Treatment for the Chiari I malformation is surgical. To date, no effective non-surgical treatment exists. While the natural history of Chiari I malformation is not known, it is generally accepted that patients with neurologic deficits referable to the Chiari, those with syringomyelia, and those who have symptoms that decrease quality of life and are thought to be due to the Chiari malformation (i.e. pain in the back of the head that is worse with coughing or valsalva), are candidates for posterior fossa decompression. In the patient with a Chiari I malformation who has none of the above symptoms or findings, it is generally agreed that observation is the preferred treatment.
- Hydrocephalus: A minority of patients with Chiari I malformation will also have hydrocephalus. If there is any uncertainty as to the coexistence of hydrocephalus, intracranial pressure monitoring is recommended prior to posterior fossa decompression.
- Craniocervical stability: Chiari I malformation often occurs with other anomalies of the craniocervical junction. Flexion and extension radiographs or MRI should be performed prior to posterior fossa decompression if there is concern that there may be instability. If this is not recognized, the posterior fossa decompression may increase the instability, causing worsening of symptoms, and possibly requiring further surgery.
- Acquired tonsillar descent: In rare circumstances, the tonsillar descent is secondary to other problems, namely pseudotumor cerebri, occult CSF leak, brain tumor, or multiple lumbar punctures. If there is clinical suspicion, these problems should be investigated/ruled out.
All surgical interventions for Chiari I involve creating more room in the posterior fossa and foramen magnum to relieve the hindbrain compression. The patient is positioned prone (face down) on the operating table with the head in a fixation device. Incision is made in the midline, and the muscles and soft tissues are separated. Bone is exposed from the occiput to the top of the C2 vertebra.
- Bony decompression: The simplest option is bony decompression only. After exposing bone, a portion of the occiput and the posterior arch of the C1 vertebra are removed. The dura and underlying brain are not disturbed. This operation is performed by a minority (but a growing number) of surgeons. Most believe that expansion of the dura is also needed.
- Bony decompression with duraplasty: This technique is probably the most common method of Chiari I decompression. Bone removal is performed as above, after which the dura is opened. Some surgeons take great care not to disturb the underlying arachnoid membrane, while others open this layer as well. Leaving the arachnoid intact may prevent the CSF from leaking out, thus presumably decreasing the incidence of aseptic meningitis and pseudomeningocele. A graft is then sewn to the dura, effectively expanding the posterior fossa. Materials used for grafting include autologous tissue (fascia from patient’s own tissues), bovine pericardium, and a variety of synthetic materials. It is not known what the best material to be used for duraplasty is.
- Tonsil resection or shrinkage: In addition to opening the bone and dura, some surgeons prefer to address the tonsilar descent directly. This can be done by either applying bipolar electrocautery to the tonsillar tissue, causing it to shrink, or by resecting the tonsils. In either case, the goal is to ensure that there is no obstruction of CSF flow out of the 4th ventricle.
- Other options: Several other surgical strategies are employed to assist in a Chiari I decompression. Some centers use ultrasound to visualize the movement of the tonsils with the cardiac and respiratory cycles, or to visualize the flow of CSF before and after decompression. This can be used as an aid to determine whether dural opening or tonsillar shrinkage is necessary. In addition, various combinations of the above procedures are sometimes used. For example, some surgeons will open the dura, shrink the tonsils, and close the dura without a graft. Others use endoscopy to shrink the tonsils through a small skin incision.
When a Chiari I malformation is severely symptomatic, causes neurological deficits, or is associated with syrinx formation, the treatment is surgical. All options for surgery have a common goal of expanding the posterior fossa allowing normalization of CSF through the 4th ventricle and foramen magnum. This goal can be accomplished in many ways, from simple bone removal, to duraplasty and tonsillar resection. So far, no one method has shown superiority over the others.