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Summary Table

Category	Details
Risk Factors	Type I: female predominance, connective tissue disorders (Ehlers-Danlos), craniovertebral junction anomalies. Type II: maternal folate deficiency, family history of neural tube defects
Etiology	Type I: underdevelopment of the posterior fossa leading to tonsillar herniation. Type II: failure of neural tube closure causing downward displacement of hindbrain structures
Patient Presentation	Type I: young adult with suboccipital headache provoked by coughing or straining. Type II: neonate with myelomeningocele and signs of brainstem compression
Classic Physical Exam	Type I: neck pain, hand weakness, dissociated sensory loss (cape-like distribution if syringomyelia is present). Type II: stridor, dysphagia, apneic spells, lower cranial nerve palsies in an infant with a lumbosacral neural tube defect
Key Diagnostic Results	MRI of brain and cervical spine showing cerebellar tonsillar descent ≥5 mm below the foramen magnum (Type I) or descent of vermis, brainstem, and fourth ventricle (Type II). Prenatal ultrasound: "lemon sign" and "banana sign" (Type II)
Management	Type I: posterior fossa decompression (suboccipital craniectomy + C1 laminectomy). Type II: myelomeningocele repair, VP shunt for hydrocephalus, posterior fossa decompression if brainstem symptoms persist
Keywords	"Headache worse with Valsalva," "syringomyelia," "cape-like sensory loss," "myelomeningocele," "lemon sign," "banana sign," "tonsillar herniation," "foramen magnum"

1. Pathophysiology

Chiari malformations are a group of structural hindbrain abnormalities defined by the downward displacement of cerebellar (and in some types, brainstem) structures through the foramen magnum into the upper cervical spinal canal. The unifying concept is a mismatch between the size of the posterior fossa and the neural structures it must contain.

In Chiari Type I, the posterior fossa is congenitally small or shallow, providing inadequate space for the cerebellum. This forces the cerebellar tonsils to herniate downward through the foramen magnum by 5 mm or more. The herniated tonsils act like a plug at the craniocervical junction, disrupting the normal flow of cerebrospinal fluid (CSF). This obstruction at the level of the foramen magnum is the central mechanism that produces symptoms. When a patient coughs, strains, or performs any Valsalva maneuver, the transient rise in intracranial pressure pushes the tonsils further downward, acutely worsening the obstruction. This is why the hallmark complaint is a suboccipital headache triggered or worsened by Valsalva. Over time, chronic CSF flow obstruction can lead to the formation of a syrinx (a fluid-filled cavity within the spinal cord), a condition called syringomyelia. The syrinx expands from the central canal outward, first damaging the anterior white commissure where the spinothalamic fibers decussate. This produces a dissociated sensory loss: the patient loses pain and temperature sensation in a "cape-like" distribution across the shoulders and upper extremities while preserving fine touch and proprioception (carried by the dorsal columns). As the syrinx enlarges, it can compress the ventral horn motor neurons, causing hand weakness and muscle atrophy.

In Chiari Type II (historically called Arnold-Chiari malformation), the pathophysiology is more severe and begins during embryonic development. The primary defect is a failure of neural tube closure, resulting in an open myelomeningocele (almost always lumbosacral). During fetal development, the open neural tube defect allows CSF to leak, which prevents normal distension of the embryonic ventricular system. Without adequate ventricular distension, the posterior fossa fails to expand to its normal size. The result is a profoundly small posterior fossa that cannot accommodate the cerebellum or brainstem. This causes herniation not just of the tonsils, but of the cerebellar vermis, the fourth ventricle, and the lower brainstem (medulla) through the foramen magnum. The downward displacement of the brainstem produces lower cranial nerve dysfunction (CN IX, X, XI, XII), manifesting as stridor, weak cry, dysphagia, and apneic episodes in neonates. Additionally, the distortion of the fourth ventricle and aqueduct of Sylvius causes obstructive hydrocephalus, present in approximately 90% of Type II patients.

