diathermy

Sometimes a clinician wants to warm a large, deep region — a whole hip or a broad area of the low back — more deeply than a hot pack ever could. Diathermy is a way to do this: it sends electromagnetic energy into the body, and the tissue itself turns that energy into heat from the inside out, like the way a microwave warms food.

The word means literally 'heating through'. The two main types are shortwave diathermy, which uses radio-frequency electromagnetic waves, and microwave diathermy. The energy passes into the body and is absorbed by tissue — especially tissue with high water content, such as muscle — producing deep, broad warming. Unlike therapeutic ultrasound, which heats a small spot, diathermy can warm a larger volume of deep tissue at once, which historically made it attractive for big joints and large muscle regions.

Diathermy is used far less today than in past decades, partly because the evidence that it outperforms simpler, cheaper heat is thin, and partly because of real safety concerns. The electromagnetic energy can dangerously heat metal, so it is avoided over implants, pacemakers, and metal jewellery; it is also avoided over growth plates in children, over the eyes, over the pregnant abdomen, and over areas with poor sensation. Like all heat, it is a comfort-and-warm-up tool, not a treatment that repairs the underlying problem.