What is Chiari malformation?

What is Chiari Malformation?

Chiari malformation is a structural abnormality of the base of the skull and cerebellum, specifically defined as the descent of cerebellar tonsils ≥3-5 mm below the foramen magnum, causing compression of neural structures at the craniocervical junction. 1

Classification and Types

Chiari malformations represent a heterogeneous group of cerebellar anomalies originally described by Austrian pathologist Hans Chiari in the early 1890s, with four distinct types that share only the commonality of cerebellar involvement. 2, 3

Chiari Type I (CM1) is the most common clinical type and the one emergency physicians and general practitioners are most likely to encounter:

• Characterized by downward herniation of the cerebellar tonsils through the foramen magnum, which can compress the brainstem and upper cervical spinal cord 1
• Prevalence ranges from 0.24% to 2.6% of the population, affecting both children and adults 1
• May be congenital or acquired 4
• Associated with syringomyelia in 25-70% of pediatric patients and 40-60% of adult cases 5

Chiari Type II is almost exclusively associated with myelomeningocele and hydrocephalus, involving herniation of not only the tonsils but also the brainstem, fourth ventricle, and cerebellar vermis into the foramen magnum. 2

Chiari Type III and IV are rare malformations, with Type III representing an encephalocele containing brainstem and posterior fossa contents, and Type IV consisting of cerebellar hypoplasia. 2

Pathophysiology

The prevalent pathophysiological theory proposes a morphological mismatch between a small posterior cranial fossa and a normally developed hindbrain that results in ectopia of the tonsils. 6

Two primary mechanisms cause symptoms:

• CSF flow obstruction occurs due to herniation of cerebellar tonsils through the foramen magnum, leading to pressure changes that trigger symptoms 1, 7
• Direct compression of neural structures, including the brainstem and cranial nerves at the craniocervical junction, directly triggers pain pathways 1, 7

Clinical Presentation

Most people with CM1 are asymptomatic and diagnosed incidentally. 6 However, when symptomatic, the presentation varies by age and severity.

Cardinal Symptoms in Adults and Older Children:

• Headache is the most characteristic presenting symptom, reported in 65% of cases 5
• The typical headache is Valsalva-induced, exacerbated by coughing, straining, or other Valsalva maneuvers 1, 7, 6
• Occipital or neck pain worsened by strain 1, 5
• Dizziness and gait impairment 5

Neurological Signs (in symptomatic patients):

• Sensory disturbances (48% of cases) 5
• Cranial nerve deficits (45% of cases), including visual disturbances such as nystagmus 1, 5
• Motor weakness (32% of cases) 5
• Autonomic disorders (35% of cases) 5
• Lower cranial nerve dysfunction causing dysphagia 1

Pediatric-Specific Presentations:

• Children younger than 3 years: oropharyngeal symptoms are prevalent, including sleep apnea and feeding problems 5
• Children older than 3 years: higher incidence of cough headache and scoliosis 5
• Respiratory irregularities and central apneas can occur in severe cases 1

Associated Conditions:

• Syringomyelia is present in 40-60% of symptomatic cases, manifesting with spinal cord dysfunction of varying severity 5, 6
• Hydrocephalus occurs in 15-20% of all Chiari I patients 2
• Chiari type II malformation is found in 98% of patients with myelomeningocele 8

Important Clinical Pitfalls

Headache in CM1 is often multifactorial, and patients can report a wide variety of non-specific symptoms and signs that may not be directly related to the malformation. 5 This creates diagnostic challenges and potential for misattribution of symptoms.

The new finding of a Chiari I malformation should be presumed acquired until proved otherwise, and should trigger an evaluation for central nervous system lesions or hydrocephalus. 4 Brain imaging to exclude increased intracranial pressure is critical in these cases.

When cerebellar tonsillar ectopia >5 mm is identified, consider pseudotumor cerebri syndrome to avoid misdiagnosis as Chiari I. 1

Special Population Considerations

Chiari type 1 malformation is detected in 25-50% of children with X-linked hypophosphatemia by cranial MRI or CT, requiring complete evaluation with fundoscopy and brain/skull imaging in any patient presenting with clinical symptoms of lower brainstem or upper cervical cord compression. 1