2. Classification and Clinical Manifestation

Type	Structural Defect	Age at Presentation	Clinical Features	Associated Conditions
I	Cerebellar tonsils herniate ≥5 mm below foramen magnum	Adolescence to young adulthood (peak 20s to 40s)	Suboccipital headache worsened by Valsalva, neck pain, hand clumsiness, sensory changes in upper limbs, oscillopsia, dysphagia	Syringomyelia (present in 25 to 85% of cases), scoliosis, basilar invagination, Ehlers-Danlos syndrome
II	Herniation of cerebellar vermis, brainstem, and 4th ventricle through foramen magnum	Prenatal or neonatal	Stridor, apnea, feeding difficulties, weak cry, opisthotonus, lower cranial nerve palsies	Myelomeningocele (nearly 100%), hydrocephalus (~90%), corpus callosum dysgenesis, cortical malformations
III	Occipital or high cervical encephalocele containing cerebellar and/or brainstem tissue	At birth	Severe neurological deficits, encephalocele visible externally, often incompatible with normal development	Encephalocele, hydrocephalus
IV	Cerebellar hypoplasia or aplasia without herniation	At birth	Severe developmental delay, cerebellar dysfunction	Considered by some classifications as a separate entity rather than a true Chiari malformation

For examination purposes, Type I and Type II are the most commonly tested. Type III and IV are exceedingly rare and are typically tested only as recognition items.

Type I Clinical Features

The headache is characteristically located in the occipital and suboccipital region. It is described as a pressure-like or aching pain that is provoked by coughing, sneezing, laughing, straining, or bending forward. Many patients also report posterior neck pain, dizziness, and unsteady gait. When a syrinx develops, the patient may notice numbness or burning pain in the hands and forearms, progressive hand weakness, and difficulty with fine motor tasks. On examination, you may find downbeat nystagmus (a classic finding), hyperreflexia or upper motor neuron signs in the lower extremities (from cord compression), and lower motor neuron signs in the upper extremities (from syrinx destroying anterior horn cells).

Type II Clinical Features

The neonate nearly always presents with a visible lumbosacral myelomeningocele at birth, and imaging reveals the hindbrain herniation. Brainstem compression symptoms include inspiratory stridor (from vocal cord paralysis, CN X), apneic episodes, weak or absent cry, and aspiration during feeding. Hydrocephalus develops in most cases, and if untreated, the infant will show a rapidly enlarging head circumference, bulging fontanelle, and "setting sun" eye sign.

3. Diagnostic Workup

Test	Role	Key Findings
MRI of brain and craniocervical junction	Best initial test and gold standard for all Chiari types	Type I: tonsillar herniation ≥5 mm below foramen magnum on sagittal view. Type II: downward displacement of vermis, brainstem, fourth ventricle; "beaking" of the tectum
MRI of entire spine	Evaluate for syringomyelia	Syrinx cavity within the spinal cord, most commonly cervical or cervicothoracic
Cine-phase contrast MRI (CSF flow study)	Assess CSF dynamics at the foramen magnum	Absent or reduced CSF flow posterior to the tonsils; used to guide surgical decision-making
Prenatal ultrasound	Screening for Type II	"Lemon sign" (scalloping of frontal bones) and "banana sign" (curved, flattened cerebellum)
CT of the head	Adjunct for bony anatomy	Evaluate for hydrocephalus, bony anomalies at the craniocervical junction (basilar invagination, os odontoideum)
Fetal MRI	Confirmatory for prenatal cases	Better delineation of posterior fossa anatomy than ultrasound

Best Initial Test: For a patient presenting with occipital headache worsened by Valsalva and neurological findings suggestive of craniocervical junction pathology, the best initial test is an MRI of the brain and craniocervical junction with sagittal views. This is also the most accurate test, making it both the screening and confirmatory study. On sagittal T1-weighted images, you will see the cerebellar tonsils extending below the foramen magnum. A descent of ≥5 mm is diagnostic of Chiari Type I. A descent of 3 to 5 mm is considered borderline and must be correlated with symptoms.

Once Chiari I is confirmed, a full spine MRI should be ordered to evaluate for syringomyelia, which alters the surgical approach and prognosis. If the syrinx is large or progressive, surgical intervention becomes more urgent.

