Watertight dural closure and careful wound closure in multiple layers are imperative to prevent cerebrospinal fluid leak and pseudomeningocele.
Avoid vascular or brainstem injury with sharp, midline dissection within the subarachnoid space and with the judicious lysis of adhesions.
Prompt treatment of cerebrospinal fluid leak, infection, or hydrocephalus can prevent further complications.
Three types of Chiari malformations were described by Hans Chiari (1851–1916) in 1891, and half a century later in 1950, W. James Gardner described the surgical treatment by posterior fossa decompression (PFD). After yet another half century, the pathogenesis of the disease remains unclear. Although bony decompression for Chiari malformations is routine, there is still no consensus on the optimal surgical techniques, which may or may not include dural opening, expansile duraplasty, use of autologous or synthetic graft, arachnoid opening and dissection, and tonsillar reduction. Patients have 70.3% clinical improvement postoperatively whether or not the surgery included duraplasty. The reported rate of complications ranges from 2.4% to greater than 20% in various studies, and the complications can vary based on the invasiveness of the surgery. There has been no direct comparison of posterior fossa decompression with duraplasty (PFDD) and without duraplasty (PFD), but several studies and a metaanalysis indicate that patients who underwent PFD had a higher reoperation rate but a lower rate of cerebrospinal fluid (CSF) related complications than those who underwent PFDD. CSF leak or pseudomeningocele is typically the most common complication when duraplasty is performed, but others include wound infection, meningitis, hydrocephalus, need for reoperation, craniocervical instability, and cerebellar sag.
Inside the dura and arachnoid, the cerebellar tonsils are visible bilaterally and the uvula superiorly. The tela choroidea and inferior medullary velum make up the inferior part of the roof of the fourth ventricle and can be seen when the tonsils are retracted laterally. The posterior inferior cerebellar artery (PICA) has a complicated and variable course. It usually arises from the vertebral artery but can arise from the basilar artery; it may also be absent on one side, or duplicate. The location of the origin may vary as well from the extradural vertebral artery below the foramen magnum to the vertebrobasilar junction. It travels dorsally around the medulla and passes by, or through, the hypoglossal, glossopharyngeal, vagus, and accessory nerves, then enters the cerebellomedullary fissure, making a caudal loop at the inferior pole of the tonsil. The caudal loop of the PICAs is noted to be inferiorly displaced in angiograms performed on patients with Chiari malformation, due to the tonsillar herniation. Perforating arteries arise from the PICA as it passes around the medulla. Vermian branches of PICA or the tonsils sometimes lie in a more medial position and cause obstruction of the foramen of Magendie.
Several factors predict favorable or unfavorable outcomes after PFD for Chiari malformation. Young age at time of surgery and signs of paroxysmal intracranial hypertension are associated with favorable outcomes. Better preoperative status also indicates a better prognosis. Prior history of Chiari decompression, arachnoiditis, older age at time of surgery, and long-tract signs are associated with unfavorable outcomes. Young age at the time of initial surgery, complex bony foramen magnum anatomy, and syndromic craniosynostosis are associated with reoperation.
Several studies have found associations between the type of dural graft used and frequency of need for reoperation, CSF leak, and aseptic meningitis, but the results have varied from study to study, and more optimal results have been reported for both allogenic and autologous graft materials. Untreated hydrocephalus is associated with increased incidence of CSF leak or pseudomeningocele. Klippel-Feil syndrome, atlantoaxial assimilation, and basilar invagination can be predictive of craniocervical instability.
Imaging should be examined closely preoperatively. Surgeons can get an idea of the extent of bone removal required. The torcular and transverse sinuses may be low-lying, especially in Chiari II malformation, and should be avoided during bone removal. Hydrocephalus, which can contribute to a higher rate of CSF leak, can be treated with CSF diversion before a decompression. Anomalies associated with Chiari malformation—Klippel-Feil syndrome, atlantoaxial assimilation, and basilar invagination—can indicate possible craniocervical instability. Some surgeons routinely obtain preoperative cervical x-rays. Patients may need posterior occipitocervical stabilization, possibly preceded by ventral decompression. Instability should also be kept in mind postoperatively because rapid treatment can prevent neurologic damage.
