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Just as the pores on one's face can become blocked and cause acne, the glands in the anus can also become infected, causing painful pus-filled eruptions on the skin.
Anal glands produce secretions that lubricate the anal canal for the smooth passage of stool. A gland can be clogged by faecal matter and infected by bacteria to form an anal abscess full of pus.
The abscess can become an anal fistula, or abnormal tract out of the body, if it forms in a deeper, intermuscular gland.
Most glands are not so deep, but are located in the submucosa, a spongy layer of tissue between the lining of the anal canal and the muscle layers, said Dr Lim Jit Fong, a senior consultant surgeon at Fortis Colorectal Hospital.
When infection occurs in a submucous gland, the resulting abscess bursts through the mucosa into the anal canal and heals spontaneously, without the patient even realising it, he said.
But 10 per cent of anal glands lie deeper, piercing the muscular layers of the anal canal.
An infection in these could track in several directions, following the fibres of the internal anal sphincter muscles that open and close the anus, said Dr Francis Seow-Choen, medical director of Fortis Colorectal Hospital.
The fistula follows the path of least resistance out to the skin of the buttocks next to the anus, appearing as a small, raised bump discharging yellow or blood-stained pus. This pimple-like swelling causes pain intermittently. It can also burst to become an open wound.
Dr Dean Koh, a senior consultant surgeon at Mount Elizabeth Novena Specialist Centre, tells his patients who have had an anal abscess surgically drained that they face a 40 to 50 per cent chance of developing an anal fistula.
This is because both conditions are triggered by infections in the anal glands which have no known risk factors, nor any way of preventing them.
In nine out of 10 cases, an anal fistula develops after an anal abscess bursts or does not completely resolve.
The rest of the time, the fistula usually has a secondary cause from digestive disorders such as Crohn's disease and inflammatory bowel disease, or from infections such as tuberculosis or human immunodeficiency virus, said DrYang Ching Yu, medical director and general surgeon at Raffles Hospital.
It is not known what the prevalence is here, but large-scale overseas studies show the condition affects six per 100,000 women and 12 per 100,000 men. Most patients are around 30 years of age.
Dr Charles Tsang, a senior consultant surgeon at Mount Elizabeth Novena Specialist Centre, believes the protective effects of oestrogen may be a reason for fewer women than men developing anal fistulas.
Colorectal surgeons said anal fistulas are the second most common non-cancerous condition affecting the anal canal they see, after haemorrhoids (piles), which are swollen veins in the anal canal.
Patients sometimes mistake anal fistulas for piles, said Dr Eu Kong Weng, senior consultant surgeon and chief of Pacific Surgical and Colorectal Centre.
Living with an anal fistula is usually not life-threatening, unless there is an acute infection and the bacteria enters the bloodstream to cause a more severe infection that gives rise to fever and chills, he added.
But the condition can be debilitating, warned Dr Bettina Lieske, an associate consultant at the division of colorectal surgery at the National University Hospital.
Bacteria from the anal canal keep entering the internal opening of the tract to cause a chronic phase of anorectal infection, which causes great discomfort.
She added that a simple fistula can worsen over time if left untreated. The original tract may branch out to other pathways to result in a complex fistula, where there may be two to three different bumps on the skin near the anus.
Dr Quah Hak Mien, a colorectal surgeon at Gleneagles Medical Centre, warned that an anal fistula that is untreated for more than 10 years puts one at increased risk of glandular cancer.
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