The goal of this clinical trial is to evaluate the efficacy of botulinum toxin injection in treatment of patients with chronic anal fissures comparing between two doses of botulinum toxin injection focusing on healing rate, fissure pain, incontinence and return to daily activity. patients will be divided into two groups group A) will receive 50IU from Botulinium toxin type A in the form of two injections at 3 o'clock and 9 o'clock without any anasthesia. While group B) will receive low dose injection in the form of 20IU by the same technique.