What are the possible causes of a rash around the rectum in a patient?

Causes of Rash Around the Rectum

A perianal rash has multiple etiologies that must be systematically evaluated, including infectious causes (sexually transmitted infections, fungal, parasitic), inflammatory conditions (inflammatory bowel disease, contact dermatitis), structural anorectal disease (fistulas, abscesses), and idiopathic pruritus ani—with the diagnostic approach prioritizing identification of treatable infectious and inflammatory causes that impact morbidity and quality of life.

Infectious Causes

Sexually Transmitted Infections

• Proctitis from gonorrhea, chlamydia, herpes simplex virus, or lymphogranuloma venereum (LGV) commonly presents with perianal inflammation, discharge, and painful ulcers 1.
• Patients with HIV infection and acute proctitis may present with bloody discharge and painful perianal ulcers, requiring presumptive treatment for both genital herpes and LGV 1.
• All persons with perianal rash and suspected proctitis should be tested for HIV and syphilis 1.

Parasitic Infections

• Pediculosis pubis (pubic lice) presents with pruritus and visible lice or nits on pubic hair, transmitted by sexual contact 1.
• First-line treatment is permethrin 1% cream rinse or pyrethrins with piperonyl butoxide applied for 10 minutes 1.

Inflammatory and Dermatologic Causes

Inflammatory Bowel Disease-Associated

• Perianal Crohn's disease manifests as abscesses, fistulas, or hidradenitis suppurativa-like features, representing specific cutaneous manifestations with granulomatous histopathology 2.
• Pyoderma gangrenosum presents as tender, red inflammatory nodules progressing to deep excavating ulcerations with purulent material (sterile on culture), affecting 0.6–2.1% of ulcerative colitis patients 1, 2.
• Sweet's syndrome (acute febrile neutrophilic dermatosis) appears as tender, red inflammatory nodules or papules, usually on upper limbs but can occur perianally, with strong predilection for women and colonic involvement 1, 3.
• Erythema nodosum presents as tender, red or violet subcutaneous nodules 1–5 cm in diameter, though typically on extensor surfaces rather than perianal region 1.

Idiopathic Pruritus Ani

• Primary (idiopathic) pruritus ani occurs when no cause is identified, though 25–75% of cases have co-existing pathology requiring detailed history and examination 4.
• Management focuses on eliminating irritants and scratching, hygiene modification, and keeping perianal skin dry and intact 4.

Structural Anorectal Disease

Abscess and Fistula

• Anorectal abscesses result from infection of intersphincteric anal glands, presenting with pain, visible redness, swelling, and tenderness 1, 5.
• Anal fistulas (fistula-in-ano) arise from preexisting abscesses in the majority of cases, presenting with drainage of blood, pus, or fecal material from external perianal opening 1, 5.
• Perianal fistulas occur in 13–27% of Crohn's disease patients and may be the initial manifestation in up to 81% of those who develop perianal disease 1.
• The internal opening at the dentate line is pathognomonic for cryptoglandular fistulas 5.

Contact and Irritant Dermatitis

Topical Irritants and Allergens

• Contact dermatitis from detergents, soaps, cosmetics, or jewelry can cause perianal rash 6.
• Hydrocortisone 1% cream is FDA-approved for temporary relief of itching associated with minor skin irritations, inflammation, and rashes, including external anal itching 6.
• Apply not more than 3–4 times daily after cleaning the affected area with mild soap and warm water 6.

Critical Diagnostic Approach

Initial Evaluation

• Obtain focused history for inflammatory bowel disease symptoms (diarrhea, weight loss, abdominal pain), sexual history, and recent antibiotic or immunosuppressive therapy 1, 5.
• Physical examination should assess for fluctuance (abscess), external fistula openings, ulcers, and characteristics of any rash 1, 5.
• Digital rectal examination provides critical information but should not include probing for occult fistulas, which risks creating iatrogenic tracts 5.

Laboratory and Imaging

• Test for gonorrhea, chlamydia, herpes simplex virus, HIV, and syphilis when sexually transmitted proctitis is suspected 1.
• MRI or endoanal ultrasound is recommended for preoperative evaluation of suspected fistulas, with MRI showing higher accuracy for complex fistulae 1, 5.
• CT pelvis with IV contrast helps visualize and characterize fluid collections, abscesses, and fistulous tracts 1.

Treatment-Induced Causes

Drug Reactions

• Anti-TNF therapy can induce paradoxical skin inflammation as a class-drug effect, usually reversible upon drug cessation 1.
• Thiopurines are associated with nonmelanoma skin cancer risk 2.
• Biologics can cause psoriasis-like, eczema-like, and lichenoid eruptions 2.

Critical Pitfalls to Avoid

• Do not assume benign etiology without excluding sexually transmitted infections, especially in sexually active patients 1.
• Do not use topical steroids before excluding infection or malignancy, particularly with ulcerated lesions 6.
• Do not probe for fistulas during initial examination, as this creates iatrogenic tracts 5.
• Mandatory screening for Crohn's disease in recurrent perianal fistulas or abscesses 5, 7.
• Hydrocortisone should not be used for vaginal discharge, diaper rash, or inserted into the rectum 6.
• Stop hydrocortisone if condition worsens or persists beyond 7 days, or if rectal bleeding occurs 6.