Anhidrosis
Summary
Anhidrosis is the inability to sweat, resulting from interruption of sympathetic cholinergic innervation to eccrine sweat glands. It is a hallmark of Horner syndrome (with ipsilateral facial anhidrosis) and is also seen in autonomic neuropathies.
Detail
Eccrine sweat glands are paradoxically innervated by sympathetic postganglionic fibers that release acetylcholine (acting on M3 muscarinic receptors), an exception to the norepinephrine rule. Loss of sympathetic input causes ipsilateral anhidrosis: in Horner syndrome from a preganglionic or central lesion, hemifacial anhidrosis accompanies miosis and ptosis; postganglionic lesions (e.g., internal carotid dissection) spare facial sweating because sweat fibers to the face travel along the external carotid. Other causes include diabetic autonomic neuropathy, amyloidosis, Ross syndrome (segmental anhidrosis + tonic pupil + areflexia), anticholinergic toxicity ('dry as a bone'), and congenital ectodermal dysplasia (absent sweat glands). Boards classic: anhidrosis + ptosis + miosis on the same side = Horner syndrome, often from Pancoast tumor.
Sources
- First Aid for USMLE Step 1 2024
- Robbins Basic Pathology 10th ed
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