Rudy Bilous, MD, FRCP
Richard Donnelly, MD, PHD, FRCP, FRACP
Diabetes affects the cellular biochemistry of skin and connective tissues, in particular collagen synthesis and structure, as well as cutaneous microvascular blood flow. Several non-infective skin conditions are associated with type 1 and/ or type 2 diabetes (Box 24.1). Diabetic dermopathy (‘shin spots’) is relatively common (reportedly present on the legs in up to 10%–15% of patients) and is probably due to microangiopathy. In contrast, necrobiosis lipoidica diabeticorum is rare (<0.3% of patients) and occurs mostly in the 40–60-year age group. Whether granuloma annulare is associated with diabetes is unclear, but the strongest evidence suggests a link with type 1 diabetes. There are also a number of skin or nail infections (fungal and bacterial) associated with diabetes, e.g. paronychia.
Diabetic dermopathy (also known as spotted leg syndrome or ‘shin spots’) is characterized by hyperpigmented, atrophic macules, a few millimetres in diameter, which typically occur as clusters on the shins (Figure 24.1). They are more common in older patients with diabetes, especially those over the age of 50 years (one or two such lesions also occur in up to 3% of people without diabetes). The spots slowly become well circumscribed, atrophic, brown and scaly scars. The usual site is the pretibial region, but forearms, thighs and bony prominences may be involved. There is no effective treatment, but the spots tend to resolve over 1–2 years.
Necrobiosis lipoidica diabeticorum (NLD) is rare, but occurs almost exclusively in patients with diabetes, typically in the 40–60-year age group, and is more common in women. NLD appears as bilateral red-brown papules on the anterior surface of the shins (Figure 24.2). The lesions gradually enlarge to form yellow, atrophic plaques with a translucent lustre and stippled with telangiectasia. Ulceration occurs in about 25%. The lesions are partially or completely anaesthetic. The aetiology of NLD is unknown, and treatment options include topical or systemic steroids, antiplatelet therapy, photodynamic therapy or anti-TNF drugs (e.g. infliximab).
Histologically, NLD is characterised by hyaline degeneration of collagen in the dermis (‘necrobiosis’) with surrounding fibrosis and diffuse histiocytic infiltrate (Figure 24.3). There may be evidence of a granulomatous reaction with giant cells similar to that seen in sarcoidosis.