Histo-topographic study of the longitudinal anal muscle

The longitudinal anal muscle (LAM) has been described as a layer of muscular tissue interposed between the external and internal anal sphincters but there is no general agreement in the literature on its attachments and constitution. The aim of the study was to investigate its topography for surgical purposes, with particular reference to its origin, insertion. After in situ formalin fixation, the pelvic viscera were removed from 8 male and 8 female cadavers (age range: 52-72 years). Serial macrosections of the bladder base, lower rectum and pelvic floor complex, cut into horizontal (6 cases) and coronal (6 cases) planes, underwent histological and immunohistochemical study. The remaining 4 specimens were plastinated. The LMA was identified in 10/12 of specimens (83%). In both coronal and transverse sections, it appeared as a layer of muscular tissue. From the anorectal junction it extends along the anal canal, receives fibres from the puborectalis and medial part of the pubococcygeus muscles, and terminates with fibro-elastic septa (7-9) which penetrate the external anal sphincter, reaching the deep part of the dermis. In the transverse plane, the mean thickness of the LAM was 1.63 ± 0.44 mm. Immunohistochemical staining showed that it consists predominantly of striated muscle fibres, with scarce smooth muscle fibres. Due to its attachments, the LMA may play a role in supporting and binding the internal and external sphincter complex together