Cine-phase contrast MRI is a functional study that maps CSF flow at the foramen magnum. It is not routinely ordered for diagnosis but is valuable when symptoms are present with borderline tonsillar ectopia, or when surgical planning requires assessment of CSF dynamics.

For Type II, diagnosis frequently occurs prenatally. On second-trimester ultrasound, the lemon sign (bilateral concavity of the frontal bones giving the skull a lemon shape) and the banana sign (anterior curvature of the cerebellar hemispheres due to herniation, making the cerebellum look like a banana) are the classic indicators. These findings in the presence of a spinal defect are highly suggestive. Fetal MRI can confirm the posterior fossa abnormality in greater detail.

An important testing pearl: if a question describes a young patient with a newly diagnosed scoliosis and no obvious cause, consider ordering a spinal MRI to look for an occult syrinx, which may point back to an undiagnosed Chiari I malformation.

4. Management and Treatment

Scenario	Management	Details
Chiari I, asymptomatic	Observation with serial MRI	Follow-up imaging every 6 to 12 months; no surgery indicated if no symptoms and no syringomyelia
Chiari I, symptomatic	Posterior fossa decompression	Suboccipital craniectomy + C1 (and sometimes C2) laminectomy + duraplasty using autologous pericranium or synthetic graft
Chiari I with syringomyelia	Posterior fossa decompression (primary)	Decompression at the foramen magnum typically resolves the syrinx by restoring CSF flow; direct syrinx drainage (syringosubarachnoid shunt) is reserved for refractory cases
Chiari II, hydrocephalus	VP shunt placement	First priority in neonatal management; reduces intracranial pressure and may improve brainstem symptoms
Chiari II, myelomeningocele	Surgical repair of the defect	Ideally within 24 to 48 hours of birth (postnatal) or in utero repair (fetal surgery, as supported by the MOMS trial)
Chiari II, persistent brainstem symptoms after shunting	Posterior fossa decompression	Performed only after confirming the shunt is functioning properly

Type I Management

The decision to operate depends entirely on whether the patient is symptomatic. Asymptomatic patients with incidental findings of tonsillar ectopia do not require surgery and are followed with serial imaging. This is a commonly tested principle.

For symptomatic patients, the standard surgical procedure is posterior fossa decompression, which involves a suboccipital craniectomy (removing a portion of the occipital bone) and a C1 laminectomy to enlarge the space at the foramen magnum. Most surgeons also perform a duraplasty, in which the dura is opened and a patch graft is sewn in to expand the subarachnoid space and restore CSF flow around the tonsils. Some surgeons additionally perform tonsillar cauterization or resection if the tonsils are severely herniated.

Regarding syringomyelia: the key teaching point is that treating the Chiari malformation (the cause) typically resolves the syrinx (the effect). Direct drainage of the syrinx via shunting is a second-line intervention reserved for cases where the syrinx fails to regress after adequate posterior fossa decompression.

Type II Management

The management of Chiari II is inseparable from the management of its associated anomalies. The immediate neonatal priorities are:

