Foot Drop Case File

Foot Drop Case File

Eugene C. Toy, MD, Ericka Simpson, MD, Pedro Mancias, MD, Erin E. Furr-Stimming, MD

CASE 43

A 21-year-old female athlete presents with a 4-week history of frequent falls and difficulty jogging. She notes she often stubs her right toe and misses steps when climbing stairs. She is quite distressed because of concerns that she will not be able to function as a catcher on the intercollegiate softball team. She has no significant past medical history. On physical examination, she has decreased ankle dorsiflexion and eversion of her right foot. There is mild muscle atrophy and visible muscle twitches of the right lateral lower leg muscles not seen on the left. Sensory examination reveals decreased sensation to light touch of right lateral leg and foot. The patient has normal coordination and normal reflexes. The remainder of her physical examination is within normal limits.

▶ What is the most likely diagnosis?

▶ What is the next diagnostic step?

▶ What is the next step in therapy?

ANSWERS TO CASE 43:

Foot Drop                                                       

Summary: This is a case of right foot drop in a young, athletic woman. Abnormal examination findings are limited to her right distal lower extremity, which includes inability to raise her right foot against gravity or to evert it laterally.

• Most likely diagnosis: Peroneal nerve palsy
• Next diagnostic step: Electromyography (EMG) and nerve conduction study (NCS) of peripheral nerve and nerve roots
• Next step in therapy: Ankle-foot orthotics evaluation and rehabilitation; possible surgical intervention

ANALYSIS

Objectives

1. Know the diagnostic approach to isolated foot drop.
2. Be familiar with the causes of isolated foot drop.
3. Be familiar with the management of isolated foot drop.

Considerations

The clinical presentation of this case is straightforward for an isolated nerve palsy resulting in a foot drop. The common peroneal (L4-L5) nerve branches to form the deep peroneal nerve (motor) and superficial peroneal nerve (sensory). The deep peroneal nerve innervates the tibialis anterior, extensor digitorum longus, extensor hallucis longus, and peroneus tertius of the lower foot and ankle that mediate dorsiflexion of the foot above gravity (foot/toes raised above the ground) and lateral deviation of the foot outward (eversion). Isolated foot drop is often caused by pathology associated with the common peroneal nerve and its branches. The presence of an isolated group of weakened muscles and a sensory deficit that follows the distribution of a single nerve is indicative of a mononeuropathy. Nevertheless, other clinical conditions can also present with foot drop, including sciatic neuropathy, lumbar plexopathy, lumbar radiculopathy, muscular dystrophies, and motor neuron disease, and these require a thorough evaluation.

APPROACH TO:

Foot Drop                                                                    

DEFINITIONS

FASCICULATION: Small, local, involuntary muscle contraction (twitching) visible under the skin arising from the spontaneous discharge of a bundle of skeletal muscle fibers.

DENERVATION: Loss of nerve supply from either motor neuron, nerve root, or peripheral nerve injury or insult.

COMPRESSIVE NEUROPATHY: Neuropathy caused by compression. Compression can occur as a result of trauma, inflammation, or entrapment. The most common type of compression neuropathy is carpal tunnel syndrome or median nerve entrapment.

CLINICAL APPROACH

Foot drop is a symptom, not a diagnosis. A thorough evaluation involves consideration of neurologic, muscular, and anatomic pathology. Foot drop is associated with a variety of conditions such as peripheral nerve injuries, radiculopathy, stroke, neuropathies, multiple sclerosis, drug toxicities, or diabetes. Treatment is variable and is directed at the specific cause. The foot and ankle dorsiflexors include the tibialis anterior, extensor hