Neuropathic leg pain – Disease of the lumbosacral plexus
After exiting the intervertebral foramina, the lumbosacral nerve roots (L1–S3) come together to form the lumbosacral plexus. Disease of the lumbosacral plexus may present in a very similar manner to diseases of the nerve root and cause pain into the leg. Lumbosacral plexus lesions are usually divided anatomically into disease of the upper plexus (lumbar plexus) and of the lower plexus (lumbosacral plexus). Lumbar plexopathies affect predominantly the L2–L4 fibers resulting in weakness of the quadriceps, iliopsoas and thigh adductor muscles (femoral and obturator nerves). The knee jerk is frequently depressed. Pain develops in the pelvis with radiation into the anterior thigh.
Sensory loss may be apparent over the anterior medial and lateral thigh as well as the medial part of the calf. Lesions of the lower lumbosacral plexus affect predominantly the L4–S2 nerve fibers. Patients typically describe a deep boring pain in the pelvis with radiation posteriorly in the thigh. Sometimes, the nerve fibers destined for the peroneal nerve will be preferentially affected with the patient presenting with foot drop and sensory disturbance over the dorsum of the foot and lateral calf. This pattern may be difficult to differentiate from an isolated lesion of the common peroneal nerve, unless electrodiagnostic studies are performed.
Akin to diseases of the nerve root, the etiology of lumbosacral plexopathy is divided into structural disease and non-structural disease. Structural disease includes local invasion by tumor, most typically bladder, uterine, ovarian, prostate or colon. In addition, a mass lesion may occur as a result of a retro-peritoneal hemorrhage which commonly occurs in the substance of the psoas muscle. Such can occur as the result of an aortic aneurysm rupture or more commonly as a spontaneous occurrence in a patient who is anti-coagulated. Lumbosacral plexopathy may also occur on a structural basis in women with endometriosis with implantation of abnormal tissue on the plexus.
In addition, several non-structural causes of lumbosacral plexopathy occur. This most commonly occurs in patients with diabetes mellitus. Known also as proximal diabetic neuropathy or plexopathy, diabetic amyotrophy classic-ally affects the lumbar plexus. Patients may have either mild or longstanding diabetes. They present with severe deep boring pain in the pelvis or proximal thigh. Movement is often difficult. The pain may last for weeks. As the pain slowly dissipates, it becomes apparent that the patient also has significant weakness out of proportion to the pain. Diabetic amyotrophy commonly affects the femoral and obturator nerves with prominent wasting of the anterior medial thigh musculature. Knee reflexes are often absent on the involved side.
Despite the prominent pain, atrophy and weakness, there is often very little sensory loss. Usually, patients who develop diabetic amyotrophy will also have coexistent diabetic polyneuropathy, and thus will have some sensory disturbance distal in their legs. Lumbosacral plexopathy can also occur on a non-structural basis from radiation damage, usually administered previously for the treatment of tumor. Radiation plexopathy is typically painless in comparison with direct infiltration of the plexus, which usually causes pain. Rare cases of idiopathic lumbosacral plexitis may occur similarly to brachial plexitis in the arm. This often occurs a week or two after an immunological inciting event such as a cold, flu or immunization. In some cases, there will be no clear inciting event. Patients develop severe deep pain along with clear neurological signs or symptoms. The pain often lasts for weeks or months with all imaging studies being unremarkable.
Evaluation of lumbosacral plexopathy
If the history and physical examination suggest a lumbosacral plexopathy, then further laboratory and radiological investigation is required. Of utmost importance is to exclude diabetes mellitus with a fasting blood sugar and hemoglobin A1c. Next, most patients require imaging of the pelvis to exclude a mass lesion. Often, the back needs to be imaged as well, as symptoms and signs of lumbosacral radiculopathy may easily mimic plexopathy.
Nerve conduction studies and EMG are indicated in the evaluation of lumbosacral plexopathy. First, nerve conductions and EMG can often localize the lesion to the plexus and exclude mononeuropathies (e.g. femoral, sciatic) and radiculopathies which can mimic plexopathy. EMG may be normal in unusual lesions which only irritate nerve but do not result in any axonal loss. In the case of radiation damage to the lumbosacral plexus, the presence of myokymia on EMG is often diagnostic. Myokymia is the spontaneous grouped repetitive discharges of a motor nerve which is highly characteristic of radiation damage. In superficial muscles, myokymia can be recognized by an undulating, worm-like movement of muscle. However, myokymia is much more easily appreciated on EMG which can sample deeper muscles.
Therapy of lumbosacral plexopathy
Compressive lesions by tumor are treated in the usual way with chemotherapy and radiation. In patients with diabetic amyotrophy and idiopathic plexitis, the condition is usually self limited but may last many months. Some cases of idiopathic plexitis may respond to intravenous gamma globulin and other immunomodulating therapies. These treatments are best reserved for patients who continue to process for several months and do not improve spontaneously. Otherwise, therapy is directed at improving muscle function by strengthening and physical therapy. Pain is often difficult to treat. Treatment of neuropathic pain from plexopathy is similar to treatment of neuropathic pain from any cause.