Evaluation of upper extremity mononeuropathy

Upper extremity mononeuropathy refers to pathology affecting a single peripheral nerve of the upper limb. The peripheral nerve may be damaged anywhere along its course from the spinal nerve root, as part of the brachial plexus, or along its terminal branches. Damage results in weakness, disturbance of sensation, and/or pain. Generally, the term mononeuropathy refers to nerve damage occurring distal to the brachial plexus. There are more than 10 individual nerves in the arm distal to the brachial plexus, so many different mononeuropathies can occur. Occasionally more than one individual nerve is affected. This may be referred to as mononeuropathy multiplex, although this term virtually always connotes a vasculitic process affecting multiple peripheral nerves.

Compression neuropathies

The most common mononeuropathies of the upper extremity are carpal tunnel syndrome, ulnar neuropathy, and radial neuropathy.[1]Latinovic R, Gulliford MC, Hughes RA. Incidence of common compressive neuropathies in primary care. J Neurol Neurosurg Psychiatry. 2006;77(2):263-5. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2077603 http://www.ncbi.nlm.nih.gov/pubmed/16421136?tool=bestpractice.com These are believed to be due to mechanical injury caused by compression or trauma. Most commonly this can be seen in the median nerve at the wrist (carpal tunnel syndrome) or in the ulnar nerve at the elbow.[1]Latinovic R, Gulliford MC, Hughes RA. Incidence of common compressive neuropathies in primary care. J Neurol Neurosurg Psychiatry. 2006;77(2):263-5. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2077603 http://www.ncbi.nlm.nih.gov/pubmed/16421136?tool=bestpractice.com [2]Doughty CT, Bowley MP. Entrapment neuropathies of the upper extremity. Med Clin North Am. 2019 Mar;103(2):357-70. http://www.ncbi.nlm.nih.gov/pubmed/30704687?tool=bestpractice.com ​ Trauma is the most frequent cause of the less common radial neuropathy, although this can also occur from compression over the spiral groove (Saturday night palsy).[3]Węgiel A, Karauda P, Zielinska N, et al. Radial nerve compression: anatomical perspective and clinical consequences. Neurosurg Rev. 2023 Feb 13;46(1):53. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9925568 http://www.ncbi.nlm.nih.gov/pubmed/36781706?tool=bestpractice.com ​ These relatively benign mononeuropathies must be differentiated from more sinister causes, which tend to occur outside of compression sites and frequently involve more than one nerve (i.e., mononeuritis multiplex). Lesions in the spinal nerve root and in the brachial plexus also have different etiologic considerations and need to be differentiated from the more common compression/traumatic neuropathies.

Inflammation, malignancy, and infection

The occurrence of multiple mononeuropathies outside of compression sites suggests vasculitis or segmental demyelination in the context of antibodies against some of the peripheral myelin compounds. Although both are rare, recognizing these conditions (particularly vasculitis) is critical. Brachial plexopathies are often idiopathic. Idiopathic brachial plexopathy has many synonyms, including brachial neuritis, Parsonage-Turner syndrome, and brachial amyotrophy. However, unlike in the distal nerve where tumors are rare, extrinsic compression by malignancy needs to be considered, particularly when symptoms localize to the medial cord or lower trunk of the brachial plexus, a localization that can appear to mimic an ulnar neuropathy. Nerve root lesions can also mimic the distal mononeuropathies. In general, these result from traumatic or degenerative disk disease. However, malignancy and infection can also cause cervical radiculopathies and need to be considered in the appropriate setting.