Arnold-Chiari Malformation: Definition and Pathophysiology
Arnold-Chiari malformation (also called Chiari malformation) is a structural abnormality of the skull base and cerebellum where the cerebellar tonsils descend ≥3-5 mm below the foramen magnum, causing compression of neural structures at the craniocervical junction. 1
Anatomical Classification
Type I (Most Common in Adults):
- Characterized by downward herniation of cerebellar tonsils through the foramen magnum, compressing the brainstem and upper cervical spinal cord 1
- Affects 0.24-2.6% of the population, including both children and adults 2, 1
- Often referred to as "adult-type" Chiari malformation, typically presenting in the second or third decade of life 3
- Not all patients with anatomical findings are symptomatic 2
Type II (Arnold-Chiari Malformation):
- The most complex form involving downward displacement of the hindbrain with herniation of cerebellar vermis, tonsils, pons, medulla, and fourth ventricle through the foramen magnum 4
- Approximately 95% of infants with Type II present with associated myelomeningocele (the most severe form of spina bifida) 4
Underlying Pathophysiology
The fundamental problem is underdevelopment of the posterior cranial fossa:
- Volumetric studies demonstrate a 13.4 ml reduction in total posterior fossa volume with a 40% reduction (10.8 ml) in cerebrospinal fluid volume, while brain volume remains normal 5
- This represents a disorder of para-axial mesoderm development, resulting in overcrowding of normally developed hindbrain structures in an undersized posterior fossa 5, 3
Two primary mechanisms cause symptoms:
- CSF flow obstruction due to tonsillar herniation, leading to pressure changes and fluid disturbances 1
- Direct neural compression of brainstem, cranial nerves, and upper cervical spinal cord 1, 3
Clinical Presentation by Age
Children Under 3 Years:
- Oropharyngeal symptoms predominate: sleep apnea and feeding problems 6
- Respiratory irregularities and central apneas in severe cases 1
Children Over 3 Years and Adults:
- Occipital or neck pain worsened by Valsalva maneuvers (coughing, straining, sneezing)—the most characteristic symptom 1, 6
- Short-lasting occipital "cough headache" 6
- Lower cranial nerve dysfunction causing dysphagia and dizziness 1
- Peripheral motor and sensory defects, clumsiness, abnormal reflexes 1
- Gait impairment and dizziness 6
- Meniere's disease-like syndrome 3
Associated Conditions
Syringomyelia is the most common comorbidity:
- Present in 60-75% of symptomatic patients at diagnosis 3, 6
- Can develop secondarily after initial diagnosis 6
- Manifests with sensory disturbances (48%), motor weakness (32%), and autonomic disorders (35%) 6
Other structural abnormalities:
- Scoliosis in 25-42% of patients 2, 3
- Basilar invagination in 12% 5, 3
- Increased cervical lordosis in 8.5% 3
- Klippel-Feil syndrome in 3.3% 3
Special population consideration:
- Chiari Type I is detected in 25-50% of children with X-linked hypophosphatemia on cranial imaging 1
Diagnostic Imaging Findings
Most consistent MRI findings:
- Obliteration of retrocerebellar cerebrospinal fluid spaces (present in 70-100% of cases) 5, 3
- Tonsillar herniation ≥5 mm (though herniation <5 mm does not exclude diagnosis) 1, 5
- Varying degrees of cranial base dysplasia and posterior fossa anomalies 5, 3
Important diagnostic caveat:
- When cerebellar tonsillar ectopia >5 mm is identified, consider pseudotumor cerebri syndrome to avoid misdiagnosis as Chiari I 1