Paraneoplastic leukemoid reaction (PLR) is the extreme leukocytosis that occurs due to a non-haematolymphoid cytokine-secreting tumour (CST) in the absence of bone marrow infiltration by that solid tumour. The clinical presentation is widely variable, and therefore challenging. If the underlying malignancy is not clinically apparent, PLR could be mistaken for myeloproliferative neoplasms, altering the patient’s management. CSTs are highly aggressive tumours associated with a poor prognosis due to multiple mechanisms. Localising and treating the underlying malignancy is the mainstay of treatment. Both the treating clinician and the pathologist should keep a high level of suspicion for this entity in patients having unexplained leukocytosis. We herein discuss the underlying mechanisms, clinical presentation, pathological features, differential diagnosis and prognosis of this rare entity. An emphasis on the role of the pathologist is provided since the lack of knowledge on this entity can lead to dramatic effects on the patient, including unnecessary diagnostic testing and treatments.

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Background Eosinophilic gastrointestinal disorders, also known as eosinophilic gastroenteritis, are rare inflammatory conditions characterized by eosinophilic infiltration of different parts of the gastrointestinal tract, along with peripheral eosinophilia in most cases. Other known causes for gut eosinophilic infiltration must be excluded to confirm the diagnosis of eosinophilic gastroenteritis. Symptoms of the disorder depend on the affected gastrointestinal tract segment and depth of involvement. Treatment includes systemic glucocorticoids and/or dietary therapy with an empiric elimination diet. Second line therapies include the leukotriene receptor antagonist montelukast, and other anti-allergy drugs such as mast cell stabilizers (including cromolyn and the H1-antihistamine ketotifen), suplatast tosilate which is a selective Th-2 cytokines (IL-4 and IL-5) inhibitor, and the monoclonal anti-IgE antibody omalizumab. We report a case of eosinophilic gastroenteritis who was successfully treated and achieved remission with montelukast as an initial monotherapy. Upon extensive literature review, this represents the second reported adult case of eosinophilic gastroenteritis who responds to montelukast alone as a first line therapy. Case presentation A 49-year-old female presented with recurrent abdominal pain, vomiting, diarrhea and unexplained eosinophilia. She was diagnosed with eosinophilic gastroenteritis and was successfully treated with montelukast monotherapy. After 7 days of therapy, the patient responded well and had complete resolution of her gastrointestinal symptoms and peripheral eosinophilia. Patient remained in remission on follow-up after 12 months. We reviewed the literature for leukotriene antagonist use in the treatment of eosinophilic gastroenteritis and included the cases treated with the leukotriene antagonist montelukast as an initial therapy or as a second line therapy for refractory disease. Conclusion Montelukast may be an effective treatment for eosinophilic gastroenteritis, either alone or in combination with systemic steroids or ketotifen. Our patient is the second reported adult case of eosinophilic gastroenteritis who responded to montelukast alone as a first line therapy. Further studies and clinical trials are required to confirm efficacy compared to standard therapy.

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