Can a dermatologist diagnose erythromelalgia/erythermalgia?

Difficult to say: It is a rare disease and many of us, from all specialties, have never seen a case of it. Proper diagnosis, as with any disease, depends on what your symptoms are & what your exam and labwork show. But rarer diseases can be hard to detect.. Here's a handy page giving advice to people who think they have it and want to know who to see: http://www.erythromelalgia.org/WhatisEM/Diagnosis.aspx

Only: if they've seen it, treated carefully with aspirin, sometimes treatment difficult.