The clinician is faced initially with several problems that can be solved sequentially when dealing with this group of diseases: (1) establishing the existence of disease of the peripheral nervous system and differentiating it from a process of the central nervous system, neuromuscular junction or the muscles; (2) distinguishing by clinical examination which of the main topographic syndromes is being displayed; (3) determining by examination and nerve conduction studies if the problem is predominantly motor or sensory or autonomic in nature or is of mixed type and whether the myelin sheath, the axon, or cell body (motor or sensory neurons) is the target of disease; and (4) assessing if the neuropathy is acquired or hereditary in nature. When taken together, these features limit the likely etiologic diagnoses from a vast list of possibilities.
Topographic and Clinical Patterns of Neuropathy (Table 43-1)
At the outset it must be determined whether the neurologic findings correspond to one of the following syndromic patterns: 1. Polyneuropathy 2. Radiculopathy or polyradiculopathy 3. Neuronopathy—motor or sensory 4. Mononeuropathy 5. Multiple mononeuropathies (mononeuropathy multiplex) 6. Plexopathy (involvement of multiple nerves in a plexus)
A discussion of these patterns is given in Chap. 8, but the main facts are repeated here.
In polyneuropathy, a generalized process affecting the peripheral nerves, weakness is relatively symmetrical from the beginning and progresses bilaterally; reflexes are lost in affected parts but particularly at the ankles; sensory complaints and loss of sensation are most pronounced distally, and in the feet before the hands in most cases.
Polyradiculopathy, a disease of multiple spinal roots, differs from polyneuropathy in that the neurologic signs are asymmetrical, with an erratic distribution that may, for example, be proximal in one limb and distal in another. Weakness and zones of sensory loss correspond to involvement of one or more spinal or cranial roots. Pain in the sensory distribution of the roots is a common feature. The common single radiculopathy, most often the result of root compression by disease of the spinal column, is identified by pain, sensory, motor, and reflex change solely in the distribution of one nerve root. The distinction from mononeuropathy (see later) is not always apparent and one must resort to a reference or to memorized knowledge of the motor and sensory innervation patterns of roots and nerves as given in Figs. 8-1, 8-2, and 8-3 and on the overleafs. Most helpful is the limitation of sensory loss to one of the dermatomes, but it so happens that there is overlap between adjacent dermatomes and such a pattern is not easily discerned.
Mononeuropathy is the most circumscribed form of peripheral nerve disease. It is reflected by weakness and sensory loss in the territory of a single peripheral nerve. Specific features serve to differentiate mononeuropathy from a radiculopathy—for example, weakness in dorsiflexion and eversion of the foot is referable either to the peroneal nerve or to the L5 nerve root; however, if there is weakness of inversion of the foot, innervated by the tibial nerve, the fault must be with the L5 root, not with the peroneal nerve. Conversely, if inversion is spared in a foot drop, the lesion is in the peroneal nerve. The distribution of sensory loss also aids in distinguishing the two processes; for example, in the aforementioned case the region of sensory change corresponding to the L5 root extends almost up to the knee on the anterior surface of the foreleg whereas it ends a limited distance above the ankle in the case of a peroneal nerve lesion (see the sensory maps in Figs. 8-1, 8-2, and 8-3).
At times, particularly in advanced stages, the accumulation of multiple mononeuropathies, termed mononeuropathy multiplex, may be difficult to differentiate from polyneuropathy as discussed further on.