Anatomy

The lumbar plexus, composed of the ventral rami of the L1–L4 spinal nerve roots, travels into and is formed within the body of the psoas major muscle, coalescing to produce several prominent nerves. The obturator nerve (L2–L4) exits from the medial aspect of the psoas major and travels caudally, innervating the adductor muscles housed in the medial compartment of the thigh. The femoral nerve (L2–L4) emerges from the lateral aspect of the psoas major roughly at the level of the L4–L5 intervertebral disc and courses caudally in the groove between the psoas major and iliacus muscle, providing innervation to the latter as well as the muscles of the anterior compartment of the thigh, such as the sartorius and the quadriceps femoris.

The lumbosacral trunk (L4–L5) lies deep to the obturator nerve and joins the sacral plexus in the pelvis, forming the lumbosacral plexus. Although it is not technically part of the lumbar plexus proper, the lumbosacral trunk cannot be ignored, both because of its contribution from the L4 and L5 nerve roots and because of its proximity to the lumbar plexus. Two smaller nerves, the ilioinguinal (L1) and the iliohypogastric (L1), enter the abdominal cavity dorsal to the medial arcuate ligament and pass along the ventral surface of the quadratus lumborum muscle, eventually innervating the abdominal muscles and skin of the inguinal area. The genitofemoral nerve (L1–L2) perforates the ventral surface of the psoas major at the level of the L4 vertebra and travels caudally on its surface, splitting near its distal end into the genital branch, which supplies the cremaster muscle, and the femoral branch, which supplies the skin inferomedial to the inguinal ligament. Finally, the lateral cutaneous nerve of the thigh (L2–L3) courses inferolaterally on the ventral aspect of the iliacus muscle, eventually entering the thigh medial to the anterior superior iliac spine, supplying the skin on the ventrolateral surface of the thigh. ¹

13.3 Lesions of the Lumbar Plexus

Lesions involving the lumbar plexus include those arising from the plexus itself and those originating from adjacent structures (e.g., psoas muscle abscesses). These lesions can include compressive pathology such as neoplasms or can be non-compressive. Examples of the latter include neuropathy, infection, iatrogenic injury, and trauma.

13.3.1 Tumors

 Schwannoma

Schwannomas are a subclass of peripheral nerve sheath tumors that arise from the myelin-producing cells of the peripheral nervous system. Owing to their neural crest origin, they stain positively for S-100 immunohistochemically—an important diagnostic factor used for the histological differentiation of these tumors. Schwannomas also stain strongly with calretinin but weakly for CD34. ² They can be classified into those that occur sporadically (which are much more common) and those arising in conjunction with an underlying inherited condition, such as familial schwannomatosis and neurofibromatosis type II (NF-2). ³

Schwannomas are most commonly found in the head and neck region (e.g., bilateral vestibular schwannomas in NF-2), but they can also arise from the peripheral nerves of the mediastinum and the periphery and, less commonly, the nerves of the pelvis. ^{4,​ 5}

Rarely, these tumors have been reported to arise from within the body of the psoas major from the sheath of one of the nerves of the lumbar plexus, such as a femoral or obturator nerve. ^{6,​ 7,​ 8,​ 9} Although rare, these retroperitoneal schwannomas can grow large, possibly causing symptoms consistent with compression of the adjacent structures (e.g., urinary obstruction and hydroureteronephrosis). ⁴ Furthermore, patients with these lesions can present with chronic pelvic and abdominal pain. ^{6,​ 10} These tumors also present initially with pain that becomes worse with movement or with weight-bearing exercise, often progressing to neuropathic symptoms from the affected nerve itself (e.g., the femoral nerve). These symptoms can include paresthesia and neuralgia on the affected side as well as weakness of the muscles innervated by the nerves. ¹¹ Lesions can be treated conservatively through surveillance, or removed when symptoms develop.

