Nerve Entrapment Syndromes

Abstract

Painful conditions affecting the innervation of the groin (ilioinguinal, iliohypogastric genitofemoral, lateral femoral cutaneous, and obturator nerves) are frequent and often follow surgical interventions on the inguinal region. Although referred to in general as “entrapment neuropathies,” these conditions are often a reflection of damage to the nerves (or their smaller branches) rather than a true entrapment. The differential diagnosis is crucially important, since pain from an entrapped nerve is treated very differently from pain from a “damaged” nerve. The different conditions would be best described as “inguinal entrapment neuropathy” and “inguinal neuralgia.”

In mild or time-limited conditions, this differential might not be crucial. Chronic severe cases might represent a combination of true entrapment syndromes overlapping with “neuropathic” pain syndromes. These cases, although fortunately uncommon, can represent a formidable diagnostic and therapeutic challenge. In many of these instances, excessively aggressive treatments can actually worsen the conditions. Surgical re-explorations, decompressions, neurectomies, and neurostimulation procedures have a role in their management.

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1 Anatomy

The main nerves innervating the inguinal/groin region include the ilioinguinal, iliohypogastric, genitofemoral, and obturator nerves. Globally they can be referred to as the “inguinal nerves.”

1.1 Ilioinguinal and Iliohypogastric Nerve

The ilioinguinal and iliohypogastric nerves sometime arise as a common trunk and, in their course, usually separate between the transversus and internal oblique muscles.

Iliohypogastric nerve. The iliohypogastric nerve arises primarily from the ventral primary rami of L1 and occasionally with a twig from T12. This nerve has a pathway similar to that of the intercostal nerves in the thoracic region. The iliohypogastric nerve traverses the psoas major muscle, piercing the lateral border of the muscle anterior to the quadratus lumborum muscle and posterior to the kidney to traverse the lateral abdominal wall. Superomedial to the anterior superior iliac spine, the iliohypogastric and ilioinguinal nerves pierce the transversus abdominis to lie between it and the internal oblique muscles. After traveling a short distance inferomedially, their ventral rami pierce the internal oblique to lie between the internal and external oblique muscles before giving off branches, which pierce the external oblique to provide cutaneous sensation. The iliohypogastric nerve supplies the skin over the inguinal region as well as a small region just superior to the pubis.

Ilioinguinal nerve. The ilioinguinal nerves emanate from the first lumbar spinal root. In one study of 200 human bodies, the ilioinguinal nerve arose from the lumbar plexus in 72.5% and by a common trunk with the iliohypogastric nerve in 25%; it was absent in 2.5% (Fig. 17.1). The ilioinguinal nerve was formed from one root in 92.5% and from two roots in about 5% of cases. In 86%, the ilioinguinal carried fibers from one spinal nerve (primarily from L1) and, in 11%, from two spinal nerves (T12, L1; L1, L2; or L2, L3) [1].

Fig. 17.1

Origin of “inguinal nerves” from the lumbar plexus

Within the inguinal canal, the nerve usually lies ventral to the spermatic cord (60% of cases), but it may lie beneath (dorsal) the cord and/or within it. The ilioinguinal nerve runs anteroinferiorly to the superficial inguinal ring, where it emerges to supply the skin on the superomedial aspect of the thigh. The ilioinguinal nerve, usually smaller than the iliohypogastric nerve, arises with it from the first lumbar nerve. It emerges from the lateral border of the psoas major just below the iliohypogastric and, passing obliquely across the quadratus lumborum and iliacus, perforates the transversus abdominis, near the anterior part of the iliac crest, and communicates with the iliohypogastric nerve between the transversus and the obliquus internus. The nerve then pierces the obliquus internus and, accompanying the spermatic cord through the subcutaneous inguinal ring, is distributed to the skin of the upper and medial part of the thigh, to the skin over the root of the penis and upper part of the scrotum in the male; in the female it provides sensory innervation to the skin covering the mons pubis and labium majus in the female (Fig. 17.2). The size of this nerve is inversely proportional to that of the iliohypogastric nerve. Occasionally, it is very small and ends by joining the iliohypogastric nerve; in these instances, a branch from the iliohypogastric might take the place of the ilioinguinal, or the latter nerve may be altogether absent. The ilioinguinal nerve may partially or completely replace the genital branch of the genitofemoral nerve or the lateral femoral cutaneous nerve.

