Chiari Malformation Treatment & Management

Patients with Chiari I malformations who have minimal or equivocal symptoms without syringomyelia can be treated conservatively. Mild neck pain and headaches can be treated with analgesics, muscle relaxants, and occasional use of a soft collar. Frankly symptomatic patients should be offered surgical treatment.

The goals of surgical treatment are decompression of cervicomedullary junction and restoration of normal CSF flow in the region of foramen magnum. Considerable controversy has existed throughout the years about the surgical steps that are required to achieve these goals. The author's preferred technique of suboccipital craniectomy, cervical laminectomy, duraplasty, and arachnoid dissection is described below. Alternatives approaches are discussed under Future and Controversies.

Preparation for surgery for Chiari I decompression is the same as for any elective surgery and depends on the patient's general health. The author routinely obtains CBC, basal metabolic panel, PT, aPTT, chest radiograph, and ECG. Blood is typed and screened. The patient is restricted to nothing by mouth (NPO) after midnight and admitted on the morning of surgery.

Thigh-high anti-embolic stockings and sequential compression devices are applied. Antibiotic prophylaxis with cefazolin or vancomycin is given within 1 hour of making the incision. Dexamethasone is given. Mannitol is not given. If adequate peripheral venous access cannot be established, a central venous catheter is inserted. A Foley catheter is inserted. An arterial line may or may not be inserted, depending on the anesthesiologist's preference. Intubation is carried out, with careful attention given to the extent of neck extension.

The patient is placed on the operating table in prone position. The author avoids the sitting position because of the potential risks of venous air embolism, intraoperative hypotension, subdural hygroma or hematoma due to excessive CSF loss, and the awkwardness of surgical access. The head of the bed is elevated 20-30° and the arms are tucked by the patient's sides. All pressure points are padded. The abdomen and the male genitalia are allowed to hang free between parallel gel rolls.

The neck is flexed at the craniocervical junction and extended at the cervicothoracic junction. Adequate flexion at the craniocervical junction is of paramount importance to safe and expeditious access to the foramen magnum. The head is fixed in neutral position in 3-point skeletal fixation in a Mayfield head holder. A strip of hair is shaved along the midline of the occiput, extending above the inion. The area is prepped and draped.

A midline incision is created, extending from 2-3 cm above the inion to the upper cervical spine. A large pericranial graft is harvested from the upper pole of the incision. An adequately-sized pericranial graft is of utmost importance in achieving a satisfactory water-tight duraplasty. One must not hesitate to extend the incision superiorly, if this is necessary to harvest a proper graft.

The inion, the midline of the occiput down to the foramen magnum, the posterior arch of C1 and the upper aspect of C2 lamina are exposed. Additional laminae may be exposed, depending on the extent of tonsillar descent. Importantly, the muscular attachments to the superior nuchal line are left intact, but the muscles are stripped off the occipital bone just superior and lateral to the foramen magnum. Large epidural veins are encountered laterally between the occiput and C1 posterior arch. If bleeding occurs from these veins, it can be controlled with judicious use of bipolar coagulation and Gelfoam. The lateral exposure along C1 posterior arch may extend to vicinity of the medial rim of the sulcus arteriosus of the vertebral artery, but there is no need to expose that artery. The condylar emissary veins entering the occiput are generally too lateral to be of concern in this exposure.

A conservative suboccipital craniectomy is carried out from the inferior nuchal line to the posterior and lateral rims of foramen magnum. Occasionally, a prominent internal occipital crest extends deeply between the two cerebellar hemispheres' dura and must be resected with a small rongeur. Overzealous resection of occipital bone may result in cerebellar ptosis and should be avoided. The goal is to decompress the foramen magnum, not the entire posterior fossa. Removal of the bony edge of the foramen magnum must extend laterally until the lateral surface of foraminal dura is visualized. Similarly, a wide C1 laminectomy is carried out until the curvature of dura is appreciated. C2 and (rarely) C3 laminectomy may have to be carried out if the tonsils descend to those levels.

The dura is opened in Y-shaped fashion, with the oblique limbs of the Y transecting the paired inferior limbs of the occipital sinus. As the latter are transected, the dural edges are coagulated or oversewn. As the oblique Y limbs come together, the inferior extension of the cerebellar falx may have to be divided. The vertical limb of the "Y"-shaped incision extends down the midline of spinal dura. In patients with Chiari malformations, a tight dural band is often observed at the level of foramen magnum, constricting the dura. This is incised along the midline. The author prefers to then "T" the spinal dural incision transversely at the inferior extent of the Y. The dural leaflets are tented with sutures to the suboccipital muscles and fascia.

The cisterna magna arachnoid is opened under the operating microscope. The goal of arachnoidal dissection is to ensure unimpeded CSF flow through the fourth ventricle outlet and around the cervicomedullary junction, particularly in patients with syringomyelia. The arachnoidal adhesions between the two cerebellar tonsils and between each tonsil and the medulla are carefully divided to mobilize the tonsils superior-laterally and to expose the obex and floor of the fourth ventricle. Extreme care is taken to avoid injury to the tonsillar segments of the posterior inferior cerebellar arteries.

