Evaluation of lower extremity mononeuropathy

Lower extremity mononeuropathies are commonly encountered in clinical practice. Dysfunction of the central nervous system (CNS) or peripheral nervous system (PNS) may result in weakness, sensory deficits and/or pain. A careful history and physical exam will assist in localizing the issue. A PNS lesion may result from pathology located anywhere along the course of the peripheral nerve, from the dorsal root ganglion through to the lumbosacral plexus and the terminal individual named nerves. These entities are a significant source of neurologic referral.

Causes

Mononeuropathies can be caused by entrapment, trauma, infectious or metabolic etiologies and may be localized to the nerve root, plexus or individual nerve. While trauma and entrapment are likely to be more frequently encountered in clinical practice, mononeuropathies may be sequelae of underlying systemic disease (e.g., diabetes mellitus, malignancy, infection, and inflammatory conditions). Viral infections such as herpes zoster, herpes simplex virus (HSV), Epstein-Barr virus (EBV), and cytomegalovirus (CMV) can involve nerve roots, leading to a painful radiculitis, or may trigger a more widespread phenomenon such as acute inflammatory demyelinating polyradiculoneuropathy (AIDP), also known as Guillain-Barre syndrome. Vasculitic neuropathy usually occurs suddenly and is painful, classically presenting as the stepwise involvement of multiple individual nerves (mononeuropathy multiplex) rather than an isolated mononeuropathy.[1]Preston DC, Shapiro BE. Electromyography and neuromuscular disorders: clinical-electrophysiologic correlations. 4th ed. Philadelphia: Saunders; 2021​.​​[2]Gwathmey KG, Burns TM, Collins MP, et al. Vasculitic neuropathies. Lancet Neurol. 2014 Jan;13(1):67-82. https://www.thelancet.com/journals/laneur/article/PIIS1474-4422(13)70236-9/fulltext http://www.ncbi.nlm.nih.gov/pubmed/24331794?tool=bestpractice.com ​ Because individual nerves are affected, one does not see the distal and symmetric (e.g., stocking-glove) distribution of deficits typical of a generalized polyneuropathy, at least early on. In advanced cases, however, a confluent pattern can emerge that mimics that of a length-dependent polyneuropathy. Mononeuropathy (mononeuritis) multiplex may also be seen with infection. Cancer can produce nerve dysfunction secondary to compression by solid tumors, infiltration by malignant cells, or paraneoplastic immune-mediated attack.

Whilst lumbosacral plexopathies or radiculopathies are outside the main scope of this article, lesions located in the lumbosacral plexus or nerve root can mimic mononeuropathies and need to be considered in the differential diagnosis.