Internal anal sphincter (IAS)

The IAS is a 2 to 3 mm thick, 2.5 to 4 cm long, circular muscle composed of the distal long condensation of the inner circular muscle layer of the rectum. On physical examination the lower edges of the IAS about 1 cm distal to the dentate line.

External anal sphincter (EAS)

The EAS is a cylinder of striated muscle that envelopes the entire length of the inner tube of smooth muscles. The uppermost end of the external muscle fuses with the puborectalis part of the levator ani muscle. The EAS ends slightly distal to the IAS.

Conjoined longitudinal muscle

This muscle is composed of fanlike fibers that run through the internal anal sphincter, intersphinteric grove and also through the external anal sphincter. These fibers ultimately insert into the perianal skin. It is the distribution of this longitudinal fibers that make this complex of smooth and striated muscles a functionally solid unit, which is imperative in the defecation mechanism.

Topical nitrites

Topical nitrates work by relaxing the internal anal sphincter through the release of nitric oxide. The use of nitrates for the treatment of anal fissures was introduced in a preliminary investigation by Gorfine in 1995⁵. Nitric oxide causes smooth muscle relaxation by increasing cGMP levels via guanylyl cyclase. Exogenous nitric oxide donors, such as glyceryltrinitrate (GTN), cause reduction in sphincter spasm and show potential in healing chronic anal fissures. In a Cochrane review of including 18 randomized clinical trials (RCTs) of nonsurgical therapies for anal fissures, GTN was found to be marginally but significantly better than placebo in healing anal fissure (48.9% vs. 35.5%, p < 0.0009), but late recurrence of fissure was common, in the range of 50% of those initially cured⁶. One major side effect of the of nitrates use is severe headaches, that are disabling in over 20% of patients. It can be occasionally resolved by decreasing the amount of applied ointment, however it may require treatment termination.

Calcium channel blockers

Topical calcium channel blockers have been used as alternatives to GTN in the treatment of anal fissures. Their mechanism of action is facilitated by blocking L-type calcium channels in the internal anal sphincter muscle fibers, which cause muscle relaxation and decreased anal tone. Although oral formulations may be used, they can have adverse systemic side effects such as hypotension or orthostasis. For this reason, Nifedipine or Diltiazem are usually compounded into topical gel and are applied topically. A randomized study comparing Nifedipine to Glyceryl Nitrate in treatment of chronic anal fissure, concluded the Nifedipine was more effective at healing chronic anal fissure and had fewer side effects, however recurrences were frequent with both drugs⁷. The main side effect reported is itching, which is rare and rarely causes termination of treatment.

Botulinum toxin

Botulinum toxin (BT) is used to temporary paralyze the internal anal sphincter and subsequently improve blood flow to the anoderm. The toxin is produced by the anaerobic bacterium Clostridium Botulinum and works on the presynaptic nerve terminals at the neuromuscular junction and prevents the release of acetylcholine in synapses. The use of botulinum toxin as an effective treatment for anal fissures was first described by Jost⁸, with and the toxin injection reducing the anal resting pressure by 30%. The procedure is usually preformed as an outpatient procedure, under monitored anesthesia care (MAC) and its effect lasts for up to 3 months. In a meta-analysis of 6 RCTs, including 393 patients, 194 treated with BT and 199 with GTN, Sahebally et al. concluded that there was no statistically significant difference in fissure healing or recurrence between BT and GTN. BT was associated with a higher rate of transient anal incontinence (OR = 2.53, 95% CI 0.98-6.57, P = 0.06) but significantly fewer total side effects (OR = 0.12, 95% CI 0.02-0.63, P = 0.01) and headache (OR = 0.10, 95% CI 0.02-0.60, P = 0.01) compared with GTN⁹. There is a considerable variation among clinicians in the injection site and also in the amount of toxin used. Option for injection sites include into the internal sphincter, into the intersphinteric grove, into the fissure directly or on either side of the fissure. The amount of toxin used is typically 20 units, but some researches have shown higher success rates after injecting 40 units, with no increase in complications¹⁰. In a meta-analysis aimed to evaluate dose dependent efficacy, a total number of BT units per session ranged from 5 to 150 IU. The authors did not observe a dose-dependent efficiency, postoperative incontinence rate was not related to the BT dosage and also no difference in healing rate was observed in regard to the site and number of injections per session¹¹.

Open lateral internal sphincterotomy

This procedure is preformed while the patient is in either the lithotomy or prone jack knife positions. The authors prefer the latter as this allows better exposure. General or MAC anesthesia is administered and is supplemented by a local block with bupivacaine and epinephrine. The local block allows relaxation of the sphincter and facilitates surgery. An anal retractor is used and identification of internal sphincter and the intersphincteric groove is achieved. This is followed by an incision in the perianal skin overlying the intersphinteric groove. A fine instrument is then used and inserted into the intersphinteric plane and the internal sphincter is isolated up to the dentate line (Figure 3). The internal sphincter is then divided either with fine scissors or electrocautery. Classically, the muscle is divided up to the dentate line, but modified sphincterotomy is preformed by dividing the sphincter up to the level of the proximal extent of the fissure. The perianal incision is then closed with an absorbable suture.

Figure 3 Open Lateral sphincterotomy - exposure of muscle fibers. *Reprinted with permission, Cleveland Clinic Center for Medical Art & Photography ©2020. All Rights Reserved. 

Complications from internal sphincterotomy are rare and include pain, bleeding, urinary retention, abscess, and fistula formation and delayed wound healing. A much more worrisome complication is incontinence. The overall incidence of incontinence in a recent meta-analysis was reported at 3.4 to 4.4%¹². Surgical therapy, however, was associated with shorter time to symptomatic relief, higher rates of healing, lower rates of recurrence and with a success rate of over 95%.

Closed lateral internal sphincterotomy

With this technique, a narrow blade scalpel is inserted into the intersphinteric groove and advanced until the tip of the blade is up at the same level as the apex of the fissure (Figure 4). The tip is then turned to face the IAS, and muscle division is preformed during scalpel withdrawal. Several superficial passes may be needed to accomplish the needed muscle fiber transection. Due to the small size of the defect in the anoderm, suture closure is not needed. Complications rate are low and are comparable to those encountered with the open technique.

Figure 4 Closed lateral sphincterotomy. *Reprinted with permission, Cleveland Clinic Center for Medical Art & Photography ©2020. All Rights Re served.