Fig. 17.2

Cutaneous distribution of the branches of the “inguinal nerves”

Of note, the ventral rami of the lower intercostal nerves (T11 and T12) also pierce the transversus abdominis muscle to lie between it and the internal oblique. These latter nerves also supply sensation to the inferior abdominal wall.

1.2 Genitofemoral Nerve

The genitofemoral nerve refers to a human nerve that is found in the abdomen. Its branches, the genital branch and femoral branch, supply sensation to the upper anterior thigh, as well as the skin of the anterior scrotum in males and mons pubis in females. The femoral branch is different from the femoral nerve, which also arises from the lumbar plexus. The genitofemoral nerve originates from the upper L1-2 segments of the lumbar plexus. It passes downward and emerges from the anterior surface of the psoas major muscle. The nerve continues downward and divides into two branches, the genital branch and the femoral branch. The genital branch passes through the deep inguinal ring and enters the inguinal canal. In men, the genital branch continues down and supplies the scrotal skin. In women, the genital branch accompanies the round ligament of uterus, terminating in the skin of the mons pubis and labia majora.

The femoral branch passes underneath the inguinal ligament, traveling adjacent to the external iliac artery and supplying the skin of the upper, anterior thigh.

1.3 Obturator Nerve

The obturator nerve originates from the ventral divisions of the ventral rami of the L2 through L4 spinal nerves within the psoas major muscle. The obturator nerve descends through the psoas muscle to emerge from its medial border at the pelvic brim. The nerve then curves downward and forward, around the wall of the pelvic cavity, and travels through the obturator foramen in which it divides into the anterior and posterior branches.

1.4 Lateral Femoral Cutaneous Nerve

The lateral femoral cutaneous nerve is a branch of the lumbar plexus, exiting the spinal cord between the L2 and L3 vertebrae. It emerges at the lateral edge of the psoas muscle group, below the ilioinguinal nerve, and then passes beneath the iliac fascia and the inguinal ligament. It divides into two branches—anterior and posterior—8–10 cm below the spine, where it also emerges from the fascia lata. The anterior branch innervates the skin of the anterior and lateral regions of the thigh to the knee; the posterior branch supplies the lateral portion of the thigh, from the greater trochanter in the hip to mid-thigh, just above the knee.

Rab et al. [2] conducted an anatomical study of the groin nerves in 64 halves of 32 human embalmed anatomic specimens. Four different patterns of cutaneous branching were identified: type A, with a dominance of genitofemoral nerve in the scrotal/labial and the ventromedial thigh region. In type A, the ilioinguinal nerve gives no sensory contribution to these regions (43.7%). In type B, with a dominance of ilioinguinal nerve, the genitofemoral nerve shared a branch with the ilioinguinal and gave motor fibers to cremaster muscle in the inguinal canal but had no sensory branch to the groin (28.1%). In type C, with the genitofemoral nerve being dominant, the ilioinguinal nerve had sensory branches to the mons pubis and inguinal crease together with an anteroproximal part of the root of the penis or labia majora. In type D, cutaneous branches stemmed from both the ilioinguinal and the genitofemoral nerves. In addition, the ilioinguinal nerve provided innervation to the mons pubis and inguinal crease together with a very anteroproximal part of the root of the penis or labia majora (7.8%). These patterns of innervation were bilaterally symmetric in 40.6% of the cadavers.

Al-dabbagh [3] conducted a study in a consecutive series of 110 primary inguinal hernias repaired by the mesh technique. Particular attention was paid to early identification and recording of the course of both the ilioinguinal and iliohypogastric nerves and preserving them throughout the operative procedure. The course of both nerves was found to be consistent with that described in anatomical texts in only 41.8% of the surgical explorations. The course of one or both nerves was found to be a variant in the other 64 of 110 (58.2%) explorations and often rendered them susceptible to injury.