 Familial Schwannomatosis

Familial schwannomatosis is extremely rare, characterized by multiple schwannomas in the absence of acoustic neuromas (▶ Fig. 13.1, ▶ Fig. 13.2, ▶ Fig. 13.3, ▶ Fig. 13.4). ^{12,​ 13} Genetically, individuals with familial schwannomatosis have mutations distinct from those found in NF-2, namely a novel germline mutation in SMARCB1. ¹³ Patients frequently present with symptoms similar to those of individuals with benign schwannomas, such as pelvic and abdominal pain, and occasionally neuropathic symptoms. Suspicion for this condition should be raised if there are multiple lesions without the (bilateral) vestibular schwannomas characteristically seen with NF-2. Magnetic resonance imaging (MRI) is frequently used to visualize the schwannomas, while surgical resection is the preferred method of treatment, although chronic pain is frequently reported even after surgical resection. ¹²

 Neurofibroma

Neurofibromas, like schwannomas, are tumors that arise from the peripheral nerve sheath (▶ Fig. 13.5, ▶ Fig. 13.6, ▶ Fig. 13.7, ▶ Fig. 13.8, ▶ Fig. 13.9). Like schwannomas, they also arise from the neural crest, so they too stain for S-100 immunohistochemically. ² Unlike schwannomas, however, they test positive for CD34 because they contain increased and varied intracellular material. On the other hand, they stain poorly for calretinin, a marker that is strongly positive in schwannomas. ^{2,​ 14} While most neurofibromas are frequently discovered as isolated tumors, they can also be associated with the autosomal dominant disorder neurofibromatosis type I (NF-1). ^{14,​ 15} They can occur in the head and neck (so-called diffuse neurofibromas) and in association with major nerve trunks, as in the case of plexiform neurofibromas.

In patients with NF-1, there are several reports of neurofibromas arising within the body of the psoas muscle, affecting the lumbar plexus, and those arising from the lumbosacral plexus in the pelvis. ^{16,​ 17} Owing to the size and location of neurofibromas within the pelvis and retroperitoneal space, symptoms can vary, but are similar in presentation to those of schwannomas (discussed above). These include nonspecific symptoms such as abdominal pain and more specific ones such compressive neuropathy of the lumbar plexus (most commonly the femoral nerve), and corresponding sequelae of weakness of the lower extremities with diminished reflexes and sensation. ¹⁸

While a thorough medical and family history can often point toward a diagnosis of neurofibroma, imaging modalities remain essential for detecting these lesions. They are often detected incidentally by computed tomography (CT), but when they are suspected, more advanced imaging modalities such as MRI are needed to visualize soft tissue and bony involvement. ^{19,​ 20,​ 21,​ 22,​ 23,​ 24}Additionally, individuals with substantial lumbosacral plexus involvement can show abnormalities on electromyography (EMG) of the anterior and medial compartment muscles.

Retroperitoneal plexiform neurofibromas characteristically appear as hypodense lesions in the retroperitoneal space on CT imaging, lacking symmetry. As with schwannomas, an increase in size in a relatively short time is worrying because it suggests potential malignancy. ²⁵ Treatment, as with schwannomas, is either continued surveillance or definitive surgical resection as appropriate.

 Malignant Peripheral Nerve Sheath Tumors (MPNST)

MPNSTs can arise from plexiform neurofibromas (most commonly associated with NF-1) or secondarily due to radiation exposure. ^{26,​ 27,​ 28,​ 29} These malignant tumors are frequently found in the trunk and distal appendages, and are often associated with major nerve trunks such as the lumbar plexus. ^{28,​ 30,​ 31} Diagnosis usually depends on biopsy, but imaging modalities such as positron emission tomography (PET) can be used to differentiate benign neurofibromas from MPNSTs ^{28,​ 32,​ 33} Treatment is often multifaceted and involves surgical resection or chemotherapy for nonresectable tumors. ^{28,​ 32,​ 34}

 Lymphoma

While very rare, lymphoma can also arise directly within the plexus, leading to a plexopathy, or can spread via direct compression from an enlarged lymph node. ³⁵ Spreading from the nodes, the disease can involve the muscle (e.g., psoas or iliacus). ³⁶ The symptomatology of these lesions will depend on the severity of involvement. Imaging (such as MRI with contrast) can be used to diagnose such lesions. Treatment often includes a diagnostic biopsy and nonoperative management such as chemotherapy.

