Dermatitis Herpetiformis (DH)

Dapsone pins the pulse oximeter - Dapsone is first-line for DH at 25 to 150 mg daily and is a common cause of methemoglobinemia. Methemoglobin absorbs red and infrared light equally, so the 1:1 ratio drives SpO2 toward 85% no matter what the arterial saturation actually is — falsely low when true saturation is above 85%, falsely high when it is below. A saturation stuck in the mid-80s alongside a normal or high PaO2 on a blood gas is the tell. Confirm with co-oximetry, which measures methemoglobin directly; the pulse oximeter cannot.

Treating methemoglobinemia - Methylene blue 1 to 2 mg/kg IV over 5 minutes, repeatable once at an hour. Treat symptomatic patients or a level above 30%. Cyanosis depends on total methemoglobin rather than the percentage — above 1.5 g/dL — so an anemic patient can carry a high percentage without turning blue. Doses above 7 to 8 mg/kg, or a rapid push, paradoxically oxidize more hemoglobin.

Find the G6PD result before you need it - Methylene blue works only if G6PD is functional, and it causes hemolysis in G6PD-deficient patients. G6PD activity is checked before dapsone is started, so the number is usually already in the chart — get it in preop, not at the moment you need to treat. If the patient is deficient, treat with vitamin C 2 mg/kg IV instead. Methylene blue also precipitates serotonin syndrome in patients on SSRIs.

Do not stack oxidizers - Skip prilocaine and benzocaine topical sprays in a patient on dapsone. Both generate methemoglobin, benzocaine in a dose-independent way, and they add to a burden the patient is already carrying.

Baseline blood work - Dapsone causes hemolysis and agranulocytosis, and deaths from agranulocytosis and aplastic anemia are reported, so get a CBC. Malabsorption from the underlying enteropathy adds iron-deficiency anemia on top of that — check hemoglobin, iron stores, and ferritin before any case with meaningful expected blood loss.

Iodine sets off flares - Iodine exposure precipitates DH flares. Ask the surgeon about a chlorhexidine prep instead of povidone-iodine where the site and procedure allow it, and settle it in the preop plan rather than discovering the prep already painted on.

Skin, monitors, and tape - The lesions are on extensor elbows, knees, buttocks, and scalp, and they are excoriated rather than intact. Punctate purpura may be present on the palms and soles. Site the BP cuff, ECG electrodes, and tape on unaffected skin, pad the elbows and knees where the lesions sit, and avoid dragging tape across a scratched surface.

Nutrition, bone, and positioning - Untreated GSE causes malabsorption, weight loss, and osteoporosis from vitamin D and calcium deficiency. Check vitamin D and calcium, and look for a bone mineral density result if the case involves extreme or prolonged positioning. Transfer and position these patients as you would anyone with fragile bone.

Document neuropathy before a block - Dapsone causes peripheral neuropathy, and GSE itself is associated with ataxia and neuropathy. Do a focused sensory and motor exam and write down what you find before any regional technique, so a pre-existing deficit is not later blamed on your block.

The autoimmune company it keeps - DH runs with autoimmune thyroid disease, type 1 diabetes, Addison disease, and vitiligo, and all patients should be screened for thyroid disease. Check TSH and a fasting glucose or HbA1c. An Addison history means the glucocorticoid replacement plan needs to be settled before the day of surgery.

Look at the teeth - Dental abnormalities, particularly enamel pits, are reported in DH, and in celiac disease lack of calcium causes problems with dentition. Document chipped, pitted, or loose teeth before laryngoscopy.

Check the mouth - Mucosal change occurs in DH. It does not usually threaten the airway, but look before you instrument and note any erosions rather than assuming the disease stops at the elbows and knees.

Dermatitis herpetiformis (DH) is a chronic autoimmune blistering disease and, for practical purposes, the skin expression of gluten-sensitive enteropathy (GSE). IgA autoantibodies against transglutaminase — epidermal transglutaminase 3 is the dominant antigen in skin — deposit in a granular or fibrillar pattern in the papillary dermis, pulling neutrophils to the tips of the dermal papillae and lifting a subepidermal blister. It is tied to HLA-DQ2 and less often HLA-DQ8, and dietary gluten is the driving environmental factor. The rash is intensely pruritic grouped papules and vesicles over extensor surfaces — elbows, knees, buttocks, scalp — and intact vesicles are rarely seen because they are scratched off, leaving excoriations.

What matters perioperatively is not the rash. It is the malabsorption behind it and the dapsone used to treat it.