Upper limb 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 1 individual nerve is affected. This may be referred to as mononeuropathy multiplex, although, strictly speaking, this term virtually always connotes a vasculitic process affecting multiple peripheral nerves.
The most common mononeuropathies of the upper extremity are carpal tunnel syndrome, ulnar neuropathy, and radial neuropathy.  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.  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). These relatively benign mononeuropathies must be differentiated from more sinister causes, which tend to occur outside of compression sites and frequently involve more than 1 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.
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.
Clinical Associate Professor
University of British Columbia
HRB receives royalties from UpToDate for an article she contributed to, "Approach to sensory loss", and she is on the editorial board of Continuum, a continuing education journal produced by the American Academy of Neurology.
Dr Hannah R. Briemberg would like to gratefully acknowledge Dr Naser Alotaibi, a previous contributor to this monograph. NA declares that he has no competing interests.
Assistant Professor of Neurology
Director of EMG Laboratory & Peripheral Neuropathy Center
Department of Neurology & Neurosciences
New Jersey Medical School
NS declares that he has no competing interests.
Associate Professor of Neurology
Department of Neurology
Santa Maria Hospital/Instituto de Medicina Molecular - Faculty of Medicine
MdC declares that he has no competing interests.
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