27th June 2023

Peripheral Neuropathy

Mononeuropathies

Mononeuropathy refers to a lesion of a single nerve. The most common causes being compression or vascular. Common mononeuropathies include carpal tunnel syndrome and common peroneal palsy.

Common nerve palsies

Median nerve compression

Carpal Tunnel Syndrome is the most common upper extremity mononeuropathy. Compression of the median nerve at the carpal tunnel leads to symptoms of pain and paraesthesia of the lateral three and a half fingers. In severe cases it can also cause weakness in thumb abduction and wasting of the thenar eminence.

• Phalen’s sign: flex the wrist for about one minute, this will cause traction of the median nerve. A positive test will elicit pain and/or paraesthesia in the innervated fingers.
• Tinel’s sign: tap on top of the carpal tunnel, a positive test will elicit pain and /or paraesthesia of the innervated fingers.

The anterior interosseous nerve is the largest branch of the median nerve, originating distal to the lateral epicondyle. Palsy here can lead to inability of thumb and index finger opposition.

Common peroneal nerve palsy

The common peroneal nerve is motor to tibialis anterior and peronei muscles (peroneus longus, brevis and tertius). Neuropathy here, therefore, will result in foot drop (weakness of dorsiflexion) and weakness of foot eversion. Patients may also have sensory loss of the dorsum of the foot. Over time there will be wasting of the anterior tibial and peroneal muscles. The most common cause is the entrapment of the nerve at its origin, the fibular neck.

Other mononeuropathies

Polyneuropathies

Causes of polyneuropathy

Polyneuropathy is the damage to multiple peripheral nerves and is commonly called peripheral neuropathy. This has a wide aetiology, the most common causes being diabetes and alcohol abuse. The presentation is usually symmetrical and, depending on the cause, may present with predominantly sensory, motor, or autonomic symptoms. Causes of predominantly sensory neuropathies include diabetes, alcohol, vitamin B12 or B1 deficiency, vasculitis, uraemia, and drugs such as isoniazid and vincristine. Causes of predominantly motor neuropathies include Guillain-Barré syndrome, chronic inflammatory demyelinating polyneuropathy, Charcot-Marie-Tooth, porphyria, botulism, and lead toxicity.

Causes of peripheral neuropathies can also be classified by demyelinating or axonal causes depending on the part of the nerve affected. This can be determined by nerve conduction studies; demyelinating neuropathies classically show a reduction in conduction velocity, whereas axonal neuropathies show a reduction in amplitude.

Guillain-Barré syndrome

Guillain-Barré syndrome (GBS) is the most common cause of acute, acquired weakness, often provoked by an infection (commonly respiratory or gastrointestinal). Campylobacter jejuni is a common cause and is associated with slower recovery and residual disability. The pathogenesis involves cross-reaction of antibodies with gangliosides in the peripheral nervous system. There is an ascending progressive symmetrical muscle weakness and eventually paralysis within 2 weeks. There may also be a loss of the deep tendon reflexes. Investigations include lumbar puncture, which will show high protein C and normal white cell count in the cerebral spinal fluid. EMG will show slow nerve conduction velocity. Treatment is with plasma exchange and IV immunoglobulins.

Miller-Fisher syndrome

This is a variant of Guillain-Barré syndrome. It is associated with areflexia, ataxia, ophthalmoplegia, but the paralysis is descending rather than ascending. Anti-GQ1b antibodies can be found in many cases.

Autonomic neuropathies

These are neuropathies which may present with some autonomic symptoms such as postural hypotension, sweating, diarrhoea or constipation, urinary incontinence, and impotence. The main causes include diabetes, amyloidosis, Guillain-Barré Syndrome, renal failure, and chronic hepatic failure.