Also, it is important to remember that during the subacute evolution of a transverse spinal cord lesion, there may be a discrepancy between the level of the lesion and that of the sensory loss, the latter ascending as the lesion progresses. This can be understood if one conceives of a lesion as evolving from the periphery to the center of the cord, affecting first the outermost fibers carrying pain and temperature sensation from the legs. Conversely, a lesion advancing from the center of the cord will affect these modalities in the reverse order, in a pattern of sacral sparing, meaning that sensation is preserved over the buttocks and anal region but is absent over the trunk and legs.
Hemisection of the Spinal Cord (Brown-Séquard Syndrome)
Disease may be confined to or predominate on one side of the spinal cord; pain and thermal sensation are affected on the opposite side of the body, and proprioceptive sensation is affected on the same side as the lesion. This pattern is the result of loss of pain and thermal senses corresponding to the opposite spinothalamic tract and loss of touch and proprioceptive sense corresponding to the ipsilateral posterior column. Although rarely present in its entirety, a partial Brown-Séquard syndrome is common in practice. The loss of pain and temperature sensation begins one or two segments below the lesion. An associated spastic motor paralysis on the side of the lesion completes the syndrome (see Fig. 8-7). Touch sensation is less affected, as the fibers from one side of the body are distributed in tracts (posterior columns, anterior and lateral spinothalamic) on both sides of the cord.
Syringomyelic Syndrome (Lesion of the Central Gray Matter)
Because spinothalamic fibers conducting pain and temperature sensation cross the cord in the anterior commissure, a lesion of considerable vertical extent in this location characteristically abolishes these modalities on both sides over several segments (dermatomes) but spares tactile sensation because the posterior columns are unaffected (see Fig. 8-7). This is termed dissociated sensory loss. Because the lesion frequently involves other parts of the gray matter, varying degrees of segmental amyotrophy and reflex loss are usually present as well. If the lesion has extended to the white matter, corticospinal, spinothalamic, and posterior column signs will be conjoined. The most common cause of such a lesion in the cervical region is the centrally situated developmental syringomyelia; less common are intramedullary tumor, trauma, and hemorrhage. A pseudosyringomyelic syndrome was mentioned earlier in relation to small-fiber neuropathies that simulate syringomyelia.
Paresthesias in the form of tingling and pins-and-needles sensations or girdleand band-like sensations are common complaints with posterior column disease. In some cases there may be the additional feature of a diffuse, burning, unpleasant sensation in response to pinprick. Loss of vibratory and position sense occurs below the level of the lesion, but the perception of pain and temperature is affected relatively little or not at all. Because posterior column lesions are caused by the interruption of central projections of the dorsal root ganglia cells, they may be difficult to distinguish from a process that affects large fibers in sensory roots (tabetic syndrome discussed above); however, the tendon reflexes are spared in the former and eliminated in tabes. In some diseases that involve the dorsal columns, vibratory sensation may be involved predominantly, whereas in others, position sense is more affected.