Neuromuscular Disease and Presentation Involving the Lower Extremity
PETER Z. YAN
ALYSSA G. REHM
CAROLINE MIRANDA
DEXTER Y. SUN
This chapter provides an overview of neuromuscular diseases in the lower extremities. As entire books have been written on the subject, the goal is to provide the reader with a pertinent rather than a comprehensive review of neuromuscular disease in the lower extremities. For completeness, the chapter begins with an anatomical description of the lumbar spine and cauda equina followed by selected common complications in this region, which while not part of the lower limbs per se, produce significant neurologic deficits affecting the lower extremities. The remainder of the chapter is organized anatomically, rostral to caudal, beginning with the lumbosacral plexus, followed by thigh, leg, ankle, and foot, and ending with peripheral neuropathies. Each section begins with an anatomic description, followed by an approach to diagnosis, and ends with a discussion of specific etiologies.
The Lumbar Spine and Cauda Equina
Lumbar Spine Anatomy
Bone and Soft Tissue
The lumbar spine exhibits a normal lordosis starting at L1. As the patient bends forward, reducing the lordosis, the spinal canal straightens, giving more room to the cord, often alleviating symptoms in patients with lumbar spinal stenosis.
The vertebral body sits anterior to the spinal cord, supported by an intervertebral disc consisting of a soft nucleus pulposus and a thick annulus fibrosis. The disc slowly desiccates over time, leading to height loss, and in combination with trauma, obesity, and genetic predisposition can lead to lumbar disc herniation more frequently at L4-L5, L5-S1.1
There are five lumbar vertebrae. For each vertebra, the pedicle extends laterally and posteriorly on both sides, forming the margin of the intervertebral foramen (e.g., the L1 pedicle forms the superior margin of the L1-L2 foramen, and the L2 pedicle forms the inferior foramen margin). Superior and inferior articulating processes extend from the pedicles and form the articulating facet joints that lock the adjacent vertebrae into place. As the pedicle extends more posteriorly, they turn medially, forming the lamina. The lamina then fuses at the midline and extends posteriorly and inferiorly as the spinous process (Fig. 6-1).
The supraspinous ligament covers the spinous processes and interspinous ligament posteriorly. The interspinous ligament fills the space between the spinous processes. The ligamentum flavum covers the spinous processes and interspinous ligament anteriorly. The anterior and posterior longitudinal ligaments cover the vertebral bodies and the intervertebral discs.
Spinal Cord and Cauda Equina
The spinal cord normally terminates in the lumbar spine at L1/2-L2/3 as the conus medullaris. The filum terminale, an extension of the pia, tethers the conus medullaris apex to the dorsal coccyx. Filum traction on the conus, most commonly from developmental abnormalities, can produce a tethered cord syndrome consisting of weakness and sensory loss in the legs as well as fecal and urinary incontinence.2
Spinal nerves of the lumbar and sacral region originate from the cauda equina, the bundle of nerves that originates from the start of the conus medullaris. The nerve roots have a very specific orientation within the cauda equina, with the most anterior nerves exiting as the L5 root and exiting in an anterior to posterior order as the S1-S5 roots (Fig. 6-2).3 Compression of these nerves produces the cauda equina syndrome (CES).
Spinal Cord Vascular Supply
Arterial supply to L1 (starting from T1) is provided by the radicular arteries that branch from the intercostal arteries from the aorta. The artery of Adamkiewicz, which branches from the aorta, enters the spinal cord between T8 and L4, usually on the left, and supplies most of the lower spinal cord; emboli to this vessel usually lead to infarction of the anterior two-thirds of the spinal cord, causing weakness, pain, and loss of temperature sensation. The general orientation of arteries relative to the spinal column is shown in Figure 6-3.
Inside the spinal canal, from the radicular artery, the radiculopial artery branches and moves posteriorly to form
the posterior spinal arteries and supply the posterior onethird of the spinal cord. In addition, the radiculomedullary branch of the radicular artery forms the anterior spinal artery. These arteries supply the anterior two-thirds of the spinal cord (Fig. 6-4).4,5
the posterior spinal arteries and supply the posterior onethird of the spinal cord. In addition, the radiculomedullary branch of the radicular artery forms the anterior spinal artery. These arteries supply the anterior two-thirds of the spinal cord (Fig. 6-4).4,5
 FIGURE 6-1. Illustrative and radiographic views of the lumbar spine (From Moore KL, Agur AM, Dalley AF. Clinically Oriented Anatomy. 7th ed. Philadelphia, PA: Wolters Kluwer; 2009, with permission). |
Venous drainage from the lower spinal cord is the same as from the rest of the cord. They drain into an irregular epidural venous plexus that communicates with the pedicular veins and ultimately drains into the vena cavae (Fig. 6-5). Lumbar needle procedures can puncture these veins and produce bleeding, and can lead to significant hematomas in patients with coagulopathies.
