Educational scope notice: This is a study note for medical students, not medical advice, diagnosis, or treatment guidance. Clinical management should follow local protocols and current guidelines.
Before classifying a neuropathy or choosing treatment, it helps to understand how nerves are built and what happens when different parts fail. Much of the clinical reasoning in this topic follows from that construction.
Nerve construction in brief
A peripheral nerve bundles many axons together in three connective layers. The endoneurium wraps each axon, the perineurium groups axons into fascicles, and the epineurium encloses the whole nerve. Myelin, made by Schwann cells, insulates most axons and speeds conduction. Small unmyelinated fibres carry pain, temperature, and autonomic signals, while large myelinated fibres carry touch, vibration, position sense, and motor commands.
Axonal versus demyelinating injury
Nerve damage falls into two broad mechanisms, and the distinction shapes both test interpretation and recovery.
In axonal injury, the fibre itself degenerates. The common pattern is dying-back: the far ends of the longest nerves fail first, producing the stocking-glove distribution that starts in the feet. Recovery requires the axon to regrow from the injury site toward its target at roughly 1 mm per day, so recovery is slow and often incomplete in long nerves. Typical causes include diabetes, alcohol, toxins, and nutritional deficiency.
In demyelinating injury, the myelin sheath is damaged while the axon survives. Conduction slows markedly because myelin is what makes it fast. Recovery can be quicker when Schwann cells remyelinate the surviving axon. Typical causes include Guillain-Barre syndrome, CIDP (chronic inflammatory demyelinating polyradiculoneuropathy), and inherited demyelinating disease.
Nerve conduction studies reflect this directly: axonal loss lowers the response amplitude, because fewer fibres contribute, while demyelination slows conduction velocity and prolongs latencies, because the surviving fibres conduct slowly. Denervation changes on needle EMG, such as fibrillations and positive sharp waves, appear 2 to 3 weeks after axonal injury. How these findings are used is covered under diagnosis (/notes/neurology/neuropathies-diagnosis/).
Sensory disturbances
Sensory symptoms divide into positive phenomena, losses, and altered sensations, and the mixture hints at which fibre sizes are involved.
Positive symptoms arise from ectopic discharges in damaged nerves: tingling and pins and needles, burning, electric-shock sensations, or a feeling of walking on stones. Burning pain at skin level suggests small-fibre involvement, while deep aching pain at rest suggests large fibres.
Negative symptoms reflect lost function: reduced touch, pain, temperature, or vibration sense. When position sense is lost, gait becomes unsteady and worsens without visual compensation, which is a positive Romberg sign and indicates sensory ataxia.
Altered sensations include allodynia, where a stimulus that should not hurt does, such as bedsheets brushing the feet, and hyperesthesia, where normal stimuli feel exaggerated. Prominent allodynia with burning feet and otherwise normal strength and reflexes should raise the possibility of small-fibre neuropathy, in which routine conduction studies may be normal.
Motor signs
Weakness in polyneuropathy is typically distal and symmetric, affecting foot dorsiflexion and hand grip before proximal muscles. This is the reverse of myopathy, which is usually proximal. Reduced muscle tone, absent or diminished deep tendon reflexes, and late muscle wasting complete the lower-motor-neuron picture. Visible twitches under the skin (fasciculations) indicate spontaneous motor-unit discharge and also occur in motor neuron disease, so they are interpreted in context rather than alone.
Some presentations are immediately localizing. Foot drop with a steppage gait points to the common fibular (peroneal) nerve. Wrist drop after prolonged pressure on the upper arm points to the radial nerve. Wasting of the thenar eminence with loss of thumb opposition points to the median nerve, and clawing of the ring and little fingers points to the ulnar nerve. High arches with hammer toes and thin calves with preserved thighs suggest a long-standing hereditary neuropathy. Breathlessness, dysarthria, or dysphagia alongside limb weakness signals cranial nerve or respiratory muscle involvement and is an urgency cue, particularly in Guillain-Barre syndrome.
Autonomic signs as red flags
Autonomic involvement narrows the differential considerably. Orthostatic hypotension, persistent constipation or alternating bowel habit, erectile dysfunction, urinary retention, and reduced sweating alongside a neuropathy point most often toward diabetes or amyloidosis rather than a nonspecific polyneuropathy.
Telling root disease from nerve disease
Pain and sensory loss in one limb can come from the nerve root (radiculopathy) or from the peripheral nerve itself, and the patterns overlap. A C8 root lesion and an ulnar nerve lesion both affect the medial hand, but the root lesion also involves territories the ulnar nerve never supplies. Mapping the exact sensory boundary, testing muscles outside the suspect nerve, and using EMG to place the lesion resolve the question. Root-level localization is detailed under diagnosis (/notes/neurology/neuropathies-diagnosis/).
Evidence anchors
- NINDS. Peripheral Neuropathy: https://www.ninds.nih.gov/health-information/disorders/peripheral-neuropathy