2 Clinical

It is important to recognize the difference between conditions where the nerve is subjected to persistent compression (which can eventually lead to permanent damage) versus condition where the nerve has been intrinsically damaged (and the damage can be temporary or permanent, partial or complete). Sometimes this differential diagnosis is feasible; in other instances, it might be extremely difficult to differentiate the two conditions, particularly in situations where both might be coexisting (nerve damage and ongoing entrapment).

The different conditions would be best described as “inguinal entrapment neuropathy” and “inguinal neuralgia.” The symptomatology may start in the exact distribution of one of the “inguinal region” nerves but then spread beyond their territory, including, at times, the abdomen or the whole thigh. Since there is significant overlap in the distribution of the groin nerves, the exact contribution of the individual nerves to the pain might not be clear in advanced cases.

Entrapment occurs where the nerve passes through the muscles of the abdominal wall medial to the anterior superior iliac spine. The most frequent cause of ilioinguinal neuropathy is inadvertent damage during herniorrhaphies and, less commonly, appendectomies [4, 5]. Other operations that may be associated with this neuropathy include tubal ligation, hysterectomy, and cesarean section. Other pathological conditions in the inguinal canal that can involve either the ilioinguinal or genitofemoral nerves are lipomas, leiomyomas, and endometriosis. Injuries to the iliohypogastric nerve mainly occur if the incision extends beyond the lateral margin of the inferior rectus abdominis fibers. Nerve(s) damage can result from direct surgical trauma, such as passing a suture around the nerve and incorporating it into the fascial repair, or postoperative entrapment in scar tissue or neuroma formation. Sports injuries, such as trauma or muscle tears of the lower abdominal muscles, may also result in injury to the nerves [6]. Injury to the iliohypogastric nerve may also occur during pregnancy, owing to the rapidly expanding abdomen in the third trimester. This is called the idiopathic iliohypogastric syndrome and is rare.

Neuralgia symptoms include burning or lancinating pain immediately following the abdominal operation. The pain extends from the surgical incision laterally into the inguinal region and suprapubic region.

Pain, numbness, and paresthesiae secondary to damage to the ilioinguinal nerve involve the upper media thigh, the base of the penis and scrotum in men, and the labia majora and mons pubis in women. Discomfort may occur immediately or up to several years after the procedure and may last for months to years. The pain could be permanent. This discomfort is possibly because of the formation of scar tissue in the region. Occasionally, the pain may extend into the genitalia because of significant overlap with other cutaneous nerves.

Rarely, ilioinguinal neuralgia will occur spontaneously. Many patients with an ilioinguinal neuropathy notice minimal numbness in the distribution of the nerve that resolves during days or weeks, but others complain of more severe pain. This syndrome has been called post-herniorrhaphy neuralgia, inguinal neuralgia, and genitofemoral and ilioinguinal neuralgia. It might be preferable to refer to it as inguinal neuralgia because a specific nerve is not implicated. Ilioinguinal neuralgia is one of the most common causes of lower abdominal and pelvic pain encountered in clinical practice. These patients have severe burning and stabbing pain in the lower abdomen, inguinal area, and upper thigh often aggravated by changing position and walking. On exam, if the neuropathy has been of sufficient intensity for a long enough period, there will be an alteration of sensation in this cutaneous zone. This could result in hypoesthesia as well as hyperesthesia. In extreme cases, there is allodynia, which consists of extreme pain even to light touch. Tinel’s sign may be elicited by tapping over the ilioinguinal nerve at the point at which it pierces the transverse abdominal muscle. The pain of ilioinguinal neuralgia is made worse by extension of the lumbar spine, which puts traction on the nerve. Patients suffering from ilioinguinal neuralgia will often assume a bent-forward novice skier’s position. If the condition remains untreated, progressive motor deficit consisting of bulging of the anterior abdominal wall muscles may occur. This bulging may be confused with inguinal hernia.

The symptoms in genitofemoral neuropathy/algia usually consist of an unpleasant painful feeling in the lower abdomen and groin, with pain radiating to the inner side of the upper leg, scrotum, or greater labia.

Injuries to the obturator nerve are uncommon. Pelvic fractures are an obvious cause. The obturator nerve can be damaged due to hip surgery or by pelvic malignant neoplasms or hernias, foci of endometriosis. Pain typically is localized to the adductor origin at the pubic bone [7].

