The Chiari malformations were described in the early 1890s by Dr. Hans Chiari, a pathologist in Prague. Since that time, our understanding has progressed. Here we will discuss the 2 common anomalies: Chiari I and Chiari II. Chiari III and IV are rare.
The Chiari anomalies are not numbered to denote increasing severity but, rather, they represent four distinct and different anomalies of the hindbrain.
In general, Chiari malformations are thought to be congenital, and may be associated with more serious conditions such as brainstem dysfunction or syringomyelia.
Cerebellar tonsillar descent into the foramen magnum with resulting impairment of cerebrospinal fluid flow and/or brainstem composition.
Commonly occurs in children (40% under age 5, 25% age 5-10 and 30% age 10-15).5 Common presenting symptom is pain (60-70%):6,14,15 usually occipital or upper cervical headache exacerbated by Valsalva (straining, cough, sneezing). Other symptoms vary greatly and may include motor (40-74%) and sensory (50%) changes in the extremities, clumsiness (15%), and dysphagia (10%).6,7,14 Syringomyelia may coexist in 60-70% of Chiari patients and may be associated with progressive scoliosis in 30% of these patients.10,14
MRI is gold standard. Descent of the tonsils through the foramen magnum is measured from a line drawn from the basion to opisthion. Normal tonsillar position has been correlated with age.9 Normal when they extend into the foramen magnum up to 6mm in children 0-10 years old, 5mm in 10-29 years old, 4mm in 30 to 79 years old, and 3mm in those patients above 80 years old. In Chiari patients, the cerebellar tonsils are generally pointed, rather than normally rounded. Supratentorial mass lesions, hydrocephalus, or history of multiple lumbar punctures may all cause tonsillar descent, which is considered an acquired Chiari.4 Other possible findings: Cervical syrinx (50-70% of patients8), evidence of skull base abnormalities such as platybasia, basilar invagination, and ventral compression. Cine-MRI was associated with conflicting results regarding sensitivity and specificity.
Posterior fossa decompression should include a suboccipital craniectomy and C1 laminectomy. Treatment may vary in children compared to adults. Approximately 5-10% of surgeons choose to stop at this point; 45% open the dura and perform a duraplasty, leaving the arachnoid intact; and the other 45% open the arachnoid, lyse arachnoidal adhesions, and often shrink or resect the cerebellar tonsils. Currently, there is no alternative to surgical therapy for Chiari I. Adequate CSF flow from the foramen of Magendie should be visualized at the time of surgery.5 Intracranial pressure monitoring should precede decompression if there is a question of ICP elevation, as pseudotumor cerebri occasionally accompanies Chiari I. Rule out craniocervical instability with dynamic cervical spine films prior to surgery. Basilar invagination or ventral compression should alert the surgeon to the possibility of chronic craniocervical instability that might need a concomitant craniocervical fusion with or without transoral odontoid resection. Three indications for surgical treatment: (1) The presence of neurological deficits, especially if progressive. (2) The presence of syringomyelia. (3) Severe intractable symptoms that are debilitating to the patient. Make sure that the symptoms are typical for Chiari I (e.g., valsalva-induced occipital headaches and others).
Outcome data on Chiari I are quite difficult to interpret. They are also dependent on age of presentation and surgery. In a series of 71 adult patients, 80-90% of patients presenting with any symptoms improved initially but patients with foramen magnum syndromes and patients with central cord syndrome were more likely to relapse later (33% and 25% respectively).15 Syringomyelia should decrease after Chiari decompression. Re-exploration of the posterior fossa is preferred before consideration is given to shunt the syrinx directly to the subarachnoid space, pleura, or peritoneum. Symptoms that most consistently improve with surgery are scoliosis of less than 30 degrees in young adolescents, occipital headache, cervical pain, and sleep apnea.11Patients with predominantly brainstem and spinal cord symptoms are less likely to improve than patients with headaches alone.3