Chiari Malformation
Overview and Classification
Chiari malformation (CM) is a structural defect characterized by herniation of the cerebellar tonsils (and in some types, the brainstem) through the foramen magnum into the cervical spinal canal. The foramen magnum is the opening at the base of the skull through which the brainstem passes into the spinal cord; when posterior fossa structures descend through it, they compress the brainstem, cerebellum, and cervical spinal cord while obstructing cerebrospinal fluid (CSF) flow.
Chiari malformations are classified into four types based on the structures involved and severity:
- Type I: Cerebellar tonsillar ectopia ≥5 mm below the foramen magnum; no brainstem herniation; the most common form, often presenting in adolescence or adulthood.
- Type II (Arnold-Chiari): Displacement of both the cerebellar vermis and brainstem through the foramen magnum; almost always associated with myelomeningocele (open spina bifida); diagnosed at birth.
- Type III: Rare; posterior fossa encephalocele containing herniated cerebellum and sometimes brainstem through the occipital bone; severe neurological consequences.
- Type IV: Extremely rare; absence of posterior fossa structures (cerebellar aplasia/hypoplasia); not technically herniation but classified with Chiari historically.
The eponym honors Hans Chiari, an Austrian pathologist who described the condition in 1891. Arnold-Chiari specifically refers to Type II, acknowledging Julius Arnold's early contributions.
Type I: The Adult Form
Chiari Type I is the most commonly diagnosed form. The cerebellar tonsils extend at least 5 mm below the McRae line (a reference line across the foramen magnum on MRI). Many individuals with mild tonsillar ectopia (3–5 mm) remain asymptomatic throughout life; tonsillar descent greater than 5 mm is more likely to obstruct CSF flow and produce symptoms.
Type I has no associated brain malformation and is often discovered incidentally on MRI obtained for unrelated reasons (headache workup, trauma). It can manifest at any age but most commonly comes to clinical attention in the second through fourth decades. The exact cause is debated: a relatively small posterior fossa (insufficient space for the cerebellum) is the leading structural hypothesis, causing the tonsils to be “pushed” downward. Secondary Chiari can occur from intracranial hypotension (low CSF pressure pulling the brain downward) — this distinction is clinically important because treatment differs.
Type II: Arnold-Chiari and Myelomeningocele
Chiari Type II almost universally accompanies myelomeningocele, the most severe form of spina bifida in which the spinal cord is exposed through an opening in the lower back. The association is so strong that virtually all patients with myelomeningocele have radiographic evidence of Type II Chiari.
In Type II, both the cerebellar vermis and the brainstem (medulla) descend through the foramen magnum. This produces more severe brainstem dysfunction than Type I, including:
- Lower cranial nerve palsies (CN IX–XII): dysphagia, aspiration, hoarse cry, vocal cord paralysis in neonates.
- Respiratory dysfunction: apnea, stridor (life-threatening in severe cases).
- Hydrocephalus: nearly universal; caused by aqueductal stenosis and impaired CSF flow; managed with ventriculoperitoneal shunt or endoscopic third ventriculostomy.
In utero surgical repair of myelomeningocele (Management of Myelomeningocele Study, MOMS trial, 2011) reduced the need for VP shunting and improved motor outcomes compared to postnatal repair. In utero repair does not cure the Chiari malformation but reduces its severity.
Rare Types (III and IV)
Type III Chiari is characterized by a posterior fossa encephalocele — a sac protruding through a defect in the occipital bone (occiput) or upper cervical vertebrae containing cerebellar tissue and sometimes brainstem. It is extremely rare, associated with severe neurological impairment, and carries high mortality. Surgical repair is complex with limited neurological recovery.
Type IV (cerebellar aplasia or hypoplasia) is the rarest category. The cerebellum fails to develop, leaving the posterior fossa largely empty. It is not truly a herniation disorder and is often discussed separately in current classifications.
Clinical Features
Symptoms in Chiari Type I arise from compression at the foramen magnum and CSF flow obstruction.
Cough Headache — The Classic Presentation
A brief but severe headache in the suboccipital region (base of skull, back of head) provoked by Valsalva maneuvers — coughing, sneezing, laughing, straining with bowel movements. The pain typically lasts seconds to a minute and radiates to the occiput, neck, or arms. Any patient reporting Valsalva-triggered suboccipital headache should be evaluated with brain MRI with attention to the posterior fossa and foramen magnum.