Common Lumbar Spine and Cauda Equina Complications
Epidural Steroid Injection
Low back pain is a common complaint, has its highest incidence in the third decade, with 1-year incidence of any back pain between 1% and 36% and recurrence between 24% and 80%.6 Epidural steroid injections (ESIs) remain a popular nonsurgical intervention for low back pain. Although there is literature supporting its efficacy,7,8 the Food and Drug Administration has not approved epidural corticosteroid injections for low back pain at this time because serious neurologic events, including paraplegia, quadriplegia, and brain and spinal cord infarction, have been reported with and without use of fluoroscopy.9
Complication rates for ESI remain low overall at 2.4%,10 for intralaminar and interlaminar and transforaminal approaches, with conflicting reports comparing relative complication rates between the techniques.10,11 Infectious complications, including epidural abscess, meningitis, discitis, and osteomyelitis, are about 1% to 2%.12 The incidence of epidural hematoma is about 1 per 150,000 and increases to 2% in patients using anticoagulants.13 Ideally, oral anticoagulation should be discontinued, and prothrombin time normalized. This usually takes 1 to 2 days. In patients taking coumadin, the International Normalized Ratio (INR) should be definitely less than 3, but there are no definitive guidelines for
INR between 1.5 and 3. Platelet function should also be normalized, which can mean holding the antiplatelet agent for 5 to 10 days.14
INR between 1.5 and 3. Platelet function should also be normalized, which can mean holding the antiplatelet agent for 5 to 10 days.14
 FIGURE 6-2. Illustrative and radiographic views of the lumbar spinal cord and cauda equina relative to their surrounding osseous structure (From Moore KL, Agur AM, Dalley AF. Clinically Oriented Anatomy. 7th ed. Philadelphia, PA: Wolters Kluwer; 2009, with permission). |
 FIGURE 6-3. The spinal arteries relative to the lumbar spine (From Moore KL, Agur AM, Dalley AF. Clinically Oriented Anatomy. 7th ed. Philadelphia, PA: Wolters Kluwer; 2009, with permission). |
Surprisingly, intravascular injection can be frequent, as high as 21.3% in the transforaminal approach with fluoroscopic guidance.15 Although this does not lead to an adverse outcome per se, instilling the local corticosteroid intravascularly subjects the patient to pain and anxiety associated with needle procedures without ensuing benefits. Dural puncture is another notable complication in ESI. Cerebrospinal fluid (CSF) flashback indicative of dural puncture may not necessarily be present, especially in the transforaminal approach.16 Aside from a CSF leak that may require blood patching, injection of intrathecal anesthetic can lead to transient ascending weakness or sensory loss, which can spread as high as C2. Rarely, serious complications such as respiratory depression can also occur. Notably, the usual 6 to 8 mL of anesthetic used in ESI is not typically sufficient to produce significant adverse events.12
Epidural Anesthesia
Epidural anesthesia can lead to dural puncture in 0.4% to 6% of patients when used in a variety of abdominal and lower extremity surgeries.17 A persistent CSF leak after dural puncture can lead to a postural headache alleviated with recumbence. There is no evidence correlating the length of postpuncture recumbence to incidence of postdural puncture headache. However, there is an association with needle gauge (11% to 28% with 20 gauge, 3% to 25% with 25 gauge, 3% to 8% with 26 gauge, and 0% to 2% with 29 gauge).18 In cases of persistent headache, an epidural blood patch is usually applied, with a success rate of 36% to 57% for the first patch, increasing with subsequent patching. Interestingly, although theorized to spread, clot, and occlude the leak, the exact mechanism
for blood patching has not been determined.19 Further, there is no consensus between blind patching and patching after a radiologic CSF study. Although targeted patching seems to provide higher symptom relief rate, in cases of indeterminate leak location, traditional blind patching in the lumbar spine is generally applied. In addition, the lumbar epidural space can accommodate a larger volume of blood without concern for compressive effects on the spinal cord.20
for blood patching has not been determined.19 Further, there is no consensus between blind patching and patching after a radiologic CSF study. Although targeted patching seems to provide higher symptom relief rate, in cases of indeterminate leak location, traditional blind patching in the lumbar spine is generally applied. In addition, the lumbar epidural space can accommodate a larger volume of blood without concern for compressive effects on the spinal cord.20
 FIGURE 6-4. The major arteries supplying the spinal cord and the associated veins (From Moore KL, Agur AM, Dalley AF. Clinically Oriented Anatomy. 7th ed. Philadelphia, PA: Wolters Kluwer; 2009, with permission). |
