17 Chiari Malformation
Penny P. Liu, Krystal Tomei, and J. Brad Bellotte
KEY POINTS
1. The Chiari malformations should be viewed as a spectrum of defects involving the brainstem, cerebellum, cervical spine, spinal cord, clivus, and subocciput.
2. Chiari I malformations are more common and the least severe of the forms. It most often presents in young adulthood.
3. Chiari II malformations are less common and more severe in this spectrum of defects. It is frequently associated with myelomeningocele, hydrocephalus, and other associated spinal cord malformations such as syrinx and tethered spinal cord.
4. Important anesthetic considerations include positioning, establishing adequate intravenous access, venous air embolism, invasive blood pressure monitoring, and neurophysiologic monitoring.
5. The type of anesthetic technique and drugs used in maintaining general anesthesia is not nearly as important as the appreciation that the ultimate anesthetic goal is a rapid, though carefully planned emergence. It should be the goal of both the anesthesiologist and surgeon to optimize postoperative neurologic assessment.
1
THE CHIARI MALFORMATIONS (CM) represent four types of hindbrain anomalies. It can be said that they are a spectrum of defects involving the craniocervical junction including the brainstem, cerebellum, cervical spine, spinal cord clivus, and subocciput.
I. Historical considerations
A. In 1883, Cleland [1] described nine cases of infants found at autopsy to have hindbrain malformations.
B. In 1891, Chiari described what was later classified as the Chiari II (CM-II) malformation [2].
C. In 1894, Arnold reported on a collection of individuals with congenital defects in the hindbrain and hydrocephalous (consistent with the later named Chiari I malformation CM-I).
D. The term Arnold–Chiari malformation was used interchangeably throughout the 1950s and 1970s to refer to CM-I and CM-II malformations. However, there are differences in the etiology of the conditions and this grouping should be avoided.
II. Chiari malformation I
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A. Anatomy
1. Cerebellar tonsils descend through the foramen magnum due to overcrowding of the hindbrain.
2. The cerebrospinal fluid (CSF) is displaced by the abnormal anatomy. This causes alterations in the flow of the CSF. It is believed that this altered CSF flow can lead to the development of a cervical spine syrinx (fluid-filled segment within the spinal cord) in a number of instances.
3. The cerebellar tonsils can cause direct compression of the brainstem leading to neurologic signs and symptoms.
B. Signs and symptoms
1. Valsalva-induced suboccipital headache are common.
2. Visual complaint such as blurred vision, flashing lights, diplopia, and sensitivity are associated with Chiari I malformation.
3. Otologic complaints can include tinnitus, feelings of pressure, vertigo, or decreased hearing.
4. Dysphagia, apnea, nausea, and ataxia are other complaints that can arise from the brainstem compression by the cerebellar tonsils.
5. A syrinx in the cervical spine will cause motor and sensory deficits of the extremities.
6. Many patients with Chiari I malformation may have various other complaints that are not as easily attributable by the local anatomy.
C. Diagnosis
1. MRI scan of the brain is regarded as the definitive test in the diagnosis of Chiari I malformation. It will show descent of the tonsils through the foramen magnum. This causes crowding of the brainstem at the foramen and even at the C1 and C2 vertebrae at times.
2. MRI scan of the cervical spine is useful in the diagnosis of a syrinx associated with the Chiari I malformation. Sometimes the disorder is first detected on a cervical MRI ordered to work up cervical complaints. Usually the tonsils and foramen magnum are imaged on the sagittal images. A close inspection of the area around the brainstem can identify patients with Chiari I malformation.
3. It is important to appreciate that there is not just a sole criterion used to make the diagnosis of Chiari I malformation. There is increasing evidence that careful interpretation of imaging studies, clinical picture, and even the quantitative analysis of CSF dynamics aids in the accurate diagnosis of this condition [3–6].
D. Treatment
1. Surgical considerations
a. The goal of surgical treatment is to provide more room at the base of the brain and posterior cervical spine. This involves a suboccipital craniectomy in which a window of occipital bone down to the foramen magnum is removed. If necessary, a C1 and C2 laminectomy can provide additional room. There are several accepted techniques to provide additional room. This often involves opening the dura and resecting the herniated cerebellar tissue. An expansion duraplasty is commonly used as part of the closure.
2. Risks
a. General surgical risks
(1) These risks include, but are not limited to, bleeding, venous air embolism, infection, pulmonary, and cardiovascular risks.
b. Specific procedure
(1) The vertebral artery is vulnerable during the initial dissection as it courses over the arch of C1 laterally. Injury to the vertebral artery at this level will result in profuse bleeding. A stroke may develop depending on collateral flow.
(2) The posterior inferior cerebellar artery (PICA) lies deep to the cerebellar tonsils. The PICA supplies blood to the lateral medulla, posterior cerebellum, and tonsils. An infarct due to an injury to PICA will result in Wallenberg syndrome. Signs and symptoms of Wallenberg syndrome include dysphagia, hoarseness, nausea/vomiting, vertigo, nystagmus, and gait and balance disturbances.
(3) The nuclei of the sixth and seventh cranial nerves are vulnerable in the floor of the fourth ventricle. Injuries to these structures will result in occulomotor dysfunction and facial paresis, respectively.
(4) The eleventh cranial nerve enters the foramen magnum on the lateral side of the spinal cord. Injury to this nerve is less common. However, it is very excitable and will result in muscle contraction with even very little manipulation.
E. Anesthetic considerations
1. Positioning
a. Typically, the corrective procedure is performed with the patient in prone position.
(1) The patient is placed in three-point head fixation.
(2) The neck is flexed to create an opening of the craniocervical junction.
(a) Check the position of the endotracheal tube after neck flexion by auscultation to rule out a mainstem intubation.
(b) The goal is to achieve a balance of neck flexion that facilitates optimal exposure and the reassurance of adequate venous outflow.
CLINICAL PEARL
Venous obstruction of the head and neck can lead to intracranial hypertension. Ideally, both surgeon and anesthesiologist should agree on the extent of neck flexion acceptable for surgical exposure, adequacy of ventilation, as well as venous outflow. Poor venous outflow of the head and neck region could potentially lead to macroglossia and intracranial hypertension.
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