• Multiple Sclerosis (MS): Uhthoff Phenomenon - Temporary worsening of neurological deficits (e.g., vision) due to heat impairing conduction in demyelinated nerves.
• Polycythemia Vera (PV): Aquagenic Pruritus - Intense itching after contact with water (especially hot), often without rash. Linked to mast cell/histamine release. Classic PV symptom.
• Systemic Mastocytosis: Symptoms like flushing, hives, itching, GI distress, syncope triggered by heat, causing widespread mast cell degranulation and mediator release (histamine).
• Cholinergic Urticaria: Small, punctate hives triggered by heat/sweating. Caused by acetylcholine stimulating mast cells.
• Situational Syncope: Fainting/lightheadedness due to heat-induced peripheral vasodilation, leading to decreased venous return and transient cerebral hypoperfusion.
• Hot Tub Folliculitis: Pruritic papules/pustules caused by Pseudomonas aeruginosa infection from contaminated warm water.