In this retrospective analysis of 215 patients who underwent evaluation for fistula-in-ano, eleven patients (5%) had histopathological evidence of a specific aetiology. Complex fistula and presence of an abscess/ collection were found to be associated with a specific aetiology. Other factors such as type of fistula, level of internal opening, recurrence and presence of haemorrhoids were not associated with the diagnosis of a specific aetiology.
Fistula-in-ano is a common cause of perineal sepsis which can be very challenging to treat. Management of fistula-in-ano is complex and includes control of infection, assessment of the anatomy of the fistula tract by examination under anaesthesia and imaging followed by the definitive treatment [13]. Most fistulae-in-ano are idiopathic or cryptoglandular in origin which are usually simple, uncomplicated and respond well to treatment. Surgical treatment is the treatment of choice for idiopathic fistulae. However, surgery carries a risk of injury to the anal sphincter complex and can result in complications such as anal incontinence which can be extremely disturbing to the patient [14, 15].
However, fistulae occurring secondary to a specific causes is known to occur infrequently, which can be managed pharmacologically. These usually present as recurrent, complex fistulae-in-ano which are often difficult to manage. Some of the known specific causes are tuberculosis, Crohn’s disease, actinomycosis, chlamydia and malignancies which can present as recurrent fistulae-in-ano [16]. The incidence of TB and acquired immunodeficiency syndrome (AIDS) are on the rise especially in developing countries [17]. Therefore, excluding the possible specific cause is necessary in order to decide on the definitive treatment. Previous studies that have analysed the histopathological findings have been summarised in Table 2 [7–10].
Table 1
Associated factors of specific aetiology in anal fistulae
| Specific Cause | Odds ratio (95% CI) | P value |
Present | Absent |
N | % | N | % |
Age (Median/range) | 34(20–60) | 40(14–73) | - | 0.201 |
Sex | Male | 11 | 100% | 168 | 82.4% | - | 0.121 |
Female | 0 | 0% | 36 | 17.6% |
Complexity | Complex | 9 | 81.8% | 45 | 22.1% | 15 (3.3–76) | < 0.001 |
Simple | 2 | 18.2% | 159 | 77.9% |
Type of fistula tract | Superficial | 0 | 0.0% | 28 | 13.7% | - | 0.217 |
Intersphincteric | 1 | 9.1% | 50 | 24.5% |
Transphincteric | 9 | 81.8% | 121 | 59.3% |
Suprasphincteric | 1 | 9.1% | 4 | 2.0% |
Extrasphincteric | 0 | 0.0% | 1 | .5% |
Level of internal opening | Below the dentate line | 6 | 54.5% | 109 | 53.4% | - | 0.408 |
At the dentate line | 3 | 27.3% | 79 | 38.7% |
Above the dentate line | 1 | 9.1% | 11 | 5.4% |
Rectum | 1 | 9.1% | 3 | 1.5% |
Could not be located | 0 | 0.0% | 2 | 1.0% |
Recurrent Fistulae | Yes | 7 | 63.6% | 138 | 67.6% | 0.8 (0.2–2.9) | 0.782 |
No | 4 | 36.4% | 66 | 32.4% |
Abscess/Collections | Yes | 5 | 45.5% | 24 | 11.8% | 6.3(1.8–22) | 0.001 |
No | 6 | 54.5% | 180 | 88.2% |
Haemorrhoids | Yes | 1 | 9.1% | 23 | 11.3% | 0.78 (0.09–6.4) | 0.823 |
No | 10 | 90.9% | 181 | 88.7% |
Prevalence of TB in fistula-in-ano
In a study from Sri Lanka involving 84 patients, TB was confirmed in two (2.4%) patients. Both patients were suspected to have a specific causes clinically as they presented with recurrent fistulae with poor response to the surgical treatment [10]. In a prospective study of 96 patients from South Africa, 7 (7.3%) were found to have tuberculosis. Of those, none had systemic manifestations of TB and only one had some evidence of TB in the chest radiograph. [9]. Sainio et al conducted a study in Helsinki and noted that 0.2% of all fistulae are tuberculous in origin [8]. However, in that population based study, those with previous history of TB were not excluded from the analysis. In a study by Shukla et al from India, 122 cases of fistulae in-ano were analysed and a considerable proportion (15.6%) were diagnosed as tuberculosis [7]. Anorectal TB is usually associated with pulmonary TB but the occurrence of anorectal TB without pulmonary involvement in also reported in literature [18, 19]. In our study 3 patients (1.4%) were found to have anorectal TB. Of which 2 were complex fistulae and all three were recurrent fistulae. Furthermore, they did not have any other systemic features of TB and thus histology was useful to clinch the diagnosis.
