Identification and Treatment of Chiari Malformation

Scott W. Soleau, M.D. is an Adult and Pediatric Neurosurgeon at The NeuroMedical Center Clinic

Patients of all ages, from infants to adults, are frequently referred to neurosurgery for evaluation of a Chiari Malformation. In fact, the Chiari 1 (C-1) subtype is responsible for virtually all office-based referrals. The true incidence of C-1 is unknown, however its recognition has increased dramatically since the advent of magnetic resonance (MR) imaging.

The term “Chiari Malformation” applies to a heterogeneous spectrum of hindbrain and/or posterior fossa developmental anomalies. Five subtypes (Chiari 0,1,2,3,or 4) are currently described. Chiari 3 and 4 are exceptionally rare and will never be encountered in the general population. Chiari 2 is exclusive to patients with spina bifida, and is readily diagnosed in-utero or at birth. Chiari 0 is clinically similar to C-1, but lacks the classic imaging findings of C-1. Chiari 0 is an unusual diagnosis of exclusion that should be made cautiously.

The underlying abnormality of C-1 is bony underdevelopment of the posterior fossa skull. This results in an inadequately small posterior fossa that cannot properly contain its neural contents. The majority of cases are idiopathic, although some metabolic and osseous disorders are linked to C-1. The volumetric mismatch and compression of the posterior fossa contents causes herniation of the cerebellar tonsils through the foramen magnum (usually greater than 5mm). This tonsillar herniation is readily observed on imaging and is the diagnostic hallmark of C-1 that usually prompts a neurosurgical consultation.

Presenting symptoms are wide-ranging and non-specific. Rarely does one initiate an imaging evaluation of the neuraxis looking specifically for C-1. Direct compression of posterior fossa contents may lead to various patterns of cerebellar, brainstem or cranial nerve dysfunction. Impediment of CSF flow across the foramen magnum may cause development of a spinal syrinx leading to long-tract signs or a suspended sensory level. Children younger than five years old, in particular, often display apnea or feeding problems due to brainstem compression. Beyond five the presenting symptoms between children and adults are similar. A sub-occipital headache that worsens during a valsalva maneuver is a common and classic symptom.

Once C-1 is suspected, thorough imaging of the neuraxis is necessary. Brain imaging is mandatory to rule out hydrocephalus or a space-occupying mass. MR imaging of the cervical and thoracic spines will detect a syrinx. Children and adolescents should be evaluated for scoliosis, as there is a clear association of C-1 with some forms of scoliosis.

Surgical intervention is not necessary in every individual diagnosed with C-1. The decision to treat a patient medically or surgically depends on multiple imaging and clinical factors. Patients with headaches as their only symptom and mild tonsillar descent should be enrolled in a comprehensive headache management program. This is often successful. Surgery for these individuals should be considered a last-resort. Mild tonsillar descent of less than 4mm in asymptomatic individuals is usually a normal anatomic variant that requires no treatment beyond a comprehensive imaging evaluation. Despite pressure from advocacy groups, “Chiari surgery” is not an accepted treatment for fibromyalgia.

Surgery is mandatory, however, for individuals experiencing brainstem dysfunction or valsalva induced syncope. These symptoms indicate a severe degree of posterior fossa compression. Patients with a spinal syrinx also require rapid surgical intervention. Spinal syrinxes caused by C-1 will progressively enlarge without surgery. Left untreated, a syrinx can cause irreversible spinal cord damage. Surgery is recommended if signs of cerebellar or lower cranial nerve dysfunction are present. Individuals with significant tonsillar herniation (greater than 1cm) and headaches refractory to a brief course of medical management are also candidates for surgery.

The goal of surgery is to reverse the root cause of C-1 by enlarging the volume of the posterior fossa. This will both decompress neural tissue and restore normal CSF flow dynamics across the foramen magnum, leading to resolution of a spinal syrinx. All surgical strategies include a sub-occipital craniectomy and opening of the foramen magnum. Cervical laminectomies must be carried out to the inferior margin of tonsillar descent.

Debate exists regarding the need to perform intra-dural exploration following bone removal. Good arguments can be made for opening or not opening the dura. I personally favor the more tedious process of opening the dura and performing an expansion duraplasty. This, in my opinion, virtually guarantees generous decompression of the posterior fossa. Additionally, only intra-dural exploration allows for direct inspection and identification of pathology within the fourth ventricle, such as arachnoid veils, that could continue to obstruct the proper flow of CSF post-operatively and hinder resolution of a spinal syrinx. Under no circumstances should a spinal syrinx be shunted initially, no matter how large, since 85-90% will resolve following the posterior fossa expansion.

Surgical outcomes are very satisfying in properly selected patients. Neurologic dysfunction may improve immediately post-operatively. Headaches take longer, ranging from days to weeks, to improve. Most spinal syrinxes are smaller within 3 months, however these patients need careful follow-up to make sure there is no recurrence of the syrinx requiring further intervention.

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