Teaching Case

Sex-working HIV-infected male-to-female transsexual with recurrent episodes of bloody diarrhoea and pelvic pain for 3 months, anaemia (Hb 7.5 g/dL), mildly raised inflammatory markers (C-reactive protein 41 mg/L). Hypotonic anal sphincter with severe tenderness and increased consistency of the anterior rectal wall at digital exploration, without identifiable perianal fistulous orifices.

At proctoscopy, profuse purulent discharge originated from the oedematous ventral rectum without evidence of active bleeding. Colonoscopy did not detect significant abnormalities in the upstream large bowel. Ultrasound (not shown) excluded significant abnormalities of the abdominal solid viscera.
Pelvic and perianal MRI, requested to investigate the possible presence of perirectal abscess collections before surgical consultation, detected marked, circumferential thickening (measuring up to 14 mm) of the rectal wall showed, with abnormal signal intensity on both T1- and T2-weighted sequences and homogeneous contrast enhancement. Associated findings included presacral fluid and multiple enlarged lymph nodes in the right perirectal and ischioanal spaces.
Microscopy and cultures on stools and blood yielded negative results for bacteria (particularly Shigella and Clostridium difficile), mycobacteria, fungi and toxins. Days after hospitalisation, positive serology for Entamoeba histolytica infection allowed diagnosing amoebic proctitis. Oral metronidazole treatment led to clinical remission within a week.

Endemic in many developing countries, amoebiasis is increasingly recognised as an opportunistic infection in homosexual people and travellers or immigrants from endemic countries. Entamoeba trophozoites invade the bowel wall causing characteristic “flask-shaped” ulcers. Invasive amoebiasis usually involves the caecum and ascending colon, sometimes the rectum, presenting with intermittent episodes of bloody or mucus-containing diarrhoea, rarely as severe dysentery with abdominal pain, fever and weight loss [1, 2].

Anoperineal abnormalities occur in approximately one-third of HIV-infected patients, particularly in male homosexuals [3]. Intestinal complaints and sexually-transmitted infections are usually assessed with microbiological tests, stool cultures and endoscopy. Colonoscopy findings range from sparse ulcerations to diffuse mucosal abnormalities, resembling those observed in chronic inflammatory bowel diseases. Sometimes, firm inflammatory endoluminal masses known as “amoebomas” develop. Diagnosis of amoebiasis is obtained through detection of trophozoites, alternatively with serological tests (anti-amoebic antibodies) more useful in Western, non-endemic areas. Amoebiasis is successfully cured with antibiotic metronidazole administration [1, 2, 4].

Scarce reports exists about imaging features of amoebic colitis, invariably imaged with CT and typically manifesting as a right-sided colitis, with diffuse bowel wall thickening and characteristic sparing of the distal ileum. Amoebomas may appear as endoluminal masses, as segmental tumour-like involvement or as non-specific diffuse bowel wall thickening; alternatively, the formation of a rectal cystic-like amoebic abscess has been described [4-8].
Increasingly employed to investigate the anorectal tract, MRI provides comprehensive assessment of the extent and topography of disease and of associated perirectal inflammatory changes, detects possible complications such as abscess collections and differentiates inflammatory changes from solid neoplastic lesions [9].

No literature reports exists about MRI findings in amoebiasis: in this first description of amoebic proctitis, MRI allowed comprehensive visualisation of the inflammatory involvement of the entire rectum including marked circumferential mural thickening with abnormal signal intensity and positive homogeneous contrast enhancement: these imaging appearances parallel those reported with CT in amoebic colitis with superior contrast resolution intrinsic to MRI. Furthermore, associated MRI findings suggesting proctitis included identification of inflammatory stranding of the mesorectal fat, presacral fluid effusion and enlarged perirectal lymph nodes [4-7].
Often, with invasive amoebiasis both endoscopic and cross-sectional imaging findings suggest proctitis, ulcers or masses, and differentiation from neoplastic processes is not always possible: suggesting amoebic proctitis as a possible diagnosis for rectal abnormalities in homosexuals is helpful because extensive laboratory investigations and sometimes biopsy may be necessary, but amoebiasis is a treatable disease [1, 2, 4, 7, 8].