Types of Neuropathy: Classification, Symptoms, and Causes

Neuropathy = nerve damage. Classification by distribution, etiology, or fiber type guides diagnosis and treatment.

By Distribution (Most Common Clinical Classification)

• Peripheral Neuropathy (Polyneuropathy): Symmetric, length-dependent (stocking-glove). Most common. Causes: diabetes, alcohol, B12 deficiency, chemo, toxins, idiopathic.
• Mononeuropathy: Single nerve (median = carpal tunnel, ulnar = cubital tunnel, peroneal = foot drop). Compression, trauma, entrapment.
• Mononeuritis Multiplex: Multiple non-contiguous nerves. Vasculitis, diabetes, sarcoid, infections (leprosy, HIV, CMV).
• Cranial Neuropathy: CN III, IV, VI (diabetic ophthalmoplegia), VII (Bell's palsy), VIII (vestibular schwannoma).
• Autonomic Neuropathy: Orthostatic hypotension, gastroparesis, neurogenic bladder, erectile dysfunction, anhidrosis. Diabetes, amyloidosis, autoimmune.
• Radiculopathy/Plexopathy: Root (herniated disc, stenosis) or plexus (brachial/lumbosacral: trauma, radiation, tumor).

By Fiber Type

• Large Fiber: Vibration, proprioception loss → sensory ataxia, pseudoathetosis. NCS shows ↓amplitude, ↓conduction velocity.
• Small Fiber: Pain, temperature, autonomic. Normal NCS. Skin biopsy (IENFD) or QSART for diagnosis. Causes: diabetes, Sjögren's, amyloidosis, idiopathic.

By Etiology (Major Categories)

• Diabetic (most common overall): Distal symmetric polyneuropathy, mononeuropathies, autonomic.
• Alcoholic: Nutritional (thiamine) + direct toxicity. Painful sensory > motor.
• Nutritional: B12, B1 (thiamine), B6 (excess), E, folate, copper.
• Autoimmune/Inflammatory: CIDP, GBS, vasculitic, paraneoplastic, Sjögren's, lupus.
• Inherited: CMT (Charcot-Marie-Tooth), HNPP, Friedreich ataxia, familial amyloid polyneuropathy (TTR).
• Toxic/Drug: Chemo (platinum, taxanes, vincristine, bortezomib), INH, metronidazole, dapsone, heavy metals.
• Infectious: HIV, leprosy, Lyme, diphtheria, syphilis.
• Systemic: Amyloidosis, paraproteinemia (MGUS, myeloma), uremia, hypothyroidism, critical illness.
• Idiopathic: Cryptogenic sensory polyneuropathy (common in elderly).

Diagnostic Approach

1. History/Exam: pattern, tempo, modality, autonomic, UMN signs.
2. Labs: Glucose/HbA1c, B12, folate, TSH, SPEP/IFE, ANA/ESR, HIV, Lyme, heavy metals, genetic panel if indicated.
3. NCS/EMG: Demyelinating vs. axonal; distribution; chronicity.
4. Skin biopsy (IENFD) for small fiber.
5. Autonomic testing (tilt, QSART).
6. Nerve/skin biopsy (rare, for vasculitis, amyloid, CIDP).

Treatment: Address cause (glucose control, B12, immunosuppression, remove toxin), symptomatic pain control (gabapentinoids, SNRIs, TCAs), rehab, fall prevention.