Disease of large myelinated fibres produces loss of touch and joint position perception. Patients complain of difficulty in discriminating textures. Their hands and feet feel like cotton wool. Gait is unsteady, especially when in darkness where vision cannot compensate for loss of joint position sensation (proprioception).
Disease of small unmyelinated fibres produces loss of pain and temperature appreciation as a consequence of which painless burns/trauma result. Damage to joints without pain results in a 'neuropathic' joint (Charcot's joint) in which traumatic deformity is totally painless.
Disease of large myelinated fibres produces paraesthesia - a 'pins and needles' sensation with a peripheral distal distribution.
Disease of small unmyelinated fibres produces painful positive phenomena:
- Burning extremities
- Dysaesthesia - when touching is painful
- Hyperalgesia - when threshold to pain appears lowered
- Hyperpathia - when threshold to pain appears elevated but, once reached, the painful stimulus is excessively felt. Lightning pains take the form of sudden, very severe shooting pains and are virtually pathognomonic for tabes dorsalis.
Causalgia results from nerve trauma. A spontaneous burning sensation in the distribution of the injured nerve is associated with an increased sensitivity to painful stimulation.
The patient notices weakness:
- When distal, e.g. difficulty in clearing the kerb when walking
- When proximal, e.g. difficulty in climbing stairs or combing hair
- Cramps may be troublesome
- Twitching of muscles (fasciculation) may be felt.
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