 FIGURE 6-5. The spinal venous plexuses relative to the lumbar spine (From Moore KL, Agur AM, Dalley AF. Clinically Oriented Anatomy. 7th ed. Philadelphia, PA: Wolters Kluwer; 2009, with permission). |
The process of epidural puncture can itself injure the vertebral venous plexus, around 6% incidence, leading to repeat procedure half the time,21 and sometimes epidural hematomas. In these cases, the majority were associated with anticoagulant use or clotting disorder. Patients frequently present with motor weakness (46%) and back pain (38%). Notably, back pain can be masked in cases of continuous anesthesia.22
Lumbar Puncture
Headache after lumbar puncture is very common; roughly one-third of patients experience it.23 These headaches are typically associated with rapid removal of roughly 15% (20 mL) of total CSF volume, provoked by sitting or standing, and relieved by recumbence.24 The mechanism involves a combination of meningeal traction from intracranial hypotension as well as dilation of cerebral veins and venous sinuses.25 Spontaneous intracranial hypotension has a similar presentation; however, its pathophysiology and diagnostic criteria differ (see Table 6-1). Conservative treatment with hydration and intravenous caffeine often offers symptomatic relief, but in refractory cases, blood patching is required.
The infection rate from lumbar puncture is low (1 to 2 per 10,000), the most common organism is Streptococcus twothirds of the time.26 Bleeding is a significant complication in coagulopathic and thrombocytopenic patients and can lead to paraplegia. The diagnosis is often delayed, with about 50% of lumbar puncture-induced hematomas discovered after 12 hours of paraplegia.27 Complication rates are significantly higher in anticoagulated patients who undergo lumbar puncture.28 Guidelines for anticoagulation and lumbar puncture are the same as those of ESI and other epidural/dural puncture procedures. Notably, clear CSF does not preclude risk of hematoma formation, as up to half of patients who later develop hematomas have initially clear CSF.27
Table 6-1. Internal Headache Society Diagnostic Criteria for Headaches Related to Intracranial Hypotension | ||||
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Degenerative Disc Disease and Spinal Stenosis
The cause of degenerative disc disease is multifactorial and includes lifestyle (smoking, occupation), mechanical loading, nutrition, and genetics (aggrecan gene polymorphism), which all lead to degenerative changes beginning in the 20s to early 30s.29 Symptomatic lumbar disease contributes to an estimated 60% to 80% of low back pain cases and is most commonly caused by lumbar disc herniations.30
Sciatica is a common complication of degenerative disc disease. The hallmark presentation is radiating pain down the buttock, following the course of the sciatic nerve. Disc compression of the nerve roots at L4-S1 can produce unilateral sciatica symptoms. However, compression of the pelvic plexus or cauda equina and lumbar stenosis can produce bilateral symptoms. Clinically, worsening pain with coughing, sneezing, or Valsalva is indicative of disc disease rather than other etiologies such as spondylolisthesis or piriformis syndrome.31 Fortunately, sciatica self-resolves in 75% of patients after 3 months.32 The mainstay of treatment is with nonopiate analgesics to allow for physical therapy. Disc surgery is another option, but in terms of pain or disability, there seems to be limited benefit compared with conservative measures at 1 year.33
Spinal stenosis typically develops in the 50s to 60s. The stenosis can occur centrally or laterally. Aside from discogenic causes, central stenosis often develops from ligamentum flavum hypertrophy, which can be from aging or mechanical instability of the spine. Patients with central stenosis often present with neurogenic claudication: radiating pain down both legs while standing or walking, relieved with sitting down and bending forward, the kyphosis of which increases the spinal canal diameter. In contrast, lateral spinal stenosis produces radiculopathies, with weakness or sensory decrease along the corresponding myotome and dermatome and decreased reflexes corresponding to the levels of stenosis. Aside from direct disc compression, foraminal stenosis from articular hypertrophy, pedicular kinking, and uncinate spur can compress the nerve root as it enters, passes through, and exits the neural foramen.34 Conservative treatment for lumbar spinal stenosis includes a combination of physical therapy, intermittent pelvic traction, oral analgesics, and epidural steroids. Depending on the degree of stenosis, 70% of patients can have symptom improvement.35 For severe stenosis or refractory cases, decompressive surgeries such as laminectomies, foraminotomies, and spinal fusion in cases of instability can be tried, and when successful, patients can have complete resolution of symptoms.