Neck and Arm Symptoms
Neck pain, occipital neuralgia, paresthesias (tingling, numbness) radiating into the upper extremities, and upper extremity weakness. These can result from direct cervicomedullary compression or from syringomyelia (see below).
Lower Cranial Nerve Symptoms
Dysphagia (difficulty swallowing), dysarthria (slurred speech), hoarseness from vagal or hypoglossal nerve involvement; vocal cord paralysis in severe cases.
Ataxia and Coordination Difficulty
Cerebellar compression produces gait ataxia, incoordination of upper extremities, and balance problems.
Downbeat Nystagmus
The eyes drift upward and then beat downward. Downbeat nystagmus is the hallmark of foramen magnum pathology (Chiari, craniocervical instability, Arnold-Chiari) until proven otherwise. It results from disruption of cerebellar floccular pathways at the foramen magnum.
Central Sleep Apnea
Brainstem compression at the respiratory centers (pre-Bötzinger complex in medulla) can cause central sleep apnea — cessation of respiratory effort during sleep, distinct from obstructive sleep apnea. Chiari should be considered in any patient with unexplained central sleep apnea.
Facial Pain and Sensory Disturbances
Trigeminal distribution numbness or pain from trigeminal nucleus compression.
Syringomyelia
Syringomyelia — a fluid-filled cavity (syrinx) within the spinal cord parenchyma — develops in 20–65% of patients with Chiari Type I. The pathophysiology involves obstruction of pulsatile CSF flow at the foramen magnum with each cardiac systole; this is believed to force CSF into the spinal cord’s central canal and eventually the parenchyma, creating an expanding cavity.
The syrinx typically develops in the cervical cord and expands over months to years, damaging anterior horn cells (lower motor neurons) and crossing spinothalamic fibers (pain and temperature sensation).
Clinical presentation of syringomyelia follows a characteristic pattern:
- Cape distribution sensory loss: bilateral loss of pain and temperature sensation across the shoulders, upper chest, and upper back (like a cape or shawl draped over the shoulders); this is dissociated sensory loss — pain/temperature affected but vibration and proprioception preserved, because the anterior white commissure (where spinothalamic fibers cross) is damaged by the syrinx while the dorsal columns (vibration/proprioception) are spared.
- Upper extremity weakness and atrophy: from anterior horn cell damage.
- Scoliosis: especially in children; syrinx-associated scoliosis (not idiopathic) should always prompt spinal MRI for underlying Chiari/syrinx.
- Loss of deep tendon reflexes in arms (lower motor neuron pattern), hyperreflexia and spasticity in legs (upper motor neuron if cord compression).
Once Chiari decompression surgery restores CSF flow at the foramen magnum, the syrinx typically collapses over weeks to months without direct surgical intervention on the syrinx itself.
Associated Conditions
Connective Tissue Disorders
Hypermobile Ehlers-Danlos syndrome (hEDS) and other connective tissue disorders predispose to craniocervical instability — excessive movement at the atlantoaxial or atlanto-occipital joints. Craniocervical instability can produce Chiari-like symptoms (and true Chiari by pulling the brain downward) even without cerebellar tonsillar descent exceeding 5 mm. This is an important and sometimes under-recognized overlap.
POTS (Postural Orthostatic Tachycardia Syndrome)
Many patients with hEDS have both POTS and craniocervical instability; some have true Chiari. The hEDS–POTS–Chiari overlap is being increasingly recognized.
Tethered Spinal Cord
In Type II Chiari, tethered cord (where the spinal cord is anchored and cannot rise normally with growth) often coexists. Surgical release improves some neurological function.
Scoliosis
Syringomyelia-associated scoliosis in children with Chiari I must be distinguished from idiopathic adolescent scoliosis because treatment differs; all children with scoliosis should be examined for Chiari/syrinx.
Diagnosis
MRI is the gold-standard diagnostic modality:
- MRI brain (sagittal T1): measures tonsillar position relative to McRae line; ≥5 mm below = diagnostic threshold for Type I, but clinical context matters (3–5 mm with symptoms is significant).
- MRI complete spine: to detect syrinx (entire spinal cord must be imaged), tethered cord.
- Phase-contrast MRI (CSF flow study): dynamic imaging of CSF pulsatility at the foramen magnum; obstructed flow confirms hemodynamically significant compression and predicts surgical benefit.
