This is most often caused by a lesion of the cerebral cortex or the spinal cord (where it causes a monoplegia of the leg). Infrequently monoplegia or fragments of it result from a restricted subcortical lesion that interrupts the motor pathways to one limb. A cerebral vascular lesion is the most common cause; a circumscribed tumor or abscess may have the same effect. Small cortical lesions in the motor strip may rarely be so selective as to cause restricted regions of weakness, for example, of parts of the hand. Multiple sclerosis and spinal cord tumor, early in their course, may cause weakness of one limb, usually the leg. Monoplegia caused by a lesion of the upper motor neuron is usually accompanied by spasticity, increased reflexes, and an extensor plantar reflex (Babinski sign). In acute diseases of the lower motor neurons, the tendon reflexes are reduced or abolished, but atrophy may not appear for several weeks.
Monoplegia with Muscular Atrophy
This is more frequent than monoplegia without muscular atrophy. Long-continued disuse of one limb may lead to atrophy, but it is usually of lesser degree than atrophy caused by lower motor neuron disease (denervation atrophy). In disuse atrophy, the tendon reflexes are retained and nerve conduction studies are normal. With denervation of muscles, there may be visible fasciculations and reduced or abolished tendon reflexes in addition to paralysis. The location of the lesion (in nerves, spinal roots, or spinal cord) can usually be determined by the pattern of weakness, by the associated neurologic symptoms and signs, and by special tests—MRI of the spine, examination of the cerebrospinal fluid (CSF), and electrical studies of nerve and muscle. If the limb is partially denervated, the EMG shows reduced numbers of motor unit potentials (often of large size) as well as fasciculations and fibrillations.
A complete atrophic brachial monoplegia is uncommon; more often, only parts of a limb are affected. When present in an infant, it suggests brachial plexus trauma from birth; in a child, poliomyelitis or other viral infection of the spinal cord; and in an adult, syringomyelia, amyotrophic lateral sclerosis, or a brachial plexus lesion. Atrophic crural (leg) monoplegia is more frequent than atrophic brachial monoplegia and may be caused by any lesion of the lumbosacral spinal cord or plexus. The mode of onset and temporal course differentiate the various diseases that affect these structures. A prolapsed intervertebral disc and several varieties of mononeuropathy almost never paralyze all or most of the muscles of a limb.
This is the most frequent form of paralysis. With rare exceptions this pattern of paralysis is a result of involvement of the corticospinal pathways. The site or level of the lesion—that is, cerebral cortex, corona radiata, capsule, brainstem, or spinal cord—can usually be deduced from the associated neurologic findings. Diseases localized to the cerebral cortex, cerebral white matter (corona radiata), and internal capsule usually manifest themselves by weakness or paralysis of the leg, arm, and lower face on the opposite side. The occurrence of seizures or the presence of a language disorder (aphasia), a loss of discriminative sensation (e.g., astereognosis, impairment of tactile localization), anosognosia, or a homonymous visual field defect suggests a contralateral cortical or subcortical location rather than at a lower level.