When a Chiari malformation is associated with syringomyelia and syringobulbia, it may be difficult to separate the effects of the two disorders. A typical example is shown in Fig. 37-4. Clinical features that favor the predominance of Chiari malformation are nystagmus, cerebellar ataxia, exertional head and neck pain, prominent corticospinal and sensory tract involvement in the lower extremities, hydrocephalus, and craniocervical malformations. In syringomyelia without a Chiari malformation but with some other type of obstructive lesion at the foramen magnum, the clinical picture is much the same, and the nature of the foramen magnum lesion can be determined only by MRI or surgical exploration.
This refers to a dilatation of the central canal that is distinct from developmental syringomyelia. The relationship between hydromyelia and syringomyelia has been the source of endless debate, in part the result of the lack of a coherent pathophysiologic explanation for either process. At least one hypothesis for the origin of syringomyelia includes an initial dilatation of the central canal (see later). Our impression is that a relatively nonprogressive, well-defined, cylindrical enlargement of the central canal over a few thoracic segments is a frequent enough occurrence in the absence of clinical changes that it represents an independent entity. In the few cases of symptomatic hydromyelia that have come to our attention, there had usually been a long-standing congenital hydrocephalus complicated years later by progressive weakness and atrophy of the shoulders and the muscles of the arms and hands. More often, there is no associated obstruction at the upper cord and no hydrocephalus for which reason it is our impression that most cases are benign and relatively nonprogressive. Proof of the existence of pure hydromyelia in the past has been based on necropsy demonstration of an enormously widened central canal, with or without hydrocephalus. Now, hydromyelia is easily diagnosable by MRI and numerous asymptomatic cases are being discovered, causing unnecessary concern and neurologic consultation.
Pathogenesis of Type I Syringomyelia
Experimental work in animals has indicated that there is a normal flow of CSF from the spinal subarachnoid space through perivascular spaces to the parenchyma of the cord and possibly into the central canal. It has been suggested that impediments to flow might explain dilatation of the central canal, or the creation of a parallel or attached syrinx cavity.
One theory of the pathogenesis of developmental syringomyelia, of which Gardner was the main advocate, is that the normal flow of CSF from the central canal to the fourth ventricle and its outlets is prevented by an obstruction of the foramina of Luschka and Magendie. As a result, a pulse wave of CSF pressure that is generated by systolic pulsations of the choroid plexuses is transmitted into the cord from the fourth ventricle through the central canal. According to this theory, the syrinx consists essentially of a greatly dilated central canal with a diverticulum that ramifies from the central canal and dissects along gray matter and adjacent fiber tracts. The frequency with which syringomyelia is linked to malformations at the craniocervical junction, that is, to Chiari and other lesions that could interfere with normal flow of CSF, lends credence to this theory.
There are many instances, however, in which