Internal anal sphincter (IAS) achalasia is a clinical condition with presentation similar to Hirschsprung's disease, but with the presence of ganglion cells on rectal suction biopsy. The diagnosis is made by anorectal manometry, which demonstrates the absence of the rectosphincteric reflex on rectal balloon inflation. The IAS is regulated by several neurogenic mechanisms, and so its pathogenesis is thought to be multifactorial, including the absence of nitrergic innervations, defective innervation of the neuromuscular junction, and altered distribution of interstitial cells of Cajal. The recommended treatment of choice is posterior IAS myectomy. Recently, however, the use of intrasphincteric botulinum toxin has been used to treat this condition, but further long-term studies are needed to determine its effectiveness.