (failed decompression): Persistent symptoms after decompression mean the surgery did not relieve the symptoms as hoped, prompting evaluation of why.
Recurrent symptoms / re-herniation: Recurrent symptoms are the return of symptoms after initial improvement, sometimes related to re-herniation or scarring after decompression.
Persistent / recurrent syrinx (post-op): A persistent or recurrent syrinx after surgery is a syrinx that does not resolve or returns, which may require further evaluation or treatment.
Cranial settling / instability (post-decompression): Cranial settling is downward shifting at the craniocervical junction after decompression, a complication related to instability that may require fusion.
Wound healing problems: Wound healing problems are difficulties with the surgical incision healing after Chiari surgery, which can require additional care.
Reoperation rate: The reoperation rate is the proportion of people who need a repeat surgery after Chiari decompression, which varies by surgical approach.
Surgical outcome (clinical improvement): Surgical outcome refers to how a person fares after Chiari decompression, with most experiencing improvement in symptoms.
Syrinx outcome (resolution rate): The syrinx outcome is how a syrinx responds to treatment, with a large majority shrinking after decompression in many studies.
Headache outcome: The headache outcome is how headaches respond to Chiari treatment, with the characteristic occipital headache often improving well after decompression.
Natural history (untreated Chiari I): The natural history of untreated Chiari I is how the condition typically progresses without surgery, which is generally benign for asymptomatic individuals.
Prognosis (by type): Prognosis in Chiari varies by type, with type I generally favorable and the more severe types (II, III) having a more guarded outlook.
Quality of life: Quality of life in Chiari reflects the overall impact on daily living, which can be affected by symptoms but often improves with appropriate care.
Pediatric outcomes: Pediatric outcomes in Chiari are generally favorable for children with type I, with treatment helping symptoms and a syrinx, though type II is more complex.
Long-term follow-up (imaging surveillance): Long-term follow-up involves ongoing monitoring after Chiari treatment, including imaging surveillance to track the syrinx and detect any changes.
Outcome (overall): The overall outcome in Chiari is generally favorable for most people, particularly those with type I who are appropriately managed.

Care Team, Therapy & Support

Neurosurgeon: A neurosurgeon is the surgeon who specializes in the brain, spine, and nervous system, central to diagnosing and treating Chiari malformation.
Pediatric neurosurgeon: A pediatric neurosurgeon specializes in nervous system surgery for children, important for treating Chiari malformation in infants and young patients.
Neurologist: A neurologist is a physician who specializes in nervous system disorders, helping evaluate and manage the non-surgical aspects of Chiari malformation.
Pain specialist: A pain specialist focuses on managing pain, helping people with Chiari malformation who have persistent or complex pain.
Physiatrist (physical medicine and rehabilitation): A physiatrist specializes in physical medicine and rehabilitation, helping people with Chiari malformation optimize function and recovery.
Geneticist / genetic counseling: A geneticist or genetic counselor helps evaluate any genetic or inherited factors in Chiari malformation, especially with associated syndromes or family history.
Radiologist / neuroradiologist: A radiologist (or neuroradiologist) interprets the imaging used to diagnose and monitor Chiari malformation, central to accurate assessment.
Otolaryngologist (ENT): An otolaryngologist (ENT) evaluates ear, nose, and throat symptoms that can occur in Chiari malformation, such as hearing, balance, voice, and swallowing issues.
Sleep medicine specialist: A sleep medicine specialist evaluates and manages sleep-related breathing problems in Chiari malformation, such as central sleep apnea.
Orthopedic / scoliosis specialist: An orthopedic or scoliosis specialist manages spinal curvature that can be associated with Chiari malformation, especially in children with a syrinx.
Physical therapy: Physical therapy uses guided exercises and techniques to support function and recovery in Chiari malformation, delivered by physical therapists.
Occupational therapy: Occupational therapy helps people with Chiari malformation perform daily activities and maintain independence, addressing fine motor and functional challenges.
Speech-language therapy (swallowing): Speech-language therapy addresses speech and swallowing difficulties that can occur in Chiari malformation, especially with brainstem involvement.
Mental health support / counseling: Mental health support helps people with Chiari malformation cope with the emotional impact of a chronic condition, an important part of comprehensive care.
Primary care / pediatrician (coordination): The primary care provider or pediatrician helps coordinate overall care for a person with Chiari malformation and manages general health needs.
Care coordination: Care coordination is the organizing of care across the multiple specialists and services involved in Chiari malformation, ensuring integrated treatment.
Symptom monitoring / when to seek care: Symptom monitoring is keeping track of symptoms in Chiari malformation and knowing when to seek prompt care for concerning changes.
Activity and lifestyle guidance: Activity and lifestyle guidance helps people with Chiari malformation make informed choices about activities, exercise, and daily life, individualized to their situation.
Bobby Jones Chiari & Syringomyelia Foundation (patient support): The Bobby Jones Chiari & Syringomyelia Foundation is a patient support and advocacy organization providing resources for people affected by Chiari and syringomyelia.
Patient advocacy & community resources: Patient advocacy and community resources connect people with Chiari malformation to information, support, and others affected, complementing medical care.
Caregiver support and education: Caregiver support and education help those caring for someone with Chiari malformation, recognizing the important role and needs of caregivers.

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