Chiari Malformations and Syringohydromyelia

In the early 1890s, the pathologist Hans Chiari described four congenital malformations that would later become known as the Chiari malformations . The four traditional varieties of Chiari malformations represent varying degrees of involvement of rhombencephalic derivatives. Three of these (types I to III) have progressively more severe herniation of these structures outside the posterior fossa as a common feature. These three also have in common a pathogenesis that involves a loss of free movement of cerebral spinal fluid (CSF) out of the normal outlet channels of the fourth ventricle. The fourth (type IV) consists of cerebellar hypoplasia or aplasia. Pathologic differences between Chiari I malformations (CIMs) and Chiari II malformations (CIIMs) can be explained with knowledge of the differences in the timing of the development of the hindbrain herniation.

Although a large majority of hindbrain hernias are congenital, acquired CIMs occur and are not rare. Not considered further in this chapter are the patients who have movement of their cerebellar tonsils into the cervical spine because of an intracranial tumor, or another mass lesion, especially within the posterior fossa, or a lumbar CSF shunt. Technically, these patients have a CIM but treatment of the cause of their hindbrain hernia usually allows resolution of their secondary CIM.

Within the large group of patients with hindbrain hernias due to some problem with equilibrating CSF across the craniocervical junction, several subclassifications have been developed and are included with the following brief descriptions of the traditional forms of Chiari malformations:

• Chiari 0— These patients do not appear to have significant hindbrain hernias, although the posterior fossa may appear “crowded”; they have large syringes that resolve with posterior fossa decompression and have a unique position in our thinking about this subject. These malformations have been informally termed Chiari 0 because they behave as though they have fourth ventricular outlet obstruction, and at surgery they frequently do have physical barriers to CSF movement but do not have caudal displacement of the cerebellar tonsils beyond a point that could be considered pathologic. It is suspected that abnormal CSF flow, which may be intermittent, as well as intermittent tonsillar herniation related to the Valsalva maneuver, underlie this condition. Studies have suggested that there may be a similar underlying developmental or genetic anomaly responsible for both Chiari 0 and CIM.

• Chiari I— This common group of patients has been found to have caudal displacement of the cerebellar tonsils more than 5 mm below the foramen magnum for adults and 6 mm for children. The brainstem is in a normal position and they may or may not have a syrinx. The 5 mm “rule” concerning the definition of pathologic extent of caudal migration of the tonsils is arbitrary. Numerous patients have tonsils well below this point and are asymptomatic, especially young infants and children. When followed over time they frequently remain asymptomatic if their initial evaluation was performed for an unrelated reason. The extent of their caudal migration may progressively improve with time and become less impressive. This, however, is not assured and the patient should be followed for the development of symptoms. Syringomyelia is often associated with the CIM.

• Chiari I.5— Although somewhat confusing, this term is applied to patients who bridge the gap between the CIM and CIIM. It may be referred to as the bulbar variant of CIM. They have characteristics of both groups and are best considered separately. They are unassociated with neural tube defects and have caudal displacement of the cerebellar tonsils similar to that seen in the CIM patient. However, their brainstem and fourth ventricle are displaced inferiorly like CIIM patients. There is no displacement of the midbrain or pons. In series of patients with a hindbrain hernia but without a neural tube defect, 17% had significant caudal displacement of the brainstem.

• Chiari II— This lesion almost always occurs in patients with neural tube defects (myelomeningoceles and encephaloceles). It consists of caudal migration of the cerebellar vermis, brainstem, and fourth ventricle. Additional structures, including choroid plexus and the basilar-vertebral circulation, may also be displaced caudally. Syringomyelia is common in this group.

• Chiari III— The Chiari III malformation is a rare and extreme form of hindbrain hernia that may be confused with an occipital encephalocele. It comprises less than 1% of all patients in this category. Patients have low occipital or high cervical sacs containing significant portions of the cerebellum and brainstem. Other intracranial anomalies such as are found with CIIM patients may be seen with this group. Hydrocephalus is common, and severe neurologic and developmental problems are almost always present.

• Chiari IV— Type IV Chiari malformation patients have cerebellar hypoplasia/aplasia without evidence of hindbrain hernia. For this reason, inclusion in a discussion of hindbrain hernias is debatable and will not be considered further.