Sudomotor function in familial dysautonomia

Patients with familial dysautonomia (FD) manifest episodic hyperhidrosis despite the reduction of sudomotor fibres and sweat glands associated with this autonomic neuropathy. We assessed peripheral sudomotor nerve fibre and sweat gland function to determine if this symptom was due to peripheral denervation hypersensitivity.

In 14 FD patients and 11 healthy controls, direct and axon reflex mediated sweat responses were determined by measuring transepidermal water loss (TEWL) after application of acetylcholine via a microdialysis membrane, a novel method to evaluate sudomotor function in neuropathy patients. Results were compared with data from conventional quantitative sudomotor axon reflex testing (QSART). Using microdialysis, interstitial fluid was analysed for plasma proteins to evaluate protein extravasation induced by acetylcholine as an additional parameter of C-fibre function.

Although reduced axon reflex sweating was expected in FD patients, neither direct or axon reflex mediated sweat responses, nor acetylcholine induced protein extravasation differed between control and patient groups. However, the baseline resting sweat rate was higher in FD patients than controls (p<0.05). TEWL and QSART test results correlated (r = 0.64, p = 0.01), proving the reliability of TEWL methodology in evaluating sudomotor function.

The finding of normal direct and axon reflex mediated sweat output in FD patients supports our hypothesis that, in a disorder with severe sympathetic nerve fibre reduction, sudomotor fibres, but not the sweat gland itself, exhibit chemical hypersensitivity. This might explain excessive episodic hyperhidrosis in situations with increased central sympathetic outflow.