3 Management

Initial treatment of inguinal neuropathy/algia should consist of simple analgesics, nonsteroidal anti-inflammatory drugs, membrane-stabilizing medications (carbamazepine, gabapentin, pregabalin), and/or tricyclic antidepressant drugs. Opioid medications are seldom indicated at this stage. Avoidance of repetitive activities thought to exacerbate the symptoms of ilioinguinal neuralgia (e.g., squatting or sitting for prolonged periods) will also help ameliorate the patient’s symptoms.

If the patient fails to respond to these conservative measures, a next reasonable step is nerve blockade with local anesthetic and steroid.

Because of the overlapping innervation of the ilioinguinal and iliohypogastric nerve, it is not unusual to block branches of each nerve when performing ilioinguinal nerve block. The clinician should be aware that due to the anatomy of the ilioinguinal nerve, damage to or entrapment of the nerve anywhere along its course can produce a similar clinical syndrome.

For patients who do not rapidly respond to nerve(s) block, or whose pain has spread beyond the mere distribution of an individual nerve, consideration should be given to epidural steroid injection of the T12-L1 segments intraspinally.

Repeated nerve blocks can be given, and sometimes they can result in cure from the condition.

If nerve blocks are effective but their efficacy is short-lived, pulsed radiofrequency lesioning (PRF) of the involved nerves might obtain long-term results [8]. Werner et al. performed a systematic review of the available data on PRF ablation in the treatment of chronic post-herniorrhaphy inguinal pain [9]. They report that there is a limited level of evidence to support the use of PRF ablation in the management of this condition, stating that the evidence is of low quality and the strength of recommendation is weak to moderate.

If the above treatments have not resulted in any improvement, a reassessment of the situation is in order.

Namely, one has to decide whether this is a nociceptive, curable pain, or whether this is a permanent neuropathic pain (Fig. 17.3). Persistent nociceptive pain is a reflection of a nerve still affected by ongoing damage by pressure or constriction (scar tissue, mesh, suture, etc.) or whether this constitutes a “neuropathic” pain, a reflection of an intrinsically permanently and irreversibly damaged nerve. To make matters more complicated, when “neuropathic” pain persists for a long time, it might become “centralized,” which means that alternative pain-generating/self-maintaining circuits originate in the central nervous system. To complicate the situation even further, sometimes both nociceptive and neuropathic pain can be present concurrently, and sometimes they can be very difficult to be teased apart clinically. In the author’s experience, three clinical signs/symptoms are almost always indicative of neuropathic pain. The first is the fact that the pain is usually present 24 h/day. In most instances of nociceptive pain, the pain is greatly lessened by rest. Secondly, neuropathic pain is usually described as a burning sensation. Thirdly, the presence of allodynia (pain even to light touch) is almost always associated with neurogenic pain. Therefore, if a patient is complaining of constant burning pain and displays allodynia, it is safe to assume that at least a component of the pain is neuropathic and most likely “centralized.”

Fig. 17.3

Neuropathic vs. nociceptive pain. Diagnostic and therapeutic considerations

The differential diagnosis is important, since the goals of treatment are drastically different. In the presence of nociceptive pain, one should try to “cure” the condition. Neuropathic pain cannot be cured, but only managed.

If one is aiming for the treatment of nociceptive pain, surgical intervention might be indicated. Different surgical approaches can be considered.

A decompression of a possibly entrapped nerve can be performed. This might entail lysis of adhesions or take down of scar tissue and/or removal of implanted hardware such a mesh or retained sutures. Aasvang and Kehlet [10] performed a literature review for data on surgical treatment of chronic pain after hernia repair. In some patients, pain may have been of inflammatory or nerve entrapment origin from the mesh, suggesting the need for mesh removal or nerve decompression. However, there were no data on how to diagnose these conditions and few on the success of mesh removal; only four patients with mesh removal alone were reported. Lee and Dellon [11] reported on their surgical management of 54 patients with groin pain history and physical examination were sufficient to relate the pain to one or more of the lateral femoral cutaneous, ilioinguinal, iliohypogastric, or genitofemoral nerves. In their series, neurolysis was performed in all instances of lateral femoral cutaneous nerve entrapment. Ninety percent of those patients experienced good to excellent results. Entrapments of the other three nerves were addressed by resection. Pain related to the genitofemoral nerve had the worst outcome.

