Examination of the anal canal is important to evaluate several common patient complaints relating to the anus including bleeding, itching, and pain.1,2 It is possible to examine parts of this area with flexible instruments or a rigid rectosigmoidoscope. The only method that will give a consistent clear view of the anal canal is anoscopy.3 To properly perform anoscopy, it is necessary to thoroughly understand the anatomy of the region, be aware of the possible causes of the symptoms evaluated, use the appropriate equipment, and position the patient correctly.
ANATOMY AND PATHOPHYSIOLOGY
It is necessary to understand the anatomy of the anal canal in order to evaluate the patient’s signs and symptoms properly.4 The anal canal is completely extraperitoneal and is approximately 2.5 to 4 cm long. The anatomy can be divided into topical anatomy and major supporting structures.5 The topical anatomy is depicted in Figure 88-1.
The topical anatomy of the anal canal.
Perineal skin covers the perineum, is fully innervated, and includes both hair follicles and apocrine glands. It can be grossly distinguished from the anoderm surrounding the anal canal by the visible hair. The anoderm is specialized squamous epithelium that lines the majority of the anal canal. It is fully innervated but does not have apocrine glands or hair follicles. This epithelium is very thin and elastic, and if it is destroyed by surgery or infection, stricture formation during healing may occur.
Looking into the anal canal, the anoderm can be seen to end in an irregular line called the dentate line (Figure 88-1). This is a demarcation of anoderm to transition zone mucosa. The anal canal proximal to the dentate line is lined with columnar epithelium. It has no cutaneous sensation and is insensitive to cutting. This allows minor therapeutic procedures (e.g., banding or suture ligation) to be done without an anesthetic agent. It is also the reason internal hemorrhoids usually do not cause pain. The transition zone continues proximally for a variable length of 6 to 12 mm before it becomes the rectal mucosa (Figure 88-1). The junction of the transitional zone with the rectal mucosa is not visible to the naked eye. The rectal mucosa decreases in diameter in the area of the transitional zone. The mucosa appears to be bunched together in columns called the columns of Morgagni at the level of the dentate line (Figures 88-1 and 88-2). Crypts are formed between the columns as the transitional zone becomes the dentate line. Under the anoderm in the crypts are multiple anal glands. Blockage of the anal glands by foreign material leads to infection. Blockage or primary infection of the glands causes the majority of abscesses that arise around the anus. The crypts ...