Anatomy and Pathophysiology

In the Chiari I malformation, the cerebellar tonsils are present within the foramen magnum, usually down to the ring of C1, and occasionally even farther down into the spinal canal to the rings of C2 or C3. Some degree of “hanging” down of the cerebellar tonsils into the cervical spinal canal is considered to be a normal variant. Radiographically, cerebellar tonsillar herniation is clearly abnormal when it is 5 mm below the foramen magnum and, especially, when it has a triangular or “beak-like” appearance ( Fig. 12.1 ). Studies using cine magnetic resonance imaging (MRI) lead to a more physiological definition of the Chiari I malformation when alteration of flow of cerebrospinal fluid (CSF) can be found at the foramen magnum. Normally, CSF exits the skull during systole and returns during diastole. This flow can become obstructed, particularly posteriorly, in symptomatic Chiari malformations ( Fig. 12.2 ).

Chiari I malformation is found as an isolated phenomenon and is associated with a great variety of other conditions. For the purpose of this discussion, tonsillar herniation secondary to an intracranial mass lesion has been omitted, although in many ways the pathophysiology is the same. Spinal imaging with and without contrast should be considered in a patient presenting with a Chiari malformation to rule out the presence of a tumor or other congenital anomaly.

Cine MRI has gained popularity for the preoperative evaluation of patients. Some evidence suggests that patients with normal CSF flow are more prone to recurrent symptoms after surgery.8 Current studies do not support the use of this imaging modality alone as an indicator of which patients will respond well to decompression; rather, it is appropriate as an adjunct for decision making. Postoperatively, failure to reestablish CSF flow at the foramen magnum in a persistently symptomatic patient is an indicator that full decompression has not been achieved.

Conditions Associated with the Chiari I Malformation

Four conditions are associated with Chiari I malformations. The first is hydrocephalus. Although it is difficult, if not impossible, to distinguish whether hydrocephalus caused a hindbrain hernia or a hindbrain hernia caused hydrocephalus, this form of Chiari I malformation was the subject of Chiari′s original studies.1,2 Second, cephalocranial disproportion exists when the skull is nondistensible and the growing brain cannot be contained in it. It occurs in craniosynostosis syndromes, such as Crouzon and Pfeiffer syndromes, in which venous hypertension related to jugular foramen stenosis may play a role.9 There is also a close association between true lambdoid craniosynostosis and a hindbrain hernia in the context of syndromic craniosynostosis.10 It is seen in patients with synostosis and microcephaly secondary to early treatment of hydrocephalus, with resultant secondary closure of multiple sutures.11 Third, abnormally low intraspinal pressure has been reported to lead to chronic herniation of the cerebellar tonsil after lumboperitoneal shunting as a result of sacral meningoceles and in the condition known as spontaneous intracranial hypotension, which is presumed to be caused by the spontaneous rupture of a perineural cyst.6,11–14 Finally, craniovertebral junction (CVJ) abnormalities with crowding and compression of structures at the skull base are discussed in more detail in the section on the management of Chiari malformation associated with basilar invagination. As reported by Aquilina and colleagues15 in a study of children receiving cranial radiation, growth of the clivus was intimately associated with tonsillar descent. Clival growth was arrested during and immediately after radiation, during which time the tonsillar descent worsened. Subsequently, the posterior fossa enlarged as clival growth returned to normal and the Chiari malformation improved.

Clinical Syndromes

Numerous clinical syndromes are associated with the Chiari I malformation, and the radiological appearance characteristic of the Chiari I malformation may be observed as an apparently incidental finding in patients undergoing MRI examinations for other reasons. In very young children, the presentation can be uncontrollable screaming, occasionally with retropulsion of the head as in opisthotonos. It is not uncommon for children under the age of 6 years to present with oropharyngeal dysfunction.16 In severe cases, slow feeding, drooling, snoring, and even apnea and stridor can herald a symptomatic Chiari malformation. In older children and adults, the most common complaint is severe sub-occipital headache. Clumsiness and unsteadiness are often reported, but true ataxia is rarely documented.

