Laboratory Studies
Workup of foot drop proceeds according to the suspected cause. In cases where a cause (eg, trauma) is readily identified, no specific diagnostic laboratory studies are required. In cases where unilateral foot drop occurs spontaneously in a previously healthy patient, further investigation into metabolic causes (eg, diabetes, alcohol abuse, and exposure to toxins) is required. The following tests may be helpful:
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Fasting blood sugar
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Hemoglobin A1c
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Erythrocyte sedimentation rate (ESR)
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C-reactive protein (CRP)
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Serum protein electrophoresis/immunoelectro-osmophoresis
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Blood urea nitrogen (BUN)
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Creatinine
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Vitamin B12 level
Radiography and Ultrasonography
If foot drop is posttraumatic, plain films of the tibia, fibula, and ankle are appropriate to uncover any bony injury. In the absence of trauma, when anatomic dysfunction (eg, Charcot joint) is suspected, plain films of the foot and ankle provide useful information.
If bleeding is suspected in a patient with a hip or knee prosthesis, ultrasonography can be helpful.
Magnetic Resonance Neurography
If a tumor or a compressive mass lesion to the peroneal nerve is being investigated, magnetic resonance neurography (MRN) may be considered. MRN has made it possible to produce high-resolution images of peripheral nerves, as well as associated intraneural and extraneural lesions.
MRN can be performed by using a standard 1.5 Tesla magnetic resonance imaging (MRI) system and special phased-array imaging surface coils. [12] Image data are acquired simultaneously from multiple receive-only surface coils. The image data from each coil in the array are combined to form a composite image with an improved signal-to-noise ratio.
Compared with standard MRI, MRN allows faster acquisition of anatomically detailed images, a smaller field of view, higher resolution, and thinner sections. By virtue of these features, MRN images are capable of showing the fascicular organization of normal peripheral nerves, thereby rendering the nerves more clearly distinguishable from other tissue (eg, tumor or blood vessels). In one study, the fascicular structure seen on MRN was found to be functional by using intraoperative electrophysiologic testing; the nonfascicular structures were nonfunctional.
Images can be processed further to allow stacking of axial sections and slicing of data in another plane of section. This is helpful in mapping the longitudinal extent of nerve involvement.
Electromyography
In addition to the metabolic disorders listed above, the differential diagnosis of spontaneous foot drop includes spasticity, dystonia, motor neuron disease, L5 radiculopathy, lumbosacral plexopathy, sciatic nerve palsy, compressive peroneal neuropathy, peripheral neuropathy, and some myopathies.
Electromyography (EMG) is useful in differentiating among these diagnoses. This study can confirm the type of neuropathy, establish the site of the lesion, estimate the extent of injury, and provide a prognosis. Sequential studies are useful for monitoring recovery in patients with acute lesions.
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Diagram of ground reaction vector during heel strike.
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Common and superficial peroneal nerves, branches, and cutaneous innervation.
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Deep peroneal nerve, branches, and cutaneous innervation.
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Incisions for Bridle procedure.
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Posterior leg with retrieved posterior tibial tendon above ankle. Window in interosseous membrane is labeled with X.
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Posterior tibial tendon (C) is pulled through slit in anterior tibial tendon (A) and inserted into second cuneiform. Posterior tibial tendon is anastomosed to anterior tibial tendon and distal stump of peroneus longus (B) that has been rerouted anterior to lateral malleolus.
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Gait of patient with foot drop.
