Small vs large fibre neuropathy
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Small-fibre vs large-fibre neuropathy
Peripheral nerves contain different sensory and motor fibre types. Neuropathy can preferentially affect one group, but mixed neuropathy is common, especially in diabetes.
| Feature | Small-fibre neuropathy | Large-fibre neuropathy |
|---|---|---|
| Fibres affected | Thinly myelinated A-delta and unmyelinated C fibres | Large myelinated sensory fibres, sometimes motor fibres |
| Main roles | Pain, temperature, itch, autonomic function | Vibration, light touch, joint-position sense, reflex arcs, coordination |
| Typical symptoms | Burning, stinging, electric-shock pain; painful sensitivity to touch; symptoms often worse at night | Numbness, tingling, “walking on cotton,” imbalance, clumsiness, falls |
| Sensory loss on exam | Reduced pinprick and temperature | Reduced vibration and proprioception |
| Reflexes | Usually preserved early | Often reduced or absent, especially ankle reflexes |
| Strength | Usually normal | May be weak if motor fibres are involved |
| Balance | Generally preserved | Sensory ataxia, worse in the dark or with eyes closed |
| Autonomic features | Can occur: altered sweating, dry skin, colour change, postural dizziness, GI or bladder symptoms | Not typical of isolated large-fibre disease |
| Nerve-conduction study / EMG | Often normal, because routine testing assesses large fibres | Often abnormal |
| Helpful confirmatory tests | Skin punch biopsy for intraepidermal nerve-fibre density; autonomic testing such as QSART; quantitative sensory testing | Nerve-conduction studies and EMG |
Typical clinical patterns
- Small-fibre: burning feet, sharp or shooting pains, allodynia, such as pain from socks or bedsheets, with preserved strength, reflexes, and vibration sense.
- Large-fibre: numb feet plus imbalance, difficulty sensing the ground, worse gait in darkness, reduced vibration at toes, impaired joint-position sense, and absent ankle jerks.
This distinction is reflected in neurological assessment: small-fibre dysfunction predominantly impairs pain, temperature, and autonomic functions, while large-fibre dysfunction impairs vibration and position sense and may cause sensory ataxia. A normal EMG/NCS therefore does not rule out small-fibre neuropathy. Diagnostic review and small-fibre testing review support this approach.
Causes
Both types can occur with:
- Diabetes or prediabetes
- Alcohol-related neuropathy
- Vitamin B12 deficiency and other nutritional disorders
- Kidney disease
- Thyroid disease
- Medication or chemotherapy toxicity
- Autoimmune disease, for example Sjögren syndrome
- Amyloidosis
- Hereditary neuropathies
- HIV and other infections
Small-fibre-predominant neuropathy is especially associated with diabetes/prediabetes, metabolic syndrome, Sjögren syndrome, celiac disease, amyloidosis, Fabry disease, and sometimes no identifiable cause. In diabetes, a person may move along a spectrum from predominantly small-fibre pain to combined small- and large-fibre sensory loss.
Diagnosis and treatment
Diagnosis starts with the pattern of symptoms and examination, then testing directed at the suspected fibre type and possible cause. Treating the underlying cause is the priority. Painful small-fibre symptoms may also need neuropathic-pain treatment, while large-fibre disease often requires fall prevention, foot care, balance/physiotherapy, and mobility support.
Seek urgent assessment for rapidly progressive weakness, new major gait loss, bladder/bowel dysfunction, marked asymmetry, or symptoms developing over days to weeks.
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