Classic presentations for the Chiari I malformation include severe headaches associated with a Valsalva maneuver and a distinctive neuro-ophthalmologic syndrome. Patients complain of not being on the floor or of the floor coming up to them. They are found to have downbeat nystagmus, which is characteristic of this condition.

Surgical Principles

The goals of surgery are to remove the compression from the brainstem and reestablish normal patterns of CSF flow. As an adjunct to surgery, it is our policy to monitor somatosensory evoked potentials (SSEPs). In one case, the disappearance of the SSEP during positioning of a patient led to the diagnosis of a previously unrecognized occipitocervical instability. Repositioning led to return of function, and a fusion was performed in association with decompression.

The patient is placed in a head holder in a slightly flexed position. We tend to use a 1-cm wide clipper to remove hair on either side of the incision to facilitate closure. The hair is prepared with povidone-iodine scrub. Next, iodine gel is applied to the hair to facilitate its staying in position after it is combed. A midline incision is made from the inion to the midcervical spine and carried down to the fascia.

At this point, the fascial planes are used as a guide and the foramen magnum is exposed. A subperiosteal dissection of one laminar arch below the intended level of laminectomy is performed, and the periosteum is removed from the occipital bone at least 5 cm up from the foramen magnum. Exposure is maximized by the use of “fish hooks” in the muscle attached to the Leyla bars (Aesculap, San Francisco, CA) on either side. This process not only pulls the muscle apart but pulls the skin down and leaves the surgeon without the normal deep hole frequently seen in such surgical procedures ( Fig. 12.3 ).

Next, the rostral extension of the ligamentum flavum and epidural fat is removed, exposing the dura from C1 to the foramen magnum. An up-angled curette is used to disconnect the dura from the undersurface of the foramen magnum. The rim of the foramen magnum is removed to a distance of 1.5 to 2.0 cm. How the rim is removed depends primarily on the patient′s age. In children, the bone is usually removed with a Kerrison rongeur. In adolescents and adults, removing the rim is quite difficult because of the thickness of the bone in that location and the difficult angle at which the operating surgeon must work. In such cases, the Midas Rex (Midas Rex Pneumatic Tools, Inc., Fort Worth, TX) instrument is used. An AM8 attachment (a cutting bur) is used to thin the bone 1.5 to 2 cm down to the dura, and the remaining bone is removed with a curette or Kerrison rongeurs. Alternatively, the footplate attachment is used to turn a craniotomy flap. If the footplate is used, care must be taken in the midline to remain extradural where a large keel can occur.

The posterior arch of C1 and, if necessary, the laminae and spinous process of C2 are removed with the ligamentum flavum using a combination of Leksell and Kerrison rongeurs. Meticulous hemostasis is essential so that minimal, if any, blood is spilled into the subarachnoid space after the dura is opened. The surgical gutters on either side of the exposure are lined with Cottonoids (DuPuy, Raynham, MA) to help control minor amounts of residual tissue ooze. Surgifoam hemostatic agent can also be applied.

Another controversial step in Chiari management is the need to open the dura. Zamel and colleagues reported that the most significant improvement in brainstem auditory evoked potentials occurred during bony decompression, without significant additional improvement upon dural opening.17 Many practitioners consider the presence of syringomyelia to be an absolute indication for dural opening. It is also widely held that arachnoid obstruction of the outlet of the fourth ventricle must be released. Conversely, others open the dura only with no disruption of the arachnoid to prevent future scarring.18

A 4–0 braided nylon dural stitch is used to tent the dura for opening in the midline, and the dura mater is opened using a no. 11 or 15 blade. The opening should be well below the foramen magnum and should extend caudally to the lowest point of the dural opening. After a small opening is made in the dura, we prefer to use a no. 12 blade to complete the opening. This blade, which has the appearance of a scimitar, allows complete control of the opening, as all movements of the blade are up, out of the wound, and away from the central nervous system (CNS) ( Fig. 12.4 ).

