Polyneuropathies (Nerve Disorders)

Peripheral nerve disorders divide into inherited and acquired forms. The distinction can be made clinically and it matters: some acquired forms respond to treatment.

Charcot-Marie-Tooth disease

This is the most common inherited neuropathy, affecting approximately 1 in 2,500. The typical childhood presentation is insidious distal weakness with foot deformity.

• Pes cavus, hammer toes, increasingly frequent ankle sprains.
• Distal atrophy: wasting of the lower calf, the inverted champagne bottle appearance.
• Foot drop and a steppage gait; late atrophy of the intrinsic hand muscles.
• Loss of deep tendon reflexes, particularly the ankle jerk.
• Distal sensory loss — the finding that separates CMT from SMA and the myopathies. Children rarely report it; it must be sought on examination.

Nerve conduction studies separate demyelinating (CMT1, markedly reduced conduction velocity) from axonal forms (CMT2, reduced amplitude with relatively preserved velocity), and this distinction guides genetic testing. The most common subtype is CMT1A, due to PMP22 duplication, so in a demyelinating picture the first test is PMP22 copy number analysis.

Other inherited neuropathies

This group includes hereditary neuropathy with liability to pressure palsies (PMP22 deletion, recurrent pressure palsies), severe congenital demyelinating neuropathies of Dejerine-Sottas type, distal hereditary motor neuropathies, and neuropathies accompanying metabolic disease. When central features accompany a neuropathy, metachromatic leukodystrophy, Krabbe disease, adrenomyeloneuropathy and mitochondrial disorders should be investigated.

Guillain-Barré syndrome

Acute, progressive, usually symmetrical and ascending weakness; loss of deep tendon reflexes; variable sensory symptoms; back and leg pain. Pain is common in children and can be misleading, presenting as refusal to walk. A preceding infection is frequent.

• The diagnosis is clinical. Albuminocytological dissociation in the cerebrospinal fluid may not yet have developed in the first week; an early normal CSF does NOT exclude the diagnosis.
• Nerve conduction studies may be normal early.
• Cord compression and transverse myelitis must be excluded.
• Red flags that argue against GBS: a clear sensory level, sphincter involvement, fever, asymmetry, brisk reflexes.
• Respiratory function (vital capacity, single-breath count), swallowing and autonomic signs are monitored closely.
• Treatment is intravenous immunoglobulin or plasma exchange; corticosteroids alone are not effective.

Chronic inflammatory demyelinating polyneuropathy

An immune-mediated demyelinating neuropathy with a course longer than eight weeks or a relapsing pattern. Distinguishing it from inherited neuropathy matters because it responds to treatment with corticosteroids, intravenous immunoglobulin or plasma exchange. In CIDP the course is faster, asymmetry and fluctuation are prominent and CSF protein is raised; in inherited neuropathy the course is insidious over years, family history is common and foot deformity indicates long-standing disease.

Differential diagnosis of acute weakness

• Acute transverse myelitis — a sensory level and sphincter involvement; early loss of reflexes can be misleading.
• Cord compression, acute flaccid myelitis.
• Infant botulism, myasthenic crisis.
• Hypokalaemic periodic paralysis, severe electrolyte disturbance.
• Tick paralysis; critical illness neuromyopathy in the intensive care setting.