Clinical and Histological Presentation and Differential Diagnosis
As the name suggests, GA is a non-infectious granulomatous skin reaction; 'annulare' refers to the morphology of the lesions, which, in localized forms of GA, appear as isolated, skin-colored to erythematous circinate papules and plaques. GA can also present in a generalized fashion, with generalized (i.e. disseminated) GA being defined as at least 10 widespread annular plaques.
Subcutaneous GA is another variant of GA and is common in the pediatric population. It typically presents as a firm, subcutaneous mass on the lower extremities. Patch type GA is a less common variant and presents as violaceous to erythematous patches that typically involve the bilateral proximal extremities. Perforating GA has also been described and presents as umbilicated papules or pustules with crusting; both localized and generalized forms of perforating GA have been reported. GA appearing on the acral surfaces is rare but has recently been reported on the palms[16,17] and the soles of the feet.
GA presents histologically with a focus of necrobiosis surrounded by palisading histiocytes. Additionally, mucin deposition is an important hallmark of GA. Multinucleated giant cells are also a common finding and eosinophils, lymphocytes, and neutrophils infiltrating the dermis may also be observed. Different subtypes of GA have been noted to have certain characteristic features, although the triad of degraded collagen, histiocytic infiltrate, and presence of mucin seem to be common findings across all subtypes of GA.
In most cases, the diagnosis of GA is straightforward. Albeit rarely, initial clinical impressions can be either ambiguous or misleading, making diagnosis difficult. Notably, GA can resemble other granulomatous skin conditions. The presence of mucin is an important histologic feature that can clinch the diagnosis in the favor of GA, as histological examination of other granulomatous skin diseases such as sarcoidosis and necrobiosis lipoidica does not reveal mucin deposition. Recently, a case of GA appearing on a scar and mimicking sarcoidosis was reported. GA has also been reported to mimic dermatofibroma, dermatomyositis, papulonecrotic tuberculid, psoriasis,[24,25] and tinea cruris. Conversely, borreliosis, Churg–Strauss Syndrome, Kaposi sarcoma, and tuberculoid leprosy have been reported to masquerade as GA. Such case reports underscore the importance of establishing a clinicopathologic correlation before rendering a diagnosis.
Dermoscopic findings may also facilitate diagnosis. Unfocused vessels of variable appearance against a pinkish-reddish background are characteristic but by no means pathognomonic.[31,32] Recently, ultrasound was reported to be of utility in the diagnosis of subcutaneous GA.[33,34] As subcutaneous GA is more common among children (who may be more averse to biopsy), ultrasound may represent a novel and useful tool in the diagnosis of this rare GA variant, although further studies are needed to validate the efficacy of ultrasound.
Am J Clin Dermatol. 2022;23(1):37-50. © 2022 Adis Springer International Publishing AG