The clinical characteristics of aquagenic pruritus (AP) based on a series of thirty-six patients are presented. AP is characterized by the development of severe, prickling-like skin discomfort that is without observable skin lesions and that is evoked by contact with water at any temperature. Other causes of pruritus associated with water contact must be excluded. In the thirty-six patients, skin discomfort developed within minutes of water contact in approximately half. In others, discomfort began 2 to 15 minutes after water exposure had ceased. The pruritus was usually generalized, lasting from 10 to 120 minutes (average, 40.6 minutes), and in 55% was associated with symptoms of acute emotional liability. There was no increased prevalence of atopy. Thirty-three percent reported a family history of water-related itching. Of fourteen patients treated with ultraviolet B phototherapy, eight (57%) noted significant relief. Of thirty-four patients, sixteen (47%) noted partial relief with oral antihistamine therapy. Patients with polycythemia rubra vera (PRV) may present with symptoms similar to those of AP, and all patients with symptoms consistent with AP should be investigated for the presence of PRV.

Merkel cell carcinoma (MCC) is an aggressive tumour with neuroendocrine differentiation. Clinically significant differences within the entity we know as MCC are apparent. This review aims to evaluate the evidence for differences in tumours within Merkel cell carcinoma and to stratify these. A literature search of research pertaining to various characteristics MCC was undertaken from 1972, when Merkel cell carcinoma was first described, to 2018, using PubMed and similar search engines. A total of 41 papers were analysed, including clinical trials, laboratory‐based research and reviews. A proportion of MCC has Merkel cell polyomavirus genome integrated (MCPyV+) while others do not (MCPyV−). Both types have a different mutation burden. MCPyV+ tumours are likely true neuroendocrine carcinomas, with a dermal origin, probably from fibroblasts which have been transformed by integration of the viral genome. MCPyV−tumours are likely derived from either keratinocytes or epidermal stem cells, are probably squamous cell carcinomas with neuroendocrine differentiation, and are related to sun damage. Prognostic factors (apart from tumour stage) include the MCPyV status, with MCPyV+ tumours having a better prognosis. P63 expression confers a worse prognosis in most studies. CD8+ lymphocytes play an important role, providing a possible target for PD1/PD‐L1 blockade treatment. The incidence of MCC varies from country to country. Countries such as Australia have a high rate and a far greater proportion of MCPyV− tumours than places such as the United Kingdom. MCC doubtlessly encompasses two tumours. The two tumours have demonstrated differences in prognosis and management. One is a neuroendocrine carcinoma related to MCPyV integration likely derived from fibroblasts, and the other is a UV‐related squamous cell carcinoma with neuroendocrine differentiation, presumptively derived from either keratinocytes or epidermal stem cells. We propose naming the former Merkel type sarcoma and the latter squamous cell carcinoma, Merkel type.

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We studied blood histamine activity (HA) and cutaneous fibrinolytic activity (CFA) in a patient with polycythaemia rubra vera (PRV) and water-induced pruritus, before and after water exposure. The results suggest that the water-induced itching in PRV is associated with an increase in HA. In addition, markedly increased levels of CFA were found both before and after water exposure. These findings have been previously reported in patients with aquagenic pruritus (AP) but not in patients with PRV. As the water-induced itching in PRV and AP share many common features, these findings suggest that the pathophysiology of the water-induced pruritus in these two conditions may be similar.