Adequate bone removal at the foramen magnum, and possible duraplasty and careful arachnoid exploration and dissection, may prevent the persistence or recurrence of symptoms. If a duraplasty is not performed, the dural band at the foramen magnum and outer dural layer should be incised to help prevent the need for reoperation. Intraoperative ultrasound can also help identify whether additional decompression is required. If the dura is opened, blood and debris should be kept out of the subarachnoid space because they could lead to aseptic meningitis or adhesions and scar formation. Arachnoid dissection should be limited to the midline and should be sharp only, to avoid damaging perforators and cranial nerves. For the same reason, tonsillar reduction should be limited to coagulation or subpial removal rather than resection. Rather than cause brainstem injury by manipulation of the medulla and obex, dense adhesions between the tonsils and brainstem may need to be left in place.
The dural graft should be carefully sutured in a watertight fashion; a Valsalva maneuver after closure can help identify gaps in the closure. Meticulous closure of the fascia and soft tissues also helps prevent CSF leak. If a CSF leak occurs postoperatively, the incision can initially be resutured and managed conservatively, but imaging to assess for hydrocephalus should be obtained if the problem persists. Timely treatment of a CSF leak can help prevent postoperative infection. Preoperative antibiotics, intraoperative sterile technique, and management of risk factors such as diabetes should be standard for infection prevention, as well as prompt treatment when an infection is identified. A postoperative infection can lead to a possible recurrence of preoperative symptoms due to inflammation and subsequent scarring.
Intraoperatively, the transverse sinus may be lacerated during the craniectomy. Additional bone removal may be needed to better visualize the injury, which can be repaired by suturing with a small piece of muscle. Dural venous lakes can be clipped to stop bleeding. An operative site hemorrhage large enough to necessitate reoperation for hematoma evacuation rarely occurs but is a consideration with a poor postoperative neurologic examination.
Patients may develop fever, headache, nausea, vomiting, and malaise postoperatively. A lumbar puncture should be done to send CSF for culture, and antibiotics may be started. Aseptic meningitis can occur as a result of irritation from blood and debris; it is generally treated with symptomatic management and steroids after bacterial meningitis has been ruled out. If bacterial meningitis or wound infection is diagnosed, in addition to antibiotics, treatment may require replacement of the dural graft. The type of graft used for duraplasty has been linked to the incidence of need for reoperation, CSF leak, and aseptic meningitis, but varying results have been reported for different dural grafts with regard to these complications.
CSF leak and pseudomeningocele are the most commonly observed complications. Even if the arachnoid is not intentionally opened intraoperatively, it can be torn during dural opening, and CSF leaks can still occur. Initially, a leak can be managed conservatively with a dressing and additional sutures, but if the leak persists, imaging should be obtained and examined for hydrocephalus. If hydrocephalus is present, it must be treated with CSF diversion, or the leak will persist. In the absence of hydrocephalus, a leak can be treated with lumbar drainage and/or reoperation with revision of the duraplasty and improved soft tissue closure. A pseudomeningocele similarly can be treated with conservative measures at first if there is no evidence of hydrocephalus, and small pseudomeningoceles may resolve without any intervention. With a persistent pseudomeningocele, the duraplasty should be repaired or replaced. Hydrocephalus can also present after a PFD without an accompanying CSF leak and can be delayed months postoperatively. It may be possible to observe for development of symptoms, but placement of a shunt should be performed for symptomatic patients or those with worsening imaging findings.
When the preoperative symptoms persist or recur, imaging should be obtained. Mild recurrent symptoms may be followed initially. If compression is still evident on magnetic resonance imaging (MRI), reoperation is likely needed. Based on the imaging and intraoperative findings and the extent of the initial surgery, the second surgery could include additional bone removal, a more expansile duraplasty, wide lysis of adhesions, and coagulation of the tonsils. Reoperation can be technically challenging due to adhesions and scar formation, and intraarachnoid dissection must be performed meticulously.
An overly aggressive decompression can lead to the rare complication of cerebellar sag, in which the cerebellum herniates into the area of decompression when the craniectomy extends too far laterally. Patients present with symptoms of cerebellar or brainstem compression or return of syrinx several weeks to months postoperatively. For patients with signs of increased intracranial pressure, the surgical treatment includes placement of a ventriculopleural shunt. The other intervention is a revision with cranioplasty to partially reconstruct the posterior fossa. Occasionally, both are required.
Patients presenting with pain and progressive neurologic deficits after Chiari decompression may have craniocervical instability evident on MRI and cervical x-rays. Surgical treatment involves occipitocervical fusion stabilization.