Diagnosis: Pretibial myxedema
The patient’s history, paired with the results of the punch biopsy, were consistent with a diagnosis of pretibial myxedema, part of the triad of Graves disease along with thyroid ophthalmopathy and acropachy (soft-tissue swelling of the hands and clubbing of the fingers). Histopathologic findings revealed wide separation of collagen bundles throughout the entire reticular dermis without fibroplasia (FIGURE 2A). The spaces contained basophilic strands (FIGURE 2B), and the strands stained strongly positive on Alcian blue (FIGURE 2C), confirming the presence of dermal mucin. Widely separated collagen fibers and deposited mucin are indicative of pretibial myxedema. No granulomas or lymphoid proliferations were seen.
The pathogenesis of pretibial myxedema is widely postulated to be due to the stimulation of dermal fibroblasts by anti–TSH antibodies, causing overproduction of glycosaminoglycans and hyaluronic acid1 and obstructing lymphatic microcirculation, resulting in nonpitting edema.2
There are 5 distinct clinical variants of pretibial myxedema1,3:
- The diffuse form is the most common. It manifests on the lower leg with hard, nonpitting edema and cutaneous thickening.
- The plaque form manifests on the lower leg as well-demarcated erythematous or pigmented flat-topped lesions.
- The nodular form, which our patient had, typically manifests on the lower leg as well-demarcated erythematous, pigmented, or skin-colored raised, solid lesions. There may be 1 lesion or several.
- The mixed form manifests as 2 or more of the other variants.
- The elephantiasic form is the rarest and the most severe. There are widespread swollen nodules and plaques on the lower legs and/or arms.
A rare, late manifestation
Although pathognomonic for Graves disease, pretibial myxedema is a late manifestation that occurs in less than 5% of these patients.4 The most common site of involvement is the pretibial region, although less common sites include the face, arms, shoulders, abdomen, pinna, and the location of previous scars.4
While pretibial myxedema usually is associated with hyperthyroidism, it can occur after treatment (as was the case here), while the patient is in a euthyroid or hypothyroid state. Radioiodine therapy has been reported to be a trigger for pretibial myxedema in 1 case report, although the pathophysiology is not fully understood.5
Continue to: More serious conditions must be ruled out