Inguinal hernia repair is one of the most commonly performed surgical procedures. Nowadays, the most frequent hernia repair is in the outpatient setting, which requires the use of local anesthesia, and the most frequent postoperative complication is late pain in the inguinal region [10]. Failure to identify inguinal nerves during the surgery has been correlated with the higher incidence of postoperative pain [7].

In this review we included 26 studies with 5265 half-bodies examinations. Fourteen studies were performed during inguinal hernioplasty and 12 during cadaveric dissections. We analyzed the identification rates of the nerves at the inguinal canal. The IIN nerve-identification rate was evaluated in 20 studies and its presence was reported in the 84.6% of the dissections. The IHN identification rate was evaluated in 14 studies and its presence was reported in 74.2% of the half-bodies’ examinations. The identification rate of the GNF was evaluated in 14 studies and the presence of nerve was reported in 47.34% of the cases. The analysis of the identification rates show that it is not always possible to correctly locate all of inguinal nerves and that the nerve that is most difficult to locate is the GNF. Identification rates obtained in this study were lower than the rates reported by a narrative review performed on 13 studies: 96% for IIN, 94% for IHN and 90% for GNF [1]. In addition, the identification rate was higher in cadaveric studies (identification rate for IIN: 97.27%, for IHN: 97.8%, for GNF: 37.83%) than in inguinal hernioplasty studies (identification rate for IHN: 63.52%, for IIN: 82.43%, for GNF: 47.8%) for all the nerves. This suggests in both cases the difficulty of identification of the GNF and that the different techniques used in anatomical and surgical procedures provide different outcomes.

Moreover, the identification rates of nerves varied across different geographic regions. There was a relatively higher identification rate of nerves reported in Asian studies and North American studies. In South America, the identification rate was very high for GNF. The data were very heterogeneous in other regions. In Africa, there was a identification rate of zero for IHN and GNF. In Europe, the identification rate was 4.4% for GNF, 72.35% for IHN, and 81.9% for IIN. We suspect the heterogeneity of patients and settings may have resulted in much of the heterogeneity between studies.

Finally, we also found that the study size was a predictor of the identification rate. Larger studies tended to have lower identification rates. Large sample size study being correlated with outcomes is a phenomenon that Sterne et al. [13] hypothesize could be the result of “interventions being implemented less thoroughly in larger studies, resulting in smaller effect estimates compared with smaller studies”. The outlier study by Lange et al. [25] with a low nerve identification rate may be explained by the small sample size of patients included (n = 40). However, in this study, the authors used methods to increase reliability, with each identified nerve being photographed by the operating theatre nurse as proof which was rechecked by the surgeon and then reviewed by an anatomist.

In modern abdominal wall surgery, inguinal nerve identification plays an increasingly important role and represents a source of significant benefits. Nonetheless, inguinal nerve variants have always been a pitfall for surgeons and the fact that all structures cannot be located in all cases, as also this review demonstrated, has important repercussions for surgical practice. The European Hernia Society guidelines [10] recommend the identification of the three inguinal nerves (ilioinguinal, iliohypogastric and genital branch of the genitofemoral) for the reduction of late postoperative pain deriving from nerve injuries.

The first description of abdominal pain after inguinal surgery was reported as “genitofemoral causalgia” from Magee in 1942 [37]. Heise and Starling [38] described the chronic pain after hernioplasty treated with partial or total prosthesis removal as “mesh inguinodynia”. There are many controversies about the treatment of the identified inguinal nerves: Lichtenstein et al. [39] for example, proposed the preservation of the inguinal nerves after identification; other surgeons suggest the prophylactic neurectomy [40]. However, there is no evidence of the superiority of one of the two techniques in postoperative pain reduction.

Surgeons who mainly perform hernioplasty surgery have the best outcomes in terms of identifying nerve structures [41]. The success in surgical identification of the three nerves has been found to be largely associated with surgical skills [41], but sometimes some anatomical variations of the nerve topography makes the surgical identification difficult no matter the skill level, especially in the cases were some of inguinal nerves are not present. For these reasons, standardization of education and training in nerve identification in hernia surgery is needed [41, 42].

Our meta-analysis reported statistical analysis of nerve course variations to provide more reliable points of reference for safe and correct local anesthesia, that would allow lowering of the incidence of chronic postoperative inguinal pain. For most nerve length outcomes, there was a large and statistically significant amount of heterogeneity. The data on anatomical reference points were in contrast with the data reported in classic anatomical textbooks, such as Clinical Anatomy by Regions [43]. Those authors suggest performing an anesthetic block of IIN and IHN 2.5 cm above the anterior superior iliac spine on the spinoumbilical line [43]. To ensure proper identification of inguinal nerves, ultrasonographic confirmation of their location should be attempted [4, 9]. In cases of abnormal nerve courses, the successful application of blind anesthetic blocks may be impossible.