A 36-year-old woman had a 2-month history of pain in her left hip and back radiating to her left leg, particularly during straining.
Past medical history was unremarkable.
Her general and neurologic examinations were normal except for mild weakness of the toe dorsiflexion of the left. Reflexes and sensory examination and straight leg raising test were normal.
What is the Differential Diagnosis in this Case?
The presentation with pain when straining suggests a lumbosacral radiculopathy likely caused by a ruptured disk, but other causes need to be ruled out. A plexopathy and a focal neuropathy are somewhat unlikely because of back pain and lack of sensory deficits. Other possibilities include spinal canal stenosis and facet hypertrophy, but the lack of classical symptoms of stiffness and back pain in the early morning and the patient’s young age are against these diagnoses. The lack of more diffuse findings and sphincter disturbances is somewhat against a lumbosacral tumor.
What would be the Most Appropriate Diagnostic Test?
The most important diagnostic tests include imaging of the spine, particularly by MRI. Electrodiagnostic tests are helpful in delineating the affected structures. For example, motor nerve conduction velocities are usually abnormal in focal neuropathies and normal in radiculopathies and plexopathies, but both may show low-amplitude CMAPs, and the SNAPs may be abnormal in plexopathies. Testing of H-reflex and F-responses is also helpful in the evaluation of patients with radiculopathy and plexopathy, although these studies provide variable results. Somatosensory responses with stimulation of nerve trunks or dermatomal stimulation can be of help, but these are not done routinely, and the results are variable. Needle electromyography studies are most useful in differentiating radiculopathy from plexopathy or peripheral nerve disease, particularly based on the myotomal distribution of denervation, which is usually present in the paraspinal muscles in radiculopathy.
A Nerve Conduction Study was Performed
| Nerve and Site | Latency (ms) | Amplitude (mV) | Conduction Velocity (m/s) |
|---|---|---|---|
| Peroneal Nerve L. | Normal ≤ 5.7 | Normal ≥ 3 | Normal ≥ 40 |
| Ankle | 3.5 | 11 | – |
| Fibular head | 9.9 | 15 | 52 |
| Lateral malleolus | 3.7 | 7 | – |
| Nerve and Site | Latency (ms) | Amplitude (mV) | Conduction Velocity (m/s) |
|---|---|---|---|
| Tibial Nerve L. | Normal ≤ 5.3 | Normal ≥ 4 | Normal ≥ 40 |
| Ankle | 2.9 | 18 | – |
| Pop. fossa | 11.2 | 18 | 49 |
| Peroneal Nerve R. | Normal ≤ 5.7 | Normal ≥ 3 | Normal ≥ 40 |
|---|---|---|---|
| Ankle | 3.7 | 9 | – |
| Fibular head | 10.1 | 9 | 52 |
| Nerve | Latency (ms) | Normal Latency ≤ (ms) |
|---|---|---|
| Peroneal nerve L. | 54.0 | 54 |
| Tibial nerve L. | 47.6 | 54 |
| H-reflex L. | 30.3 | 34 |
| H-reflex R. | 30.1 | 34 |
| Nerve | Onset Latency (ms) | Normal Onset Latency ≤ (ms) | Peak Latency (ms) | Normal Peak Latency ≤ (ms) | Amp (μV) | Normal Amp ≥ (μV) | Conduction Velocity (m/s) | Normal Conduction Velocity ≥ (m/s) |
|---|---|---|---|---|---|---|---|---|
| Sural nerve L. | 2.8 | 3.5 | 3.3 | 4.0 | 17 | 11 | 50 | 40 |
What were the Nerve Conduction Test Findings?
The left peroneal nerve had normal motor conduction velocity and distal and F-wave latencies, but the CMAP response on knee stimulation was larger than on ankle stimulation ( Fig. 20-1A ). The left tibial motor nerve had normal conduction velocity, CMAP amplitude, distal latency, and F-wave latency. The right peroneal conduction test, left sural SNAP, and both H-reflexes were normal.
What is the Significance of the Findings of this Test and What should be Done Next?
The tests were normal except for low-amplitude distal peroneal CMAP response compared with the CMAP obtained with proximal stimulation. This could be technical from poor stimulating techniques, as the distal CMAP normally should be close to 100%, compared with results obtained during proximal stimulation; this could also be caused by an accessory peroneal nerve.
