Sensory Polyneuropathies
The differential diagnosis for polyneuropathy can be categorised in several ways
1. Large fibre vs small fibre: Large fibre neuropathies will typically show reduced reflexes, weakness, and reduced vibration and position sense. On the other hand, small fibre neuropathies have normal reflexes and strength, and more minimal findings of reduced sensation to pin prick and temperature.
2. Hereditary vs acquired: Hereditary polyneuropathies can manifest in adulthood or childhood and there is often a family history. Charcot-Marie-Tooth can be suspected with the presence of a distinctive pes cavus foot.
3. Primary vs secondary: Primary polyneuropathy e.g. chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), and secondary polyneuropathy e.g. diabetes, toxin, and monoclonal gammopathy.
4. Axonal vs demyelinating: Axonal loss for example in diabetic polyneuropathy, or demyelination for example in CIDP. Polyneuropathies can be associated with axonal loss (e.g., diabetic polyneuropathy) or demyelination (e.g., CIDP)
- Characteristics of peripheral nerve fibres
| Nerve Fibre | Myelin | Diameter (ยตm) | Conduction velocity (m/s) | General Function |
|---|---|---|---|---|
| Aฮฑ (I) | Yes | 13-20 | 80-120 | Proprioception: muscle spindle primary endings (Ia), golgi tendon organs (Ib), and alpha motor neurons |
| Aฮฒ (II) | Yes | 6-12 | 35-75 | Discriminative sensitivity to mechanical stimuli (touch, vibration), proprioception, pain modulation (block nociceptive information, allodynia in sensitisation) |
| Aฮณ | Yes | 4-8 | 15-40 | Touch, pressure, and gamma motor neurons. |
| Aฮด (III) | Thin | 1-5 | 5-30 | "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). |
| B | No | 1-3 | 3-14 | preganglionic autonomic |
| C (IV) | No | 0.2-1.5 | 0.5-2.0 | "second" pain, mechanical, chemical, thermal, pruritis, and postganglionic autonomic. polymodal |
