5A. Polyneuropathies

Polyneuropathies are disorders affecting multiple peripheral nerves, which usually affect the most distal parts of the body symmetrically.

Etiology

There are many types and causes of polyneuropathies. Here are some of the most common.

• Diabetic neuropathy
• Alcoholic neuropathy
• Toxic neuropathy (heavy metal, solvents)
• Deficiency neuropathy (B1, B6, B12, copper)
• Hypothyroid neuropathy
• Immune-mediated neuropathies
  • Guillain-Barre syndrome (acute inflammatory demyelinating polyradiculopathy, AIDP)
  • Chronic inflammatory demyelinating polyradiculoneuropathy (CIDP)
• Hereditary neuropathies
  • Charcot-Marie-Tooth disease

Diabetic and alcoholic are the most common types.

Classification

They can be classified as either axonal or demyelinating. Axonal neuropathy occurs due to damage to the nerve axon, causing degeneration of the distal nerve and eventually denervation. Demyelinating neuropathy occurs due to demyelination, which causes slow conduction or conduction block.

We can also distinguish small fibre neuropathy, which affect the thin unmyelinated and poorly myelinated nerve fibres, and large fibre neuropathy, which affects the larger nerve fibres.

We can also distinguish polyneuropathies according to the modality they affect, either as sensory, motor, or sensorymotor. Autonomic fibres may also be affected.

Clinical features

Symptoms usually affect the distal parts of the body symmetrically. The clinical features depend on the specific type, but generally we can see weakness, atrophy, sensory loss, neuropathic pain, paraesthesia, and/or loss of reflexes.

In Charcot-Marie-Tooth disease, we can see foot drop, pes cavus deformity, and calf muscle atrophy.

Diagnosis and evaluation

ENG shows slow conduction and temporal dispersion in demyelinating neuropathies, and low amplitude potentials (denervation) in axonal neuropathy. However, ENG can only measure the function of thick nerves. For thin nerves, a skin biopsy is required.

In inflammatory neuropathies we can find high levels of protein but normal levels of cells in the CSF (albuminocytologic dissociation).

Treatment

Treatment involves treating the underlying condition. Demyelinating neuropathies usually respond better to treatment.

Chronic inflammatory neuropathies are treatable and are treated with steroids, plasma exchange or IVIG.