Sixty-one studies were included in the review (49,568 patients, range eight to 20,545 unilateral patients). Study follow-up ranged from one to 36 years.
Incidence of metachronous contralateral inguinal hernias ranged from 1.1% to 38%, leading to a pooled incidence of 5.76% (95% CI 5.55% to 5.97%). The number needed to treat to prevent one metachronous contralateral inguinal hernia was 18 patients (95% CI 16.8 to 18).
Subgroup analysis indicated a non-significant trend for a slight increased risk of developing metachronous contralateral inguinal hernias in females compared with males. Incidence of metachronous contralateral inguinal hernias generally decreased with age; patients less than six months old had the greatest incidence (NNT=9, 95% CI 7.1 to 9.3). Patients with an initial left inguinal hernia repair were much more likely to develop a metachronous contralateral inguinal hernia than those with initial right inguinal hernia repair (NNT=9, 95% CI 7.8 to 8.8 versus NNT=15, 95% CI 13.7 to 15.6; p<0.0001). More than half of the metachronous contralateral inguinal hernias occurred within one year, although the proportions increased steadily over time and 86.2% occurred within four years.