Prevalence of Crohn’s disease in fistula-in-ano
Anorectal fistula is a known manifestation of Crohn’s disease and it usually occurs with other classical symptoms of Crohn’s disease. The presentation of perianal Crohn’s disease is variable as some (5%) may even develop anal fistulae prior to other manifestations of Crohn’s disease [19–21]. Although Crohn’s disease was previously regarded as a disease of the Western world, it is increasingly being reported in South Asia [22]. In a study by Sainio et al involving 458 patients with anal fistulae, 1.3% were secondary to Crohn’s disease [8]. Interestingly, a higher proportion (1.5%) were associated with ulcerative colitis. In our study, there was no anal fistulae associated with ulcerative colitis, although an increasing incidence of ulcerative colitis is noted in the South Asian region [23]. However, the study conducted by Sainio et al was a population based study, therefore it is possible that those with previous confirmed ulcerative colitis were also included in the analysis [8]. In a similar study from Sri Lanka including 84 patients, one patient (1.2%) was diagnosed of Crohn’s disease [10]. In the present study, 5 patients (2.3%) were found to have Crohn’s disease of which, four patients (80%) presented with a complex fistulae. However, the remaining patient presented with a non-recurrent simple fistula without any associated symptoms to suspect the diagnosis Crohn’s disease. This finding is clinically significant as it is evident that non-recurrent simple fistulae can also have a specific aetiology. Interestingly, two out of five patients did not have any other associated symptoms to suggest a possibility of Crohn’s disease at the time of presentation. However, they later manifested other symptoms of Crohn’s disease. Therefore, histological evaluation in these patients was helpful to diagnose Crohn’s disease early.
Malignancy associated with a recurrent anal fistula is a rare entity and it is reported to occur in long standing fistulae [24]. In our study, one patient was found to have adenocarcinoma with mucinous differentiation. The patient was a 57-year-old male with a history of long standing fistula for four years. There was an induration in association with the fistula on rectal examination and thus, the history was suggestive. Other studies summarised in table 1 did not find an associated malignant histological finding.
Although a few studies have analysed the histological findings in fistula-in-ano (Table 2), the associated factors of fistulae with a specific aetiology was not previously studied. In this study, we have shown that in few patients, histology was useful in detecting the specific aetiology in the absence of clinical features. This was not reported in previous studies.
Table 2: Summary of previous studies comparing histological findings of anal fistula
Author | Year | Country | Sample | Study type | TB | Crohn’s | Malignancy | Others |
Sainio | 1983 | Finland | 458 | Population based study | 1 (0.2%) | 6 (1.30%) | None | UC: 7(1.5%) |
Shukla | 1988 | India | 122 | Retrospective | 19 (15.6%) | None | None | None |
Stupart | 2009 | South Africa | 96 | Prospective | 7 (7.3%) | None | None | None |
Wijekoon | 2010 | Sri Lanka | 84 | Retrospective | 2 (2.4%) | 1 (1.2%) | None | None |
Present study | Sri Lanka | 215 | Retrospective | 3 (1.4%) | 5 (2.3%) | 1(0.5%) | NCG: 2 (0.9%) |
UC: Ulcerative colitis, NCG: Non-caseating granuloma |