The stimuli strength of the peroneal nerve was increased by 25% without an increased CMAP amplitude. The stimulating electrode was then moved to the lateral aspect of the ankle and then behind the lateral malleolus when a small CMAP appeared in the extensor digitorum brevis (EDB) muscle (see Fig. 20-1B ).
These findings indicate an accessory deep peroneal nerve, a normal variant that occurs in up to 28% of normal people. In this, one or both EDB muscles are partially innervated by an accessory deep peroneal nerve, a branch of the superficial peroneal nerve ( Fig. 20-2 ). This is the most common cross-innervation of the lower extremity.
A Needle EMG Test was Performed
What were the EMG Findings?
Electromyography showed denervation potentials in the left tibialis anterior, tensor fasciae latae, and tibialis posterior muscles; some large motor unit potentials were seen in the extensor hallucis longus. These muscles are innervated mainly by L4 and L5 roots. The lack of denervation in the vastus lateralis suggests that the L4 was not affected, and the negative findings in the gastrocnemius ruled out an S1 root involvement. Thus all denervated muscles originate from the L5 root, particularly the tensor fasciae lata which is innervated primarily by this root. Although the paraspinal muscles were normal, the electrophysiologic diagnoses were a left L5 radiculopathy and an accessory deep peroneal nerve.
What should be the Next Test for this Patient?
MRI has become the most important imaging study of the lumbosacral spine; CT, lumbar myelography, and post-myelogram CT are also useful. Because spinal abnormalities can be found in normal persons, the findings of these tests should be correlated with the clinical presentation and the results of electrophysiologic tests.
An MRI of the lumbosacral spine showed a herniated lateral L5–S1 disk compressing the L5 root ( Fig. 20-3 ).
The patient was treated with conservative management with physiotherapy without improvement. She was referred to neurosurgery and underwent removal of the disk using laser surgery.
Summary
A 34-year-old female with leg pain was found to have an L5 radiculopathy caused by a herniated disk seen on MRI. She also had an accessory deep peroneal nerve.
Important Points
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An accessory deep peroneal nerve is a normal anatomic variant that occurs in up to 28% of individuals. The diagnosis is suspected because of lower-amplitude CMAP obtained with distal peroneal nerve stimulation compared with the CMAP obtained with proximal stimulation. To confirm this variation, the examiner should stimulate the nerve laterally, behind the lateral malleolus.
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The most important electrodiagnostic finding in radiculopathy is the presence of myotomal denervation, particularly if there is sparing of the paraspinal muscles. Paraspinal denervation helps in the diagnosis, but its absence does not rule out radiculopathy.
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MRI of the spine is valuable in the evaluation of patients with a lumbosacral radiculopathy, but as abnormalities can be seen in normal people, the interpretation of the findings should be correlated with the clinical presentation and the results of the electrophysiologic tests.
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Radiculopathies caused by herniated lumbar disks can be treated conservatively. Surgery should be considered if there is persistent pain or progressive and prolonged deficits.
Discussion (Cases 18–20)
In Case 18 , a woman presented with a history of back pain and leg weakness and numbness in the territory of the L4 root, and muscles supplied by L4 were denervated on EMG. Thus she was diagnosed to have an L4 radiculopathy; however, while these findings are most useful in the diagnosis, they should always be interpreted with some caution for a definitive anatomic diagnosis because of the variable innervation of the lower extremity muscles.
In radiculopathies, motor nerve conduction studies and SNAPs are usually normal, but occasionally the SNAPs may be abnormal from large lateral lesions that affect the posterior root ganglia. The routinely tested SNAPs do not evaluate the upper lumbosacral plexus and thus are not useful in differentiating upper lumbosacral plexus lesions from root lesions. Needle EMG is the most important study for lesions of the upper roots.
An L4 radiculopathy is most likely caused by a central or paracentral herniated L3–L4 disk or a laterally herniated L4–L5 disk ( Fig. 20-4 ). Other causes of radiculopathy include, among others, osteoarthritis with facet hypertrophy, tumors, and infections. MRI is the best imaging technique in lumbar radiculopathy, and CT myelogram can also be useful.
