Sciatica: An Extremely Rare Complication of the Perianal Abscess

Background: Sciatica has been classically described as pain in the back and hip with radiation in the leg along the distribution of the sciatic nerve, secondary to compression or irritation of the sciatic nerve. Spinal abnormality being the most common etiology, is one of the most common indications for MRI of the lumbosacral spine. Here we describe imaging findings secondary to a supralevator perianal abscess causing irritation of the sciatic nerve, which was diagnosed on MRI of the lumbosacral spine.

Case report: A 47-year-old male patient presented to the emergency department with severe acute pain in the right hip and right leg which was aggravated by limb movement. Clinically, a possibility of sciatica was suggested and MRI of the lumbosacral spine was ordered. The MRI did not reveal any abnormality in the lumbosacral spine; however, on STIR coronal images, a right perianal abscess with air pockets was seen. The perianal abscess was extending above the levator ani muscle with and was seen tracking along the sciatic nerve, explaining pain along the distribution of the sciatic nerve. The abscess was surgically drained, followed by an antibiotic course. The patient was symptomatically better post-surgery. Post-operative scan done 3 days later revealed significant resolution of the infra- and supralevator perianal abscess. The patient was discharged from hospital on post-operative day 3 on oral antibiotics for 7 days. On 15^th post-operative day, the patient was clinically completely asymptomatic with good healing of the perianal surgical wound.

Conclusions: Extra-spinal causes are rare and most often overlooked in patients with sciatica. While assessing patients with sciatica, extra-spinal causes for the radiation of pain along the distribution of the sciatic nerve should always be looked for if abnormalities in the MRI of the lumbar spine are not found. Inclusion of STIR sequences in the imaging of the lumbosacral spine, more often than not, helps to identify the extra-spinal cause of sciatica when MRI of the lumbosacral spine does not reveal any abnormality.