In a syringomyelic patient, if the tonsils are densely adherent to the medulla and cannot be readily resected, a limited subpial resection of the tonsils may be carried out. In a patient with high cervical syringomyelia or syringobulbia whose cervicomedullary neural tissue has been thinned down to a thin membrane, a limited midline myelotomy may be carried out to directly decompress the syrinx.

A careful watertight duraplasty is carried out using the pericranial graft harvested at the beginning of the operation. The intradural compartment is filled with warm saline before the last suture is tied. Any leakage is meticulously repaired. The duraplasty is covered with fibrin glue or DuraSeal glue. After the retractors are removed, meticulous muscle hemostasis is secured to avoid a postoperative epidural hematoma. The wound is then closed in layers.

Chiari II malformations are decompressed in a similar fashion, except that a multilevel cervical laminectomy may be required. No attempt is made to dissect the tightly adherent cerebellar tonsils from the brainstem in Chiari II.

The patient is carefully observed during the first 24 hours after surgery for any signs of brainstem dysfunction, particularly apnea, which is a rare but serious complication of Chiari surgery. In the author's practice, all Chiari patients spend the first postoperative night in the ICU. The patients are then rapidly mobilized and usually discharged home by the end of postoperative day 2, as long as they are neurologically intact, ambulatory, and able to eat without vomiting. Incisional pain and muscle spasms are common and are controlled with opiate analgesics and muscle relaxants. Some patients obtain partial pain relief by use of a soft cervical collar. Nausea and vomiting are also common and may prolong the hospitalization.

During postoperative visits, patients are questioned about persistence or improvement of their preoperative symptoms. The incision is inspected for CSF leakage and pseudomeningocele formation. A small pseudomeningocele presenting as fullness in the back of the neck is managed conservatively and often resolves after a couple of months. A large symptomatic pseudomeningocele may necessitate percutaneous drainage or surgical repair. Heavy lifting and strenuous exercise are avoided for the first 2-3 weeks after surgery. Recovery is usually complete in 4-6 weeks for those with good preoperative neurological function. Patients who started with a major neurological deficit require postoperative rehabilitation. Patients with syringomyelia undergo repeat MR imaging to confirm that the syrinx has responded to Chiari decompression.

The most common complications of Chiari decompression are pseudomeningocele formation and CSF leakage. Early detection and repair of CSF leakage prevent the more serious complication of meningitis.

Wound infection and meningitis are rare. Patients with a high postoperative fever for which no other etiology can be found undergo a lumbar puncture. A CSF formula consistent with bacterial meningitis warrants initiation of broad-spectrum intravenous antibiotic treatment while awaiting CSF culture growth. Aseptic meningitis is treated with corticosteroids.

In the immediate postoperative period, lower brainstem dysfunction, apnea, and epidural hematoma are rare but serious complications. Close observation of the patient in the immediate postoperative period is warranted.

Other rare complications include vertebral artery injury and increased neurological deficit as a result of surgical manipulation of the brainstem and the spinal cord or due to vascular occlusion.

Cerebellar ptosis may occur when a large suboccipital craniectomy has been performed and may result in recurrent interruption of CSF flow across the foramen magnum and recurrent syringomyelia.^[11] Pre-existing craniocervical hypermobility syndromes and basilar invagination may worsen and may require craniocervical fusion.

Persistent symptomatic syringomyelia after adequate decompression of the cervicomedullary junction may require direct shunting of the syrinx.

Prognosis after surgery for Chiari I is generally good and depends on the extent of preoperative neurological deficits. Those with little or no neurological deficit, symptomatic primarily with pain, can expect an excellent outcome. Those with severe and longstanding weakness and muscle atrophy are less likely to improve.

The main controversy is treatment of Chiari I malformation centers around the question of what surgical steps are necessary to achieve decompression of the cervicomedullary junction and restore CSF flow across foramen magnum. The most widely accepted approach consists of limited suboccipital craniectomy, C1 laminectomy, duraplasty and arachnoidal dissection as described above.^[11] On the other side of the spectrum are recommendations for bony decompression without opening the dura.^{[12, 13]}

Most surgeons who recommend this approach resect or incise the thick fibrous band that constricts the dura at foramen magnum without opening the dura. Some recommend serial incisions of the outer layer of the posterior fossa dura, with the expectation that this may expand the posterior fossa volume.^[13] Others recommend opening the dura, but not the arachnoid, and then performing a duraplasty. Still others recommend keeping the arachnoid closed but leaving the dura open, without duraplasty.^[14] Some recommend performing an occipitocervical fusion at the time of decompression, citing up to 19% rates of craniocervical instability in pediatric patients after Chiari decompression.^[15]

Those who advocate the more conservative extradural and extra-arachnoidal approaches seek to avoid CSF-related complications (CSF leak, pseudomeningocele, aseptic meningitis), particularly in the pediatric population.^[13] However, other studies indicate that the incidence of such complications is too low to justify a positionally suboptimal extradural decompression.^[16]

The neurosurgery literature is replete with studies that support all of these approaches, but, unfortunately, most of the studies provide only class III or limited class II evidence. Prospective controlled trials are needed to compare the various techniques in adult and pediatric patients and in those with and without syringomyelia.