Dermatomes

Similarly, within the leg and foot, when there is numbness and tingling of the toes, either the fibular (peroneal) or tibial nerve are affected or nerve roots L5 and S1 are damaged. Lumbosacral (LS) back pain, that is, sciatica, is a common accompaniment when a LS nerve root lesion is responsible for partial foot numbness. More generalized peripheral neuropathies usually lead to bilaterally symmetric symptoms initially having a stocking-like, and later glove-type, distribution. These are related to disorders predominantly affecting the distal portions of multiple peripheral nerves; there may or may not be a motor component, and thus some degree of weakness is sometimes detected. When there is a pure distal, sometimes more global, symmetric sensory loss, one needs to consider the remote possibility of a disorder primarily affecting the posterior root ganglion cells, also known as a sensory ganglionopathy.

Two unusual peripheral nerve variants occasionally occur. One includes the initial acute involvement of an individual nerve, soon thereafter followed by involvement of another anatomically distinct mononeuropathy, and subsequently, in short order, yet another individual nerve becomes involved; these soon fuse into what might appear to the unwary historian/examiner to be a primary generalized polyneuropathy because all nerves eventually become involved, leading to a picture mimicking a much more common generalized polyneuropathy. Usually, there is a concomitant motor component that helps define the specific affected peripheral nerves. After more than one nerve is semiacutely involved in a steplike fashion, it becomes clear that the patient has an evolving mononeuritis multiplex (MNM).

Typically, these patients note both an acute sensory deficit as well as acute weakness, such as a footdrop, in the distribution of one specific peripheral nerve, here the fibular nerve. These disorders are quite uncommon. The acuity of onset mimics a stroke, and indeed that is exactly what is occurring as the small arterioles, vasa nervorum, that supply the individual nerve develop a vasculitis, leading to nerve infarcts. The two most common MNM etiologies are an arteritis, such as occurs with diabetes mellitus, or a systemic vasculitis, such as polyarteritis nodosa.

Hereditary neuropathy with predisposition to pressure palsies (HNPP) is a different pathologic entity that can also affect multiple individual peripheral nerves in a varied sequence. However, this is subacute and rarely leads to a clinical fusion of findings as with a vasculitis. HNPP is an autosomal dominant form of demyelination that is secondary to a PMP-22 deletion on the short arm of chromosome (17p11.2). Of interest, duplication here leads to Charcot-Marie-Tooth disease, the most common hereditary neuropathy.

Some peripheral mononeuropathies manifest predominantly with weakness, particularly the wristdrop of radial nerve lesions or footdrop of fibular (peroneal) nerve lesions. Sometimes these are mistaken for a stroke. A clear appreciation of each individual peripheral nerve’s motor distribution ultimately aids in the correct diagnosis. Rarely, lesions as high as the parasagittal region of the brain may also manifest with foot weakness.

Atrophy of muscles innervated by the involved nerve occurs with significant chronic denervation. Concomitantly, fasciculations may also be present. Measuring extremity circumference may document significant side-to-side asymmetries and, by inference, muscle atrophy secondary to anterior horn cell, nerve root, or peripheral nerve damage. It is most important here to carefully search for sensory loss, such as one finds with the ulnar nerve lesion, often manifesting with painless intrinsic hand muscle atrophy, to exclude the possibility of a pure motor nerve lesion mimicking amyotrophic lateral sclerosis (ALS) or syringomyelia.

In summary, careful understanding of the precise distribution of peripheral nerve or dermatomal innervation may be immensely helpful in differential diagnosis; when this is not clear, electromyography can help define the problem.

Only gold members can continue reading. Log In or Register to continue