The nature and severity of headache that are reasonably attributable to a Chiari malformation is somewhat unclear. Occipitonuchal pain with coughing, position change, or the Valsalva maneuver is the most dependable association, but even then, decompression may not relieve the symptoms. Exertional headache alone is a questionable association. Only large and genuine malformations, not minor descent of the tonsils should be considered causative. More generalized headaches may or may not be explained by the finding of a Chiari malformation and the advisability of a surgical treatment then depends on the degree of disability created by other aspects of the malformation. Further discussion of this subject in relation to headache is found in Chap. 9.
The tongue of cerebellar tissue and the kinked cervical cord obstruct the flow of CSF and give a highly characteristic imaging profile, particularly on sagittal MRI (Fig. 37-4). Inspection of the axial sections of scans at the level of the foramen magnum demonstrates crowding of the upper cervical canal by inferiorly displaced cerebellar tissue, but one must be aware of the variations in the normal position of the cerebellar tonsils at this level. A slight descent of the cerebellar tonsils that is reversible is also seen with low cerebrospinal fluid pressure and not indicative of a Chiari malformation. Recent inceptions in phase-contrast MRI technology allow the imaging of CSF flow in the region of the foramen magnum but the relevance to choosing patients for surgical decompression has not been clarified (see summary by Menick). The CSF in Chiari malformation is usually normal but may show an elevated pressure and protein level in some cases for unexplained reasons.
The treatment of Chiari malformation and any associated basilar impression is far from satisfactory. If clinical progression is slight or uncertain, it is probably best to do nothing. If disability by way of spasticity, ataxia, pain in the shoulders or arms, or lower cranial-nerve disease is increasing, upper cervical laminectomy and enlargement of the foramen magnum are indicated. The proper course for patients that have headaches alone is uncertain but many such patients are operated upon if their cranial pain has been progressive, or if it is consistently and markedly worsened by cough or similar Valsalva actions, or if there is fainting or another associated symptom that can be reasonably related to the Chiari abnormality. The outcome of surgery, in our experience, has been less satisfactory when decompression was performed mainly for intractable headache, but there have been exceptions, especially when exertion or Valsalva maneuver elicits the symptoms.
The basic operation is suboccipital and C-1 decompression; various forms of shunting may be added if there is syringomyelia (the shunt is to the adjacent subarachnoid space) or hydrocephalus. The surgical procedure must be done cautiously. Opening of the dura and extensive manipulation of the malformation or excision of herniated cerebellum may aggravate the symptoms but is performed by some neurosurgeons to decompress the lower brainstem. Often, surgery halts the progress of the neurologic illness, arrests the hydrocephalus, or results in some other clinical improvement. The surgical series reported by Alzate and colleagues is representative. Emphasis was placed on proper patient selection, but the analysis of 66 cases, as in most other reports, was retrospective. (This reference is part of a very informative monograph). Most series report that a CSF leak follows surgery in about 5 percent of patients. The fate of an associated syringomyelia has been uncertain but most series report optimistic results.