 Other Malignant Tumors

Malignant tumors of three possible origins have been reported within the lumbar plexus. First, a primary tumor can spread from its original adjacent location and involve the plexus. ³⁷ Second, tumors can metastasize to the tissue surrounding the plexus. Third, they can metastasize directly to the plexus itself. These tumors include gastrointestinal, colorectal, and genitourinary carcinomas as well as a wide range of sarcomas. ³⁸ Although the grade and cell of origin of tumors can differ, pain has been reported as a most consistent symptom. ^{38,​ 39} Imaging findings of these malignant lesions are not always pathognomonic. While MRI is better at visualizing nerve and soft tissue involvement, CT is extremely useful for assessing osseous involvement. Even with contrast, these lesions can have a nonspecific appearance and cannot easily be distinguished from other pathologies in the region. ⁴⁰ Treatments are often multimodal and include radiation, chemotherapy, or surgery, although newer agents such as monoclonal antibodies have been used for specific conditions (e.g., giant cell tumors). Other lesions that arise in the region can include primary bone tumors such as sarcomas or chordomas.

13.3.2 Infection

Lumbar plexus pathologies can also arise from infection by organisms such as Staphylococcus aureus, Escherichia coli, Mycobacterium tuberculosis (TB), and a number of other less common etiologies. Compressive neuropathy of the lumbar plexus and its branches can result from infections of the psoas because the plexus is embedded within the psoas muscle. Infections of the psoas muscle can be primary or secondary. If secondary, they are frequently due to spread from the gastrointestinal or genitourinary systems, the colorectal area, or the spine. In fact, discitis osteomyelitis of the lumbar vertebrae is a potent cause of psoas muscle abscesses with lumbar plexus involvement and subsequent compression. The two most common etiologies of primary infection of the psoas muscle are S. aureus and E. coli, which are also the two most common infectious agents in the United States, ⁴¹ while TB remains the most common cause worldwide.

 Staphylococcus aureus and Escherichia coli

S. aureus is a gram-positive coccus, a frequent asymptomatic colonizer of human body surfaces. While many etiologies can cause psoas abscess formation (the pathogenesis of which is suspected to be related to hematological spread), the most common cause of primary psoas abscesses is S. aureus. ^{42,​ 43}

Classical physical symptoms of a psoas abscess, whether caused by S. aureus or E. coli, are fever, pain radiating to the flank (most common) and thigh, and pain upon movement of the psoas muscle (i.e., hip movement). ^{44,​ 45} However, patients can also present with more generalized symptoms such as a nausea, vomiting, and fatigue. Upon physical examination, specific signs can include a physical mass that is palpable in the region of the psoas muscle, or extreme pain upon flexion of the hip.

Laboratory values are frequently consistent with those of acute infection, including elevated CRP, ESR, and leukocytosis. ⁴⁵ Imaging modalities such as intravenous contrast-enhanced CT scans of the abdomen and pelvis are particularly useful for visualizing these abscesses. Following initial visualization, MRI can be used to visualize soft tissue and hence local involvement. ^{20,​ 21,​ 45,​ 46,​ 47} Definitive microbiological diagnosis involves obtaining a biopsy via needle aspiration or by surgery. For an uncomplicated abscess, percutaneous drainage with placement of a pigtail catheter is the preferred treatment option, along with long-term antibiotic therapy.

