Patients presumed to have lower limb symptoms localizing to the lumbar or lumbosacral plexus require rigorous electrophysiological evaluation. Entities that cause lumbosacral plexopathies may be patchy, asymmetrical and more diffuse than initially suspected. As a result, bilateral nerve conduction studies and needle examination outside those routinely tested and clinically affected may be needed to document the extent of involvement including needle examination of the thoracic paraspinals and consideration of upper limb studies. This article outlines the lumbar and lumbosacral plexus anatomy, and discusses a differential diagnosis and electrophysiological approach in assessing patients with presumed lumbosacral plexopathies.
Key points
- •
Pure lumbosacral plexopathies are rare.
- •
Lumbosacral radiculoplexus neuropathies are more common than pure lumbosacral plexopathies.
- •
Inflammatory diabetic and nondiabetic lumbosacral radiculoplexus neuropathies are among the most common causes of lumbosacral plexopathies and are pathologically due to ischemic injury and microvasculitis.
- •
Lumbosacral plexopathies often do not occur alone but are found in association with thoracic and cervical radiculoplexus neuropathies.
Introduction
The lumbosacral roots and the lower extremity peripheral nerves are commonly involved in peripheral nervous system diseases (radiculopathies, length-dependent peripheral neuropathies). The lumbosacral plexus (composed of both the upper lumbar plexus and lower lumbosacral plexus) as a primary target for peripheral nervous system disease is less common. Despite this lesser frequency, the scope of processes that may be implicated in lumbosacral plexopathies is vast, ranging from compressive causes (ie, hematoma) and neoplastic diseases to inflammatory conditions secondary to systemic disease (ie, diabetic radiculoplexus neuropathies [DRPN]).
In all cases of lumbosacral plexopathy, electrodiagnostic studies incorporating nerve conduction studies and needle electromyography (EMG) can be helpful for localization and characterization of the underlying process. Localization to the lumbar plexus as opposed to nerve roots or individual roots is important, because diagnostic implications, evaluation, and treatment options may be different. Disorders that clinically present as a lumbosacral plexopathy often are not isolated to the lumbosacral plexus electrophysiologically and can extend into the roots (paraspinal involvement) as well as peripheral nerves. The concept of a radiculoplexus neuropathy (a process involving roots, plexus, and peripheral nerves) may therefore perhaps be more fitting when considering an electrodiagnostic approach to evaluating these patients.
This article focuses on nerve conduction studies and needle EMG in the diagnosis of lumbosacral plexopathies. In some clinical scenarios, there may be usefulness in other electrodiagnostic testing, including autonomic and quantitative sensation testing, to more firmly establish a diagnosis of lumbosacral plexopathy.
Introduction
The lumbosacral roots and the lower extremity peripheral nerves are commonly involved in peripheral nervous system diseases (radiculopathies, length-dependent peripheral neuropathies). The lumbosacral plexus (composed of both the upper lumbar plexus and lower lumbosacral plexus) as a primary target for peripheral nervous system disease is less common. Despite this lesser frequency, the scope of processes that may be implicated in lumbosacral plexopathies is vast, ranging from compressive causes (ie, hematoma) and neoplastic diseases to inflammatory conditions secondary to systemic disease (ie, diabetic radiculoplexus neuropathies [DRPN]).
In all cases of lumbosacral plexopathy, electrodiagnostic studies incorporating nerve conduction studies and needle electromyography (EMG) can be helpful for localization and characterization of the underlying process. Localization to the lumbar plexus as opposed to nerve roots or individual roots is important, because diagnostic implications, evaluation, and treatment options may be different. Disorders that clinically present as a lumbosacral plexopathy often are not isolated to the lumbosacral plexus electrophysiologically and can extend into the roots (paraspinal involvement) as well as peripheral nerves. The concept of a radiculoplexus neuropathy (a process involving roots, plexus, and peripheral nerves) may therefore perhaps be more fitting when considering an electrodiagnostic approach to evaluating these patients.