This approach to opening the dura is desirable because of the potential risk of severe bleeding when the dura of the posterior fossa is opened. In normal patients, a “circular sinus” exists at the level of the foramen magnum. Dural sinuses are collections of flowing blood between the leaves of the dura. These sinuses do not occur within the spinal dura, but their size and location in the posterior fossa vary greatly, especially in small children. This point is extremely important in the discussion of the Chiari II malformation. If the surgeon opens one of these sinuses directly over the dura, it is likely that only one wall of the sinus will be opened. Consequently, it becomes very difficult to stop the bleeding, and significant blood loss can occur even in a small and relatively unimportant circular sinus. In this situation, use of monopolar or bipolar cauterization often leads to continual expansion of the hole, and the rate of bleeding increases and makes control more difficult. However, if the dura is opened with the no. 12 blade from the cervical dura to the cranial dura, both leaves of the dura are opened simultaneously. In this situation, bleeding can be controlled simply by putting an instrument under the dura and lifting upward, compressing the two leaves of the dura together. At this point, definitive hemostasis can be obtained by coagulating the two leaves, thus sealing them together. When the sinus is so large that it is difficult to execute this technique, titanium clips can be applied to assure the seal. To minimize artifact on postoperative imaging studies, we use 4–0 braided nylon sutures to sew the dural leaves together and then remove the titanium clips.

Because the presumed pathophysiology of the symptomatic Chiari I malformation relates to the isolation of the intracranial pressure compartment from the intraspinal pressure compartment, one of the goals of this surgical procedure is to reestablish normal CSF dynamics at the CVJ. For this reason, we believe that part of the surgical approach to this anomaly involves assuring that the foramen of Magendie is patent. Using the microscope, the surgeon either retracts upward or removes a small portion of the lower tip of the cerebellar vermis with bipolar cauterization. A veil, which is typically present, must be opened to visualize the porcelain white floor of the fourth ventricle.

This opening into the fourth ventricle is one of the controversial points in this surgical procedure. Some neurosurgeons believe that this step is unnecessary unless hydrocephalus or syringomyelia is present. It is difficult to prove or disprove this point. We perform this step primarily for theoretical reasons, believing that it does not add to the morbidity of the procedure. Another group of neurosurgeons advocates that the opening into the fourth ventricle be assured using a stent, usually a piece of shunt tubing.19 However, we believe that these stents become a nidus for scar formation, almost always occluding with time and potentially leading to postoperative tethering of the brainstem.

The most controversial aspect of this procedure, with the possible exception of surgical indications, is the method for closure. Williams20 and Krieger and colleagues21 believe that the dura should be left widely open. However, most neurosurgeons believe that the dura should be closed with a patch graft to create a large CSF space around the cervicomedullary junction. Choice of material is also controversial. Available materials include autologous fascia, Durasis porcine graft (Cook Biotech, West Lafayette, IN), and Durepair synthetic material (Medtronic, Inc., Minneapolis, MN). Both the Durasis and Durepair require soaking in irrigation for at least 30 minutes to prevent chemical meningitis.

Autologous material can be obtained from the pericranium, contiguous fascia of the paracervical musculature, or fascia lata of the thigh. The use of the paracervical musculature leads to more postoperative discomfort and occasionally leads to a troublesome pseudomeningocele related to the difficulty in creating a watertight seal. Use of fascia lata results in an un-sightly scar. At this time we are using pericranium, which can be harvested with only a minor increase in the size of the incision and has handling properties more similar to dura. The graft can be sewn in place with absorbable or nonabsorbable suture ( Fig. 12.4 ).

After the graft is sewn in place, its integrity should be checked by having the anesthesiologist perform several Valsalva maneuvers. To prevent the graft from scarring to the underlying tonsils, a titanium plate has been developed to span just above the previous foramen magnum ( Fig. 12.5 ). The Codman Chiari plate (Codman Corp., Raynham, MA) is attached with screws bilaterally, and a stitch is placed in the center of the graft to tent it up to the plate. This maneuver re-creates the CSF space dorsal to the tonsils. The deep cervical musculature is then closed, and the fascia is closed with absorbable suture. The integrity of the fascial closure is essential in preventing pseudomeningocele. The remainder of the closure is routine.

Postoperatively, there is no need for cervical immobilization, although the patient will be quite uncomfortable and will require a significant level of analgesia for a few days. We tend to use frequent small doses of intravenous morphine sulfate, which may be supplemented with muscle relaxants in adolescents and adults.