Plexopathies (brachial or lumbosacral) create the most confusing patterns of motor and sensory involvement; only one limb is affected, but the motor, sensory, and reflex loss does not conform to a pattern of several adjacent nerve roots or nerves. Knowledge of the innervation of the involved muscles at the level of the plexus usually clarifies the situation.
In sensory neuronopathy, the ganglion cells rather than the peripheral sensory nerves are predominantly affected. This gives rise to symptoms and signs of sensory loss in both a proximal and distal distribution, including the scalp, thorax, abdomen, and buttocks as well as the extremities; sensory ataxia is a common accompaniment. There is no weakness, but movements may be awkward as a result of a sensory ataxia. Motor neuronopathy is essentially the obverse condition, a disorder of the anterior horn causing weakness, fasciculations, and atrophy in a widespread distribution and, therefore, not properly included as a process of the peripheral nerves.
The apparent complexity of peripheral nerve disease is greatly simplified by recognizing that, of the multitude of diseases, each manifests itself by one or another of above-described topographic and sensory-motor patterns for which reason the pattern of neuropathy sets limits on the etiologic possibilities.
In the analysis of a polyneuropathy, it is of further value to determine whether the process is predominantly motor with less sensory involvement or the converse, or purely sensory, motor, or mainly autonomic. The time course of the disease also informs diagnosis. An acute onset (i.e., rapid evolution) is nearly always an inflammatory, immunologic, toxic, or vascular polyneuropathy. The other extreme, a polyneuropathy evolving over many years, is indicative of a hereditary or, rarely, a metabolic disease. Most of the toxic, nutritional, and systemic diseases of nerve develop subacutely over several weeks and months. In addition to the patient’s report of the progress of symptoms, signs such as muscle atrophy signify a process of relatively long duration, at least several months.
The etiologic diagnosis of polyneuropathy is next guided by deducing whether the myelin sheath or the axon is primarily involved (i.e., demyelinating or axonal neuropathy). The neurologic examination alone may be sufficient to make this distinction, but greater precision is attained from nerve conduction studies and needle examination of muscles (EMG). The latter test also helps separate primary disorders of muscle (myopathies) and neurogenic denervation of muscle or neuromuscular block (myasthenia). The electrical examinations of nerve and muscle described in Chap. 2 greatly reduce the number of possible diagnoses. These EMG and nerve conduction abnormalities may be so characteristic as to virtually define a neuropathy, for example, chronic demyelinative motor neuropathy with multifocal conduction block.
Other useful laboratory procedures are (1) biochemical tests to identify metabolic, nutritional, or toxic states; (2) CSF examination (increase in protein and in cells that indicate radicular or meningeal involvement); (3) nerve, and occasionally accompanying muscle biopsy (the latter aids in the diagnosis of vasculitic causes of neuropathy); (4) measurement of immunoglobulins and antineural antibodies that relate to immune-mediated neuropathies; and (5) genetic testing for several of the inherited neuropathies. These are discussed in the context of each of the main diseases of nerve and in the later parts of
Chap. 2.