Other common causes of lumbar plexopathy are discitis and osteomyelitis. Bacteria can seed the intervertebral disc via hematogenous spread and then radiate outward into the vertebrae. ⁴⁸ Infection can also spread from contiguous structures (e.g., aorta or bowel) into the disc space and bony structures. ⁴⁹ Once an infection is established, it can spread into the adjacent soft tissue structures including the retroperitoneal space and psoas muscle, producing an abscess or posteriorly causing an epidural abscess compressing the spinal cord or cauda equina. ⁵⁰ The most common cause of discitis–osteomyelitis and subsequent abscess formation is S. aureus, while other etiological agents include Pseudomonas spp., Candida spp., group A and B Streptococcus spp., and Brucella spp. ^{51,​ 52} Patients with discitis–osteomyelitis frequently present with pain and fever, along with pain upon palpation of the overlying infection. As expected, they also have leukocytosis and elevated CRP and ESR values. Diagnosis is established once again by positive cultures obtained via needle biopsy or from surgical drainage of the abscess. Treatment is long-term antibiotic therapy or surgical debridement and fixation if conservative treatment has failed or symptoms have dramatically worsened with neurological deficits.

 Tuberculosis (TB)

While the infectious agents discussed above are frequently the cause of psoas muscle infection and discitis–osteomyelitis leading to lumbar plexopathy in the United States, M. tuberculosis continues to be the main cause of it worldwide. ⁴¹ M. tuberculosis is an acid-fast bacterium that causes tuberculosis, which manifests as a pulmonary tract infection in the vast majority of cases. However, in approximately 10%, there is widespread extrapulmonary involvement including lymph nodes, cardiac pleura, spinal column (Pott’s disease), central nervous system, and the abdomen and pelvis. ⁵³

TB infection of the lumbar plexus, while not common in the United States, is frequently cited in the literature. Nonspecific symptoms of TB infection include weight loss and night sweats. Presentations of TB tend to be significantly less specific than S. aureus infections. In individuals with suspected psoas muscle abscess, a positive acid-fast lung smear is diagnostic of infection. ⁵⁴ As with other infiltrative lesions, abscesses caused by TB can lead to lumbar plexopathy with ensuing neurological symptoms. A large abscess can be detected on CT ^{19,​ 20,​ 22,​ 23,​ 24} while MRI can be used to visualize soft tissue, nerve, and bony involvement in great detail. ^{20,​ 21,​ 46}

 Other

Other less common causes of psoas muscle abscess and subsequent lumbar plexopathy are Klebsiella pneumonia, especially in patients with diabetes. These infections can be accompanied by gas formation at the site of infection, which is linked with a high mortality rate (80%). ⁵⁵ There are rare reports of abscess due to Streptococcus pneumoniae, Streptococcus moniliformis, and Nocardia spp. ^{56,​ 57,​ 58} Additionally, Salmonella spp. and Candida spp. have been described as causes of psoas abscesses and subsequent lumbar plexopathy, although these are exceedingly rare. ^{43,​ 59,​ 60} As above, definitive diagnosis is obtained using needle biopsy of the lesion, with treatment frequently including percutaneous drainage and long-term antibiotic therapy.

13.4 Neuropathy And Plexopathy

Lumbar plexopathy can also arise due to idiopathic or autoimmune-mediated degenerative changes. Chronic inflammatory demyelinating polyneuropathy (CIDP) is one immune-mediated cause of lumbar plexus degeneration leading to neuropathy. Neuralgic amyotrophy, a rapidly progressing degenerative condition, can often present with mixed lumbar plexus and sacral plexus symptoms, with pain being reported in the distribution of both plexuses. Unlike neurogenic plexopathies of the brachial plexus, involvement of the lumbar and sacral plexuses are often reported as progressive in nature, with an initial acute pain course followed by months or occasionally years of diminished strength in the affected muscles. ⁶¹

Another common cause of lumbar plexopathy is diabetes mellitus (DM), called diabetic amyotrophy, which is thought to be caused by an accumulation of transient microvascular ischemic events sustained over a long period. Owing to the size and distribution of the femoral and obturator nerves in relation to the others within the plexus, these two are most commonly affected, and so in consequence are the muscles within their distribution. These include atrophy and weakness of the anterior and medial muscle compartments of the thigh, with the patellar reflex frequently absent. ⁶¹