This article focuses on nerve conduction studies and needle EMG in the diagnosis of lumbosacral plexopathies. In some clinical scenarios, there may be usefulness in other electrodiagnostic testing, including autonomic and quantitative sensation testing, to more firmly establish a diagnosis of lumbosacral plexopathy.
Anatomy
Knowledge of the lumbosacral plexus anatomy is critical in assessment of the patient and planning a comprehensive electromyographic evaluation. Although often considered one entity, the lumbosacral plexus can be divided into 2 parts anatomically: the “upper” the lumbar plexus; and the “lower” the lumbosacral plexus.
Lumbar Plexus
The lumbar plexus lies within the psoas muscle and comprises the anterior rami of the T12 to L4 nerve roots ( Fig. 1 ). Many of these muscles and nerves cannot be tested by standard EMG techniques because they are deep in the abdomen or are small cutaneous nerve branches. The 6 major branches of the lumbar plexus include :
- •
Iliohypogastric nerve (T12/L1), which supplies the transverse and internal oblique muscles as well as sensation to the low abdomen.
- •
Ilioinguinal nerve (L1), a sensory branch to the inguinal region that also provides sensation to a small area of medial thigh and upper scrotum/labia sensation.
- •
Genitofemoral nerve (L1, L2) provides sensation to the skin of the femoral triangle. In men, this nerve provides muscular innervation to the cremasteric muscle and sensory innervation to the lower scrotum. In women, this nerve provides lower labial sensation.
- •
Lateral femoral cutaneous nerve (L2,L3) is a sensory branch to the lateral thigh. This sensory nerve, unlike those listed earlier, can be tested by nerve conduction techniques, although reliability of the results is questionable.
- •
Obturator nerve (L2, L3, L4 anterior rami) provides motor innervation to the thigh adductors and a small area of sensation to the medial thigh.
- •
Femoral nerve (L2, L3, L4 anterior rami, posterior divisions) is the largest branch of the lumbar plexus whose motor component can be tested by nerve conduction techniques. This branch supplies motor innervation to the iliopsoas, sartorius, and quadriceps muscles, and divides to form the saphenous nerve, which supplies sensation to the medial lower leg.
Unlike the brachial plexus, there are no subcomponents (trunks, cords) in the lumbar plexus (see Fig. 1 ).
Lumbosacral Plexus
The lumbosacral plexus primarily originates from the ventral rami L4 to S3 nerve roots (see Fig. 1 ).
- •
The lumbosacral trunk (also called the furcal nerve) has an L4 component that joins the L5 nerve root to form the lumbosacral trunk. This nerve then joins the sacral plexus within the pelvic outlet. This structure is commonly affected in postpartum lumbosacral plexopathy, thought to be secondary to compression from the fetal head.
- •
Superior gluteal nerve (L4, L5, S1) supplies motor innervation to the tensor fascia lata, gluteus medius, and gluteus minimus muscles.
- •
Inferior gluteal nerve (L5, S1, S2) provides motor innervation to the gluteus maximus muscle.
- •
Sciatic trunk/nerve (L5–S3) provides most motor innervation to the muscles of the posterior thigh and then into the leg via its 2 branches (common peroneal and tibial nerves).
- •
The pudendal nerve is formed from the anterior S2, S3, and S4 roots.
The tibial portion of the sciatic nerve innervates all the muscles of the posterior thigh except the short head of the biceps femoris, which is innervated by the peroneal division of the sciatic nerve or the common peroneal nerve branch of the sciatic nerve. This point is important when performing needle electromyogram studies for localization purposes.
History and physical examination
When presented with a patient with lower limb symptoms, careful history and examination are imperative. Many different processes can cause lower extremity plexopathy, and the responses to a few questions can be important in narrowing a differential diagnose and planning a subsequent electrodiagnostic study:
Onset
Did the process begin acutely (hours to days), subacutely (days to weeks), or is it chronic (months to years)? Most lumbosacral plexopathies have an acute to subacute onset, which is helpful in identifying the process.