Cauda Equina Syndrome
CES consists of low back pain, including radicular pain or numbness, tingling or electrical sensation traveling down the leg, as well as sensation change in the perineal region, or “saddle anesthesia,” and bowel or bladder dysfunction. In practice, many patients present with a partial syndrome.3,36
 FIGURE 6-6. The lumbosacral plexus and their associated roots and nerves (From Agur AM, Dalley AF. Grant’s Atlas of Anatomy. 13th ed. Philadelphia, PA: Wolters Kluwer; 2012, with permission). |
Compression, whether by trauma, disc herniation, abscess, hematoma, or tumors, is the main etiology of CES. The degree and timing of compression affect clinical presentation, because patients with chronic mild compression are largely asymptomatic and show only changes in electrodiagnostic studies, whereas those with acute significant compression, such as from a hematoma, can manifest the entire CES syndrome.
When CES is suspected, imaging confirmation is required. Although magnetic resonance imaging (MRI) is the study of choice, CT with myelography can be useful for determining pathology that requires immediate surgical intervention. Intervention should be urgent, as patients have significant symptom improvement with decompression within 48 hours, though earlier is often preferred in practice.37 More than half of patients who undergo decompression within 48 hours can have complete recovery of urinary incontinence versus only one third in patients outside of 48 hours.38 Overall, surgery is very effective, with 43% of patients eventually gaining complete recovery and 87% gain functional recovery.39
The Lumbosacral Plexus
Lumbosacral Plexus Neuroanatomy
All spinal nerves exit the intervertebral foramen and divide into anterior (ventral) and posterior (dorsal) rami. The anterior rami of L1-S4 form the lumbosacral plexus, which has three components: (1) the lumbar plexus (L1-L4), (2) the sacral plexus (S1-S4), and (3) the lumbosacral trunk (L4-L5) that connects the lumbar and sacral plexus (Fig. 6-6).
The lumbar plexus forms on the psoas major muscle along the posterior abdominal wall, making it prone to compression from psoas muscle damage or retroperitoneal (RP) hematoma. It then branches into an anterior and posterior division. The anterior division forms the obturator nerve, which goes into the medial compartment of the thigh. The posterior division forms the femoral nerve, which innervates the leg extensors, and, owing to medial rotation during development, ends up in the anterior compartment of the thigh. The lumbar plexus nerve, roots, and functions are summarized in Table 6-2.
Table 6-2. Lumbar Plexus, Nerves, Roots, and Their Functions | ||||||||||||||||||||||||||||
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The roots of the sacral plexus lie on the piriformis muscle and form the sciatic nerve, which also divides into an anterior and posterior division. The anterior division forms the tibial branch of the sciatic nerve, which is more medial, while the posterior division forms the peroneal branch of the sciatic nerve, which is more lateral. The sacral plexus nerve, roots, and functions are summarized in Table 6-3.
Table 6-3. Sacral Plexus Nerves, Roots, and Their Functionsa | ||||||||||||||||||||||||||||
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Diagnosing Lumbosacral Plexopathy
Diagnosis
Clinical features of a lumbosacral plexopathy (LSP) depend on location and the underlying etiology. One should consider LSP if a patient’s symptoms cannot be localized to a peripheral nerve or a single nerve root. If the lumbar plexus is damaged, there will be weakness of hip flexion, knee extension (femoral), and hip adduction (obturator). Sensory loss and paresthesia tend to occur over the lateral, anterior, and medial thigh, but may extend down to the medial calf. If pain is present, it is most often located in the pelvis, with radiation to the anterior thigh. Lesions of the sacral plexus tend to present with weakness of hip extensors (gluteus maximus), adductors, and internal rotators (gluteus medius and tensor fasciae latae), and hamstring muscles or distal foot muscles. Sensory symptoms are seen over the posterior thigh, and posterior lateral calf and foot.40 Pain may be present in the pelvis.