13.5 Hematoma

The rich vascular supply around the lumbar plexus makes ischemia rather unlikely, but also makes the plexus particularly susceptible to compression due to hemorrhage. Hematomas in the area can have etiological beginnings in many places, such as retroperitoneal hemorrhage (to be discussed below, under iatrogenic pathologies ⁶²) and hematomas directly within the psoas and iliacus muscles, including those iatrogenically created through bleeds. ^{63,​ 64,​ 65} Ruptured aneurysms in and around the distribution of the internal iliac artery have also been reported to compress the lumbar plexus, leading to deficits. ⁶¹

Hematomas of the psoas and iliacus muscles can cause lumbar plexopathy, especially in individuals on anticoagulation therapy who fall or sustain trauma. These patients are more susceptible to rupturing of retroperitoneal blood vessels, especially those in the body of the psoas muscle, forming an iliopsoas hematoma. Bleeding within the body of the muscle can compress the lumbar plexus, most commonly affecting the femoral and obturator nerves, and can lead to initial symptoms of moderate to severe pain with pain upon extension of the hip. ^{66,​ 67} (The larger a hematoma becomes, the more likely it is to compress the femoral nerve and the rest of the plexus; smaller hematomas tend to be limited to femoral nerve compression. ^{64,​ 66}) The preferred imaging modality for initial visualization of the hematoma is CT, while MRI is preferred for visualizing bony and nerve involvement. ^{19,​ 20,​ 21,​ 22,​ 23,​ 24,​ 46} Therapeutic options include observation, reversal of anticoagulation, needle aspiration or placement of an in-situ drain to assist with evacuation of the degraded blood products.

13.6 Trauma

Traumatic causes of lumbar plexus injuries are very rare because the plexus is well protected. The most common causes of traumatic injury to the lumbar plexus involve motor vehicle and motorcycle accidents. ⁶⁸ Individuals with post-MVA/motorcycle-related lumbar plexus injuries are usually found to have varied degrees of functional deficit in both the femoral and obturator nerves, leading to weakness in the iliopsoas and quadriceps muscles. Additionally, pelvic fractures secondary to high speed trauma can cause lumbar or sacral plexopathy. ⁶⁶ While often demanding surgical intervention, they are frequently treated with conservative therapy. Unfortunately, because high-speed traumatic injuries frequently involve many organs, lumbar or sacral plexopathy can go undetected unless an additional investigation is conducted such as EMG, nerve conduction study (NCS), or advanced imaging such as MRI neurography.

13.7 Iatrogenic

A frequent cause of iatrogenic retroperitoneal hemorrhage is a pre-operative lumbar plexus block in the setting of anticoagulation therapy. ^{61,​ 69,​ 70} Symptoms include an inability to move muscles innervated by the lumbar plexus, but the deficits are most easily noticed in the distribution of the femoral nerve and present as inability or weakness in knee extension and hip flexion, and potentially sensory loss in the anterior thigh.

Another common and novel mode of iatrogenic injury to the lumbar plexus is during lateral trans-psoas lumbar interbody fusion (LLIF). (▶ Fig. 13.10) This technique is considered a minimally invasive alternative to the traditional posterior (PLIF) or anterior lumbar interbody fusion surgery (ALIF). A recent study found that roughly 23.8% of those undergoing LLIF reported post-operative plexopathies while only 15.8% of ALIF patients, 7.8% of PLIF patients, and 2% of TLIF patients reported the same symptoms. ⁷¹ Additional studies have demonstrated the benefit of continuous intraoperative EMG to monitor the integrity of the nerves of the lumbar plexus during the lateral approach to the lumbar spine. ⁷² Common symptoms following a lateral transpsoas procedure such as LLIF include anterior thigh numbness and hip flexion weakness, though the other branches of the lumbar plexus can also be injured during the approach. It is suggested that the etiology of injury is related to the retraction time of the psoas muscle as opposed to the placement of a tube through which LLIF is typically performed. ⁷³

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Lateral Femoral Cutaneous Nerve Ilioinguinal Nerve Obturator Nerve Anesthetic Blockade of the Femoral Nerve Lumbar Dorsal Root Ganglia Nerve Root Anomalies with Application to the Lumbar Plexus

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