Actinomyces odontolyticus infection associated with a horse bite in an immunocompetent man

EJD, vol. 33, n° 3, May-June 2023 Initial biopsy showed deposition of the amorphous material (figure 1G) with similar histochemical and immunohistochemical features (figure 1H), accompanied by histiocytic and giant cell reaction in the dermis and features of elastorrhexis. Changes in the epidermis were unremarkable. Melanin incontinence was evident. Morphological, histochemical and immunohistochemical features were consistent with the diagnosis of LCDD. Further clinico-laboratory investigation was recommended, and the patient was referred to the haematology department. Bone marrow biopsy was carried out, revealing an infiltration (10-12%) of lambda monoclonal plasma cells. Congo red stain was once more negative. MRI findings strongly suggested restrictive cardiomyopathy. The diagnosis of LCCD was established, without involvement of any other organs. LCDD usually occurs in the setting of plasma cell dyscrasias or lymphoproliferative disorders and may even be idiopathic [2]. There is a male predilection and patients have a median age of 58 years. The diagnosis is based upon Congo red-negative deposition, staining with single immunoglobulin antibody, and an absence of fibrillar architecture ultrastructurally, or arrangement of β-pleated sheet [3]. Although LCDD primarily affects the kidneys (96% of patients), it may be implicated in the heart, liver, peripheral nervous system and, less frequently, the lungs and skin [2-4]. The deposits are usually located in the basement membrane and among elastic and collagen fibres. LCDD presents with skin involvement in 5% of patients, as recently described by Joly et al. based on a large cohort study [4]. Prior to this study, little attention had been paid to extrarenal manifestations of the disease. In 2018, Hendricks et al. [5] summarized six cases with cutaneous involvement, three of which did not manifest with any extracutaneous organ involvement (supplementary table 1). Disease can present as nodules, papules, plaques, purpura and erythematous sclerodermic lesions depending on the pattern of involvement that seems to be in accordance with the pattern of cutaneous amyloidosis [5]. Widespread deposition of globular material mainly in the upper dermis presents clinically as erythematous lesions or purpura, whereas extensive deposition also involving the deep dermis and subcutis manifests as nodules and plaques [5]. In our case, both patterns of deposition with the above-mentioned clinical manifestations were found. Differential diagnosis includes diseases with deposition of amorphous material such as amyloidosis, colloid milium, lipoid proteinosis and cutaneous macroglobulinaemia. Early diagnosis is of essence in order to achieve a better outcome. Treatment for systemic disease is based on chemotherapeutic agents with or without autologous stem cell transplantation [4, 6]. Older age, associated haematological disease and extrarenal deposition are considered high-risk indicators of an unfavourable prognosis [2]. It is important for dermatopathologists to maintain a high level of suspicion for LCDD since its clinical, laboratory and histological findings can be quite ambiguous, and renal involvement is not always evident at first sight. To further emphasize this, the diagnosis in our case was based on skin biopsy, providing the stimulus for a thorough examination. A multidisciplinary approach is crucial to reach the correct diagnosis of this rare disease, especially when there is merely skin involvement.