The lumbar plexus showing the ilioinguinal, iliohypogastric, lateral femoral cutaneous, and genitofemoral nerve. (Reprinted with permission from Dr. Maria Fernanda Rojas)
The lumbar plexus is formed by the ventral rami of nerves L1 to L3 and majority of the ventral ramus of L4, with a contribution from the T12 (subcostal) nerve. Branches of the lumbar plexus include the iliohypogastric, ilioinguinal, and genitofemoral nerves, lateral femoral cutaneous nerve of the thigh, and femoral and obturator nerves (Fig. 6.1). The IH and II nerves arise as a single trunk from the ventral ramus of nerve L1. Either before or soon after emerging from the lateral border of the psoas major muscle, this single trunk divides into the IH and II nerves.
The IH nerve emerges along the upper lateral border of the psoas major and passes across the ventral surface of quadratus lumborum muscle, travelling in the fascia lumborum towards the iliac crest. Midway between the iliac crest and twelfth rib pierces the transversus abdominis muscle to lie between it and the internal oblique muscles (Fig. 6.2).The IH nerve then runs inferomedially, piercing the internal oblique muscle above the anterior superior iliac spine (ASIS). From this point, the nerve runs between the internal oblique and external oblique muscles, piercing the external oblique aponeurosis approximately 1 inch above the superficial inguinal ring.
The IL nerve is smaller and emerges along the lateral border of the psoas major, running a similar course that is more oblique and inferior to the IH nerve. It pierces the transversus abdominis and then the internal oblique at its lower border and enters the inguinal canal, emerging through the superficial inguinal ring anterior to the spermatic cord.
It supplies the skin of the posterolateral gluteal region and the pubic region over the lower region of the rectus abdominis muscles (Fig. 6.3). Throughout the course, it also supplies branches to the abdominal musculature.
The nerve provides sensory innervation to the skin over the root of the penis and the anterior surface of the scrotum in men (mons pubis and lateral labium majora in women) and superomedial thigh region, although the sensory innervation is variable. Throughout the course, it also supplies branches to the abdominal musculature.
The border nerves anatomic variations are well documented as one or more of the nerves are frequently absent in dissections. There are free communications between all these nerves and they innervate common dermatomes. The inguinal course of the II and IH may be consistent with the classic textbook description in only 42% of the dissections. Rab et al. delineate four different types of branching patterns in these nerves, and only 20% correspond to the normal distribution of the nerves. However, the location of II and IH nerves is quite consistently lateral and superior to the ASIS where the nerves are found between the transversus abdominis and internal oblique muscle layers (90%).
Most of the patients have a previous history of surgery and a small subset of patients with no history of trauma. Involvement of II or IH nerves causes pain in the lower abdominal wall, which sometimes radiates to the genital area. There are two patterns related to previous trauma: early and late. The early is self-explanatory. The late presentation may be related to reduced mobility of the nerves, which are used to glide between muscle and fascia planes with movement. With scarring, the mobility decreases which contributes to microtrauma of the nerves.
The diagnosis remains primarily clinical. There is no pathognomonic imaging and laboratory and electrophysiology investigation for inguinal neuralgia. The clinician has to differentiate this from the visceral pain that is deep seated and poorly localized. Sensory change with objective signs is sometimes present. Other physical signs include a discrete tender spot, abnormal pain sensitivity (allodynia and hyperalgesia), and Tinel’s sign.
The block serves both diagnostic and therapeutic roles. If the patient responds well to the analgesic injection with effects lasting for a few weeks, it is worthwhile to repeat the injections. Because of the potential systemic effects of steroids, it is prudent to avoid frequent repeated injections. The alternative options are pulsed radiofrequency, surgical nerve exploration and release (triple neurectomy), or implantation of peripheral nerve stimulator.
Probe: Linear 6–13 MHz