Several other morphologic features are characteristic of the true Chiari anomaly. The medulla and pons are elongated and the aqueduct is narrowed. The displaced tissue (medulla and cerebellum) occludes the foramen magnum; the remainder of the cerebellum, which is small, is also displaced so as to obliterate the cisterna magna. The foramina of Luschka and Magendie often open into the cervical canal, and the arachnoidal tissue around the herniated brainstem and cerebellum is fibrotic. All these factors are probably operative in the production of hydrocephalus, which is always associated. Just below the herniated tail of cerebellar tissue there is a kink or spur in the spinal cord, which is pushed posteriorly by the lower end of the fourth ventricle. In this fully expressed form of the malformation, a meningomyelocele is nearly always found. It should again be emphasized that hydromyelia or syringomyelia of the cervical cord are commonly associated findings and the main theories of causation of the latter are based on the change in CSF dynamics produced by the Chiari malformation.
Developmental abnormalities of the cerebrum, particularly polymicrogyria may infrequently coexist, and the lower end of the spinal cord may extend as low as the sacrum (i.e., a tethered cord). There are usually cranial bony abnormalities as well. The posterior fossa is small; the foramen magnum is enlarged and grooved posteriorly. Nishikawa and colleagues suggested that smallness of the posterior fossa with overcrowding is the primary abnormality leading to the brain malformation. Often the base of the skull is flattened or infolded by the cervical spine (basilar impression).
In the less common but more consequential type II Chiari malformation (with meningomyelocele), the problem becomes one of progressive hydrocephalus. Cerebellar signs cannot be discerned in the first few months of life. However, lower cranial-nerve abnormalities—laryngeal stridor, fasciculations of the tongue, sternomastoid paralysis (causing head lag when the child is pulled from lying to sitting), facial weakness, deafness, bilateral abducens palsies—may be present in varying combinations. If the patient survives to later childhood or adolescence, one of the syndromes that are more typical of the type I malformation may become manifest.
In the more common type I Chiari malformation (without meningocele or other signs of spinal dysraphism), neurologic symptoms may not develop until adolescence or adult life. The symptoms are those of (1) increased intracranial pressure, mainly headache, (2) progressive cerebellar ataxia, (3) progressive spastic quadriparesis, (4) downbeating nystagmus, or (5) the syndrome of cervical syringomyelia (segmental amyotrophy and sensory loss in the hands and arms, with or without pain). Or the patient may show a combination of disorders of the lower cranial nerves, cerebellum, medulla, and spinal cord (sensory and motor tract disorders), usually in conjunction with headache that is mainly occipital. This combination of symptoms is easily mistaken for multiple sclerosis or a tumor at the foramen magnum. The symptoms are usually chronic but may have an acute onset after sustained or forceful extension of the neck, as, for example, after a long session of dental work, hairdressing, or chiropractic manipulation. The physical habitus of such patients may be normal, but approximately 25 percent have signs of an arrested hydrocephalus, or a short “bull neck.” When basilar impression (a congenital abnormality of the occipital bone that invaginates the posterior atlas into the cranial cavity) and a Chiari malformation coexist, it may be impossible to decide which of the two is responsible for the clinical findings.