- Flexion-extension MRI or upright MRI: to evaluate craniocervical instability, especially in connective tissue disorder patients where instability may be dynamic (not visible on standard supine MRI).
Neurophysiological studies: somatosensory evoked potentials, brainstem auditory evoked potentials — can document subclinical brainstem or cord dysfunction.
Sleep study (polysomnography): to characterize sleep apnea (central vs. obstructive) and quantify respiratory events.
Treatment
Asymptomatic Type I (Incidental Finding)
No surgery indicated. Serial MRI every 3–5 years; earlier if new symptoms develop. Patient education about warning symptoms.
Symptomatic Type I / Type I with Syringomyelia
The primary surgical treatment is posterior fossa decompression (PFD):
- Suboccipital craniectomy: removal of a portion of the occipital bone to enlarge the foramen magnum.
- C1 laminectomy: removal of the posterior arch of the first cervical vertebra.
- Duraplasty: opening the dura and patching it with autologous or synthetic graft to expand the dural sac volume, restoring CSF flow.
Outcomes of posterior fossa decompression: Cough headache resolves in 80–90% of patients. Syrinx collapses or stabilizes in ~70% within months. Neurological deficits (weakness, sensory loss) improve variably — duration of deficits before surgery is the strongest predictor of recovery. Long-term data show durable outcomes in most patients.
Type II (Arnold-Chiari)
Management is multifaceted:
- In utero myelomeningocele repair (MOMS trial): reduces but does not eliminate need for VP shunting; improves motor outcomes.
- Postnatal myelomeningocele closure if not done prenatally.
- Ventriculoperitoneal shunt for hydrocephalus (nearly universal).
- Posterior fossa decompression if brainstem compression is symptomatic (apnea, dysphagia, vocal cord paralysis).
Craniocervical Stabilization
For patients with documented craniocervical instability (especially in hEDS), cervical fusion (occipital-cervical or atlantoaxial) may be necessary to provide stability and stop dynamic compression.
Post-Operative Monitoring
MRI at 3–6 months for syrinx regression, then annually for 2 years, then as needed.
Key Research Papers
- Milhorat TH, et al. Chiari I malformation redefined: clinical and radiographic findings for 364 symptomatic patients. Neurosurgery. 1999;44(5):1005–1017. PMID: 10232534.
- Aitken LA, et al. Chiari malformation type 1: is surgical treatment superior to conservative management? Child's Nervous System. 2009;25(10):1257–1266. PMID: 19296116.
- Tubbs RS, et al. Chiari malformation type I: operative indications. Neurosurg Clin N Am. 2015;26(4):537–541. PMID: 26408059.
- Oakes WJ. Chiari malformations, hydromyelia, syringomyelia. In: Wilkins RH, Rengachary SS, eds. Neurosurgery. McGraw-Hill; 1996. PMID: 8302741.
- Durning SJ, Trowbridge RL. Chiari malformation type I: an incidental finding or a significant diagnosis? J Gen Intern Med. 2007;22(4):544–549. PMID: 17372800.
- Alperin N, et al. MR-intracranial compliance and pressure in Type I Chiari malformation. J Magn Reson Imaging. 2008;27(4):850–855. PMID: 18383246.
- Sadler B, et al. Rare, deleterious variants in PTPN11 and other MAPK pathway genes in human neural tube defects and Chiari malformations. Hum Mutat. 2020;41(8):1473–1481. PMID: 32459372.
- Massimi L, et al. Chiari malformation type II: long-term outcome of surgical treatment. Acta Neurochir (Wien). 2019;161(6):1123–1133. PMID: 30820784.
- Adzick NS, et al. A randomized trial of prenatal versus postnatal repair of myelomeningocele. N Engl J Med. 2011;364(11):993–1004. PMID: 21306277.
- Henderson FC Sr, et al. Neurological and spinal manifestations of the Ehlers-Danlos syndromes. Am J Med Genet C Semin Med Genet. 2017;175(1):195–211. PMID: 28219064.
- Meadows J, et al. Asymptomatic Chiari Type I malformations identified on magnetic resonance imaging. J Neurosurg. 2000;92(6):920–926. PMID: 10839251.
- McGirt MJ, et al. Correlation of the extent of tonsillar descent with the surgical outcome of decompression. J Neurosurg. 2008;109(4):665–670. PMID: 18826348.