The first step in diagnosing an LSP is the exam. The physician should assess for weakness and sensory loss in the distributions discussed above. Loss of reflexes, or diminished reflexes, may indicate specific nerve root involvement (adductor [L3], patellar [L4], Achilles [S1]). One should also palpate the inguinal region to feel for hematoma or mass, and palpate the greater trochanter of the hip for bursitis. Straight leg raise (L5, S1) can also help distinguish an LSP from a radiculopathy (a common mimic). Maneuvering the hip can also help determine whether the pain is related instead to sacroiliitis.
The imaging of choice is an MRI. If abscess, neoplasm, or inflammatory changes are suspected, the MRI should be ordered with contrast. More recently, MR neurography can more closely examine at the plexus nerve roots.40 Electrodiagnostic studies help localize a lesion to the lumbosacral plexus, and exclude radiculopathies or neuropathies that may clinically mimic an LSP. Nerve conduction studies (NCS) should be performed to look for specific nerve abnormalities, and electromyogram (EMG) of lower extremities and paraspinal muscles can aid in localizing weakness or muscle denervation. Specifically on NCS, decreased sensory nerve action potentials imply that the lesion is at or distal to the dorsal root ganglion, but not at the level of the nerve roots. Important muscles to test on EMG include gluteal, thigh adductor muscles, and paraspinal muscles. Testing the gluteal muscles can distinguish a sciatic neuropathy from a lower LSP. Abnormalities in the adductor muscles (obturator nerve) in addition to femoral innervated muscles indicate an upper LSP, rather than an isolated femoral neuropathy. Abnormalities in the paraspinal muscles localize the lesion to the nerve root, rather than the plexus. An upper limb EMG should be used if there is bilateral involvement, to help exclude polyneuropathy.
The clinician should consider hemoglobin A1c, erythrocyte sedimentation rate (ESR), C-reactive protein, infectious studies (Epstein-Barr virus [EBV], varicella-zoster virus, syphilis, Lyme), and rheumatologic studies (anti-nuclear antibody [ANA], anti-neutrophilic cytoplasmic antibody [ANCA], angiotensin converting enzyme [ACE], serum protein electrophoresis [SPEP], Anti Ro/La) because these tests help rule out the common causes of neuropathy. A lumbar puncture can also be used to look for occult infection or malignancy.
Common Etiologies of Lumbosacral Plexopathy
Systemic Etiologies
Diabetes
Diabetic LSP (or diabetic amyotrophy) typically occurs in long-term type 2 diabetics. Persistently high blood sugar produces an ischemic microvasculopathy. Although only 1% of diabetics develop an LSP, those who do have significant morbidity. Patients present with acute onset of unilateral neuropathic pain (stabbing, burning, aching) and allodynia in the thigh or leg, usually lasting weeks. As the pain subsides, patients experience proximal more than distal weakness out of proportion to the pain. Over months, the symptoms become bilateral and diffuse. Autonomic involvement is common. NCS reveal multifocal primary axonal degeneration.41 Unfortunately, many patients have long-term disability requiring wheelchairs or walkers. Because proximal segments of the plexus reinnervate earlier, foot drop is the most notable chronic symptom.
Idiopathic Plexitis
For most idiopathic plexitis, the underlying pathology remains unclear; inflammation is thought to be the primary culprit. Presentation is acute-onset severe pain in the proximal pelvis or upper leg that subsides over several weeks, followed by weakness that subsides over months. Sometimes, patients report a preceding illness or vaccination. In cases of progressive LSP, ESR may be elevated, indicating a systemic inflammatory response, and steroids or immunosuppressive therapy can be used.
Infectious, Inflammatory, and Infiltrative
Infectious, inflammatory, and infiltrative causes of LSP are very rare. They should be considered in patients with HIV; who have concomitant infections with echovirus, EBV, cytomegalovirus, Lyme; or who are undergoing HIV seroconversion. Although patients most often experience radicular symptoms, LSP can also occur. Other considerations include compression from an abscess, sarcoidosis, and amyloidosis.42,43
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