Often a resection or sectioning of the nerve(s) is performed [12, 13]. Some authors have reported excellent results after sectioning of the three nerves involved (ilioinguinal, iliohypogastric, and genitofemoral nerves), called the “triple neurectomy” procedure [14,15,16]. This can be performed either laparoscopically or via an open technique. Madura et al. [17] reported on a consecutive series of 100 patients who underwent inguinal neurectomy for inguinal nerve entrapment. Seventy-two percent of the patients obtained good long-term relief. The authors emphasized the need to address the distal end of the resected nerves, in order to prevent formation of painful neuromas. These include burying the cut end of the nerve into muscle, end-to-side nerve anastomosis, and epineural ligation and flap. Experimentally, treatment of the nerve end with electro-fulguration, YAG laser destruction, and tissue bioglues also is reported to prevent neuroma formation.

One should beware of the fact that, if the original pain is exquisitely neuropathic (defined as excruciating constant burning pain and allodynia), these surgical ablative approaches can result in a severe (at times permanent) worsening of the pain.

In the author’s experience, if the pain is clearly neuropathic, the condition is a reflection of an “electrical” problem within the nervous system. In these instances, electrical stimulation through an implantable neurostimulation device can often be very effective in reducing the pain signals and can actually represent the most effective treatment option [18,19,20,21,22,23]. Certainly, it is the most reversible surgical modality and the one that would cause the least irreversible damage to the nervous system. Electrical signals are delivered through implanted electrodes. The electrodes can be implanted on the peripheral nerves, on the small subcutaneous nerve fibers of the affected painful area, or on the nerve roots or dorsal root ganglia (DRG) serving that area (usually T12-L1, Figs. 17.4, 17.5, and 17.6). The stimulation can be delivered as a test trial on a temporary basis. The electrodes are usually placed for about 7–15 days and then removed. If the patient experienced good pain relief, a complete neurostimulation system can then be implanted. This usually consists of a pulse generator and one or more electrodes. The pulse generator, very much like a pacemaker, contains a lithium battery (which can be externally rechargeable) and the electronic circuitry. The parameters of stimulation (polarity, intensity, frequency, waveform, etc.) can be modified externally via a hand-held wireless controller.

Fig. 17.4

Intraoperative images of insertion of two percutaneous leads over the ilioinguinal and iliohypogastric nerve distribution for a temporary test trial

Fig. 17.5

X-rays of implanted peripheral nerve stimulation systems for inguinal pain

Fig. 17.6

X-rays of implanted T12-L1 dorsal nerve root stimulation systems for inguinal pain

Since the neurostimulation modality can be tested in a reversible and non-damaging manner, it should be the preferred modality in cases of severe persistent pain where it is not clear of the relative contribution of nociceptive vs. neuropathic pain.

Neurostimulation can be effective for a very long time, and the author has a number of patients that were implanted 30 years ago and still maintain excellent pain relief.

Patients with chronic severe pain often develop severe depression, anxiety, and other mood disturbances. Psychological and psychosocial intervention is often of crucial importance in the support and management of these unfortunate individuals.

Conclusions

Persistent neuropathies of the nervous structures innervating the groin can present a formidable challenge. The clinical spectrum can vary from some minimal transient numbness and discomfort to life-altering permanent excruciating pain conditions. Often the exact mechanisms and contribution of different nervous structures are ill-defined. In mild cases, a stepwise approach as described above is usually successful in providing meaningful control over the condition, which is often self-limited. In severe and chronic cases, the physician must be aware of the presence of neuropathic pain, which can make all the efforts to “cure” the condition doomed to fail. In those instances, membrane-stabilizing drugs and/or neurostimulation can provide meaningful, long-lasting relief. Neurectomy, while effective in the management of chronic nerve entrapment pain, is usually not effective in cases with “centralized” neuropathic pain.