Rectal cancer mimic: a rare case of syphilitic proctitis

Background

Syphilitic proctitis is a rare condition, but over recent years, there has been a rising incidence of syphilis diagnoses, particularly among men who have sex with men (MSM). This case report highlights the need of general physicians and radiologists to be aware of this condition as it can often be misdiagnosed clinically, radiologically and endoscopically. A prompt diagnosis will avoid unnecessary morbidity resulting from inappropriate or delayed treatment.

Case presentation

A 66-year-old retired businessman presented to the adult emergency department with a 10-day history of severe generalised abdominal pain and constipation. He was diagnosed with HIV-1 infection 14 years earlier when presenting with miliary tuberculosis and was well controlled (last CD4 count 520 cells/mm³, HIV viral load <40 copies/mL, 4 months before presentation) on maraviroc 150 mg two times per day with emtricabine 200 mg, darunavir 800 mg and ritonavir 100 mg once daily. Previous routine Treponema antibody testing was negative (last performed 4 months before presentation). Examination revealed generalised abdominal tenderness and an empty rectum.

Investigations

Full blood count, urea and electrolytes and serum carcinoembryonic antigen were normal. Abnormal liver function (alkaline phosphatase 552 IU/mL, normal range 30–130; alanine transaminase 92 IU/mL, normal range 2–53) and elevated C reactive protein 60 mg/L were found. Hepatitis B and C virus serology was negative. HIV viral load was <40 copies/mL. A contrast-enhanced CT scan of the abdomen and pelvis showed non-dilated small and large bowel but diffuse mucosal thickening in the rectum with perirectal fat stranding and multiple enlarged mesorectal lymph nodes (figure 1). Rectal cancer was suspected from the CT findings. Sigmoidoscopy and CT of the chest were advised. The CT of the chest was unremarkable. Sigmoidoscopy showed two distinct low rectal lesions with surrounding oedema and erythema (figure 2).

An MRI was requested for fuller evaluation. This showed diffuse rectal mucosal and wall thickening. The mucosa showed high T2-weighted (T2W) signal with associated restricted diffusion. Abnormal mesorectal nodes also restricted (figure 3). The diffuse appearances were not considered typical for rectal cancer and the possibility of a lymphoid malignancy was raised.

Histology of the rectal biopsy (figure 4) showed chronic lymphoid and neutrophil inflammatory infiltrate and mucosal prolapse type changes with ulceration and granulation tissue . There was no dysplasia or malignancy. Abundant plasma cells were present which is a recognised feature of Treponema Pallidum. However Warthin-Starry staining (special silver staining for T. Pallidum) was negative.

Infectious diseases opinion was sought. Sexual history identified MSM and unprotected anal intercourse with a new male partner. Rectal swabs were negative by PCR for Neisseria gonorrhoea and Chlamydia trachomatis. Serum Treponemal antibody (Treponema pallidum particle agglutination assay) and Treponema IgM were positive with rapid plasma reagin (RPR) titre 1:32 indicating recent active syphilis infection. In view of a developing headache, lumbar puncture was performed which found mild cerebrospinal fluid pleocytosis, elevated protein 0.96 g/L (normal range 0.1–0.45 g/L) with positive Treponema TPPA and negative RPR. He was diagnosed with early syphilis. The rectal ulceration was felt to represent a primary chancre overlapping with manifestations of secondary syphilis including syphilitic hepatitis and meningo-vascular involvement.

Differential diagnosis

The initial diagnosis for the CT findings of diffuse rectal thickening with local lymphadenopathy was rectal malignancy in keeping with the presentation and age. MRI findings were not, however, typical for rectal malignancy and raised the suspicion of a lymphoproliferative disorder. Other potential differentials for rectal lesions in the general population would include inflammatory bowel disease but this was felt less likely given the history and imaging characteristics. Among MSM, infective pathogens should also be considered including Chlamydia trachomatis, Neisseria gonorrhoea, Herpes simplex virus and Lymphogranuloma venereum.1

Treatment

He received 2.4g of intravenous benzyl penicillin administered every 4 hours for 14 days.

Outcome and follow-up

His headache and gastrointestinal symptoms resolved following treatment. Follow-up MRI after 3 months demonstrated improvement in the rectal findings and regression of lymph nodes (figure 5). Serum RPR declined to 1:4 at 3, 6 and 9 months and was negative at 12 and 18 months post treatment, indicating acceptable serologic response (≥4 fold decline required). Liver function abnormalities had resolved by 3 months post treatment. His partner was diagnosed with active syphilis and successfully treated.