Myelomeningocele repair within 24 to 48 hours to prevent infection and further neurological deterioration. The landmark MOMS (Management of Myelomeningocele Study) trial demonstrated that prenatal (in utero) repair at 19 to 25 weeks of gestation reduces the need for VP shunting and improves motor outcomes at 30 months compared to postnatal repair. This trial result is a frequently tested pearl.
VP shunt insertion for hydrocephalus. Since approximately 90% of Type II patients develop hydrocephalus, shunt placement is often required in the first weeks of life.
If brainstem symptoms (stridor, apnea, feeding difficulty) persist after a functioning shunt is confirmed, posterior fossa decompression is considered. However, the first step when a shunted infant develops new brainstem symptoms is to evaluate shunt function (shunt series X-ray and rapid-sequence MRI or CT to assess ventricular size). Never proceed to posterior fossa surgery before confirming the shunt is working.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Tension-type headache	Both cause occipital/posterior headache in young adults	Tension headache is bilateral, band-like, not provoked by Valsalva, and has no neurological deficits on exam
Idiopathic intracranial hypertension (pseudotumor cerebri)	Both present with headache and may have papilledema; both occur in young patients	IIH causes headache with visual changes (transient visual obscurations), CN VI palsy, and shows elevated opening pressure on LP with normal MRI brain parenchyma; no tonsillar herniation
Multiple sclerosis	Both can cause nystagmus, sensory changes, and upper motor neuron signs in a young patient	MS presents with episodes that are disseminated in time and space; MRI shows periventricular white matter plaques, not tonsillar herniation or syrinx from craniocervical obstruction
Cervical spondylotic myelopathy	Both cause hand weakness, sensory loss, and upper motor neuron signs in the lower extremities	Cervical spondylosis occurs in older patients (>50), is related to degenerative disc/osteophyte disease, and MRI shows cord compression from disc or osteophyte, not tonsillar herniation
Intramedullary spinal cord tumor (ependymoma, astrocytoma)	Both can cause a syrinx and progressive myelopathy with dissociated sensory loss	The syrinx in a tumor is typically associated with an enhancing intramedullary mass on contrast MRI; in Chiari, the syrinx exists secondary to CSF flow obstruction with no enhancing mass
Dandy-Walker malformation	Both are posterior fossa anomalies; both can cause hydrocephalus	Dandy-Walker shows cystic dilation of the fourth ventricle, hypoplasia/absence of the cerebellar vermis, and an enlarged posterior fossa (the posterior fossa is too large, not too small). There is no tonsillar herniation and no association with myelomeningocele
Basilar migraine (migraine with brainstem aura)	Both cause occipital headache with vertigo, ataxia, and possible diplopia	Basilar migraine is episodic with complete resolution between attacks, often has visual aura, and MRI is normal

The most critical differential to master is Dandy-Walker vs. Chiari II, as both are posterior fossa malformations presenting in neonates. The distinguishing feature is the posterior fossa size: in Chiari II, the posterior fossa is too small and structures herniate downward; in Dandy-Walker, the posterior fossa is too large and is occupied by a cyst arising from the fourth ventricle, with an absent or hypoplastic vermis.

6. Traps and High-Yield Pearls

The most common way students lose points on Chiari questions falls into a few predictable patterns.

Trap 1: Missing the Valsalva connection. A vignette will describe a young adult with headaches and will bury the Valsalva trigger in the middle of the stem ("the headache is worse when she coughs or bears down during bowel movements"). Students who skim the vignette select tension headache or migraine. The moment you see headache provoked by coughing, straining, or laughing in a young patient, Chiari I should be at the top of your differential.

Trap 2: Confusing the direction of causation with syringomyelia. Students sometimes treat syringomyelia as a separate primary diagnosis and order direct syrinx drainage. The tested principle is that the syrinx is secondary to CSF flow obstruction at the foramen magnum. The correct next step is posterior fossa decompression, not syrinx shunting.

Trap 3: Forgetting to check the shunt before decompressing. In a Chiari II patient with a VP shunt who develops new brainstem symptoms, the next best step is to evaluate shunt function (imaging for ventricular size, shunt series). Students who jump directly to posterior fossa decompression are choosing a surgical intervention that may be unnecessary if the shunt is simply malfunctioning.

Trap 4: Confusing Dandy-Walker and Chiari II. Both involve the posterior fossa and hydrocephalus. The discriminator is direction: Chiari II herniates downward through a small foramen magnum and is associated with myelomeningocele. Dandy-Walker expands upward and outward with a cystic fourth ventricle and absent vermis, with no neural tube defect.

Trap 5: Not recognizing the prenatal ultrasound findings. When a second-trimester ultrasound shows the lemon sign and banana sign together with a spinal defect, the diagnosis is Chiari II with myelomeningocele. Students who are unfamiliar with these signs may select other neural anomalies.

The core competency being tested across all Chiari questions is the ability to connect anatomical pathology to clinical presentation: understanding that a small posterior fossa produces tonsillar herniation, which obstructs CSF flow, which causes Valsalva-sensitive headache and syringomyelia, which produces a cape-like dissociated sensory loss. Every symptom traces back to the anatomy.

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