Progression
Are the symptoms and findings worsening, stable, or improving? The disease course is a helpful feature to consider when thinking about the cause of a lumbosacral plexopathy as well as predicting prognosis. For instance, a slow, progressive course may point to a malignant cause, whereas a relapsing course may favor an inflammatory cause.
Extent
Is the process unilateral or bilateral at onset? We, and others, have described inflammatory plexopathies that are generally unilateral and focal in onset but become bilateral and widespread with time. At clinical presentation, the disease may be bilateral but it should not be assumed that symptoms were bilateral at onset. This point should be clarified when taking a medical history. Also, it is important to confirm that the symptoms are confined to the lower limb, because involvement of the upper limb may make a structural process less likely, and raise concern for a more diffuse, possibly inflammatory, process affecting cervical, thoracic, as well as lumbosacral segments.
Pain, Sensation, and other Temporally Associated Symptoms
Associated pain and sensation changes are important clues in lumbosacral plexopathies. If the weakness seems to be confined to the plexus distribution, but the sensory loss seems to follow a more dermatomal distribution, this may increase the extent of needle examination and encourage imaging and further work-up, because concern for a primary root level process (ie, radiculopathy) may be heightened. Other clinical points to consider are associated systemic features that may support an inflammatory immune disorder such as sarcoidosis. Weight loss, in addition to constitutional symptoms such as fever and night sweats, may support the diagnosis of neoplastic infiltration or a paraneoplastic cause. A rash associated with the neuropathic symptoms may occur in the context of certain vasculitides.
Electrophysiologic evaluation
Nerve Conduction Studies
The electrophysiologic presence of lumbosacral plexopathy can be defined when there is evidence for electrophysiologic abnormalities in the distribution of at least 2 different peripheral nerves in at least 2 different nerve root distributions. Sparing of paraspinals on needle examination is also helpful in localizing a pure lumbosacral plexopathy. However, most of these conditions are not pure and involve paraspinal denervation, hence our preferred term: radiculoplexus neuropathies.
Several sensory and motor nerve conduction studies are helpful in the diagnosis of a lumbosacral plexopathy ( Box 1 , Table 1 ). Several of these nerve conduction studies are not performed on routine lower limb studies and can be considered when evaluating for a lumbosacral plexopathy, especially if clinically involvement of the upper lumbar plexus is suspected. In many cases, if symptoms are unilateral, bilateral studies can be helpful in determining a relative reduction in the size of the motor or sensory response (looking for axonal loss). Our laboratory uses a 50% difference from side to side as representing a significant and potentially pathologic difference.
If the lumbar plexus is the most likely site of a lesion, then test:
- 1.
Peroneal motor nerve conduction with F-wave study
- 2.
If the peroneal compound muscle action potential (CMAP) amplitude is low or the conduction velocity is reduced then:
Consider testing the opposite side
Consider recording the peroneal motor nerve over the tibialis anterior muscle
- 3.
Tibial motor nerve conduction with F-wave study
- 4.
Femoral motor nerve conduction study with side-to-side comparisons (may require needle stimulation)
- 5.
Sural sensory nerve conduction
- 6.
Superficial peroneal sensory nerve conduction
- 7.
Saphenous sensory nerve conduction
- 8.
Lateral femoral cutaneous sensory nerve conduction
If the lumbosacral plexus is the most likely site of a lesion, then test:
- 1.
Peroneal motor nerve conduction with F-wave study
- 2.
If the peroneal CMAP amplitude is low or the conduction velocity is reduced then:
Consider testing the opposite side
Consider recording the peroneal motor nerve over the tibialis anterior muscle
- 3.
Tibial motor nerve conduction with F-wave study
- 4.
Sural sensory nerve conduction
Superficial peroneal sensory nerve conduction
In both, it is likely necessary to compare abnormalities or borderline abnormalities with the contralateral limb.
If symptoms are bilateral, consider studying the upper limb to assess for a polyradiculoneuropathy or peripheral neuropathy.
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
