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Small Fibre Neuropathy
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Small fibre sensory neuropathy can cause severe symptoms but there can be minimal objective findings on examination. Class symptoms include distal burnings, pain, numbness, paraesthesia, and autonomic symptoms. Autonomic symptoms can include sweating alterations, temperature dysregulation, dry mouth and eyes, and erectile dysfunction. Most affected patients have a combination of positive signs (e.g. hyperalgesia, and allodynia), and negative signs (e.g. diminished pin prick and temperature sense). Standard electrophysiologic testing is typically normal with disorders that affect mainly the small, unmyelinated nerve fibres. Sural nerve biopsy may be only minimally abnormal, or even normal.
- Characteristics of sensory nerve fibres
| Nerve fibre | Myelinated axons | Diameter (ยตm) | Conduction velocity (m/s) | Sensory information | Usefulness of electroneuromyography | Usefulness of QST |
|---|---|---|---|---|---|---|
| Aฮฑ | Yes | 13-20 | 80-120 | Proprioception, muscle spindle primary endings (Ia), golgi tendon organs (Ib) (and alpha motor neurons) | Yes (H reflex) | No |
| Aฮฒ | Yes | 6-12 | 33-75 | Discriminative mechanoreception (touch, vibration), proprioception, pain modulation (block nociceptive information, allodynia in sensitisation) | Yes (sensory nerve conduction) | Yes |
| Aฮณ | Yes | 4-8 | 15-40 | Touch, pressure (and gamma motor neurons) | No | |
| Aฮด | Thin | 1-5 | 3-30 | "rapid" pain, crude touch, pressure, temperature. AMH type I for rapid mechanical pain (high heat threshold >53C), AMH type II for rapid heat pain (lower heat threshold 43-47C). | No | Yes |
| C | No | 0.3-1.5 | 0.5-2.0 | "slow" pain, touch, pressure temperature (and postganglionic autonomic). Polymodal. | No | Yes |
