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 aetiological 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, recognising 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 tumours are rare, extrinsic compression by malignancy needs to be considered, particularly when symptoms localise to the medial cord or lower trunk of the brachial plexus, a localisation 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 disc disease. However, malignancy and infection can also cause cervical radiculopathies and need to be considered in the appropriate setting.
- Ulnar neuropathy at the wrist/palm
- Radial neuropathy at the spiral groove (Saturday night palsy)
- Neurogenic thoracic outlet syndrome
- Posterior interosseus syndrome
- Anterior interosseus neuropathy
- Brachial neuritis (brachial amyotrophy, Parsonage-Turner syndrome, idiopathic acute brachial neuropathy)
- Amyotrophic lateral sclerosis
- Metastatic cancer/nerve sheath tumours
- Post-irradiation brachial plexopathy
- Peripheral nerve vasculitis (mononeuritis multiplex)
- Multifocal chronic inflammatory demyelinating polyneuropathy
- Hereditary neuropathy with liability to pressure palsies (HNPP)
- Lyme disease