Paralysis of both lower extremities may occur with diseases of the spinal cord, nerve roots, or, less often, the peripheral nerves. If the onset is acute, it may be difficult to distinguish spinal paralysis, which results in flaccidity and abolition of reflexes from spinal shock, from that due to peripheral nerve disease. In acute spinal cord diseases with involvement of corticospinal tracts, the paralysis or weakness affects all muscles below a given level; if the white matter is extensively damaged, sensory loss below a circumferential level on the trunk is conjoined (loss of pain and temperature sense because of spinothalamic tract damage, and loss of vibratory and position sense from posterior column involvement). Also in bilateral disease of the spinal cord, the bladder and bowel and their sphincters are usually affected. These abnormalities may be the result of an intrinsic lesion of the cord or an extrinsic mass that narrows the spinal canal and compresses the cord.
In peripheral nerve diseases, motor loss tends to involve the distal muscles of the legs more than the proximal ones (exceptions are certain varieties of the Guillain-Barré syndrome and some types of diabetic neuropathy and porphyria); sphincteric function is usually spared or impaired only transiently. Sensory loss, if present, is also more prominent in the distal segments of the limbs, and the degree of loss is often more for one modality than another.
For clinical purposes, it is helpful to separate the acute paraplegias from the chronic ones and to divide the latter into two groups: those beginning in adult life and those occurring in infancy.
The most common cause of acute paraplegia (or quadriplegia if the cervical cord is involved) is spinal cord trauma, usually associated with fracture–dislocation of the spine. Less common causes are a vascular malformation or fistula of the cord or its overlying dura, which causes ischemia by a complex mechanism, and infarction of the cord resulting from occlusion of the anterior spinal artery or, more often, from occlusion of segmental branches of the aorta as a feature of dissecting aneurysm or atheroma, vasculitis, or nucleus pulposus embolism. Epidural or subdural hemorrhage from a hemorrhagic diathesis or warfarin therapy causes an acute or subacute paraplegia; in a few instances the bleeding follows a lumbar puncture.
Paraplegia or quadriplegia that develops more slowly, subacutely over a period of hours or days is caused by postinfectious myelitis, demyelinating or necrotizing myelopathy, or epidural abscess or tumor with spinal cord compression. Paralytic poliomyelitis and acute Guillain-Barré syndrome—the former a purely motor disorder with mild meningitis, the latter predominantly motor but often with sensory disturbances—must be distinguished from the acute and subacute myelopathies and from each other.
In adult life, multiple sclerosis and tumor account for most cases of chronic spinal paraplegia, but a wide variety of extrinsic and intrinsic processes may produce the same effect: protruded cervical disc and cervical spondylosis (often with a congenitally narrow canal), epidural abscess and other infections (tuberculous, fungal, and other granulomatous diseases, HIV and HTLV-1), syphilitic meningomyelitis, motor system disease, subacute combined degeneration (vitamin B12 deficiency and copper deficiency), syringomyelia, epidural lipomatosis, neuromyelitis optica, and degenerative disease of the lateral and posterior columns. (See Chap. 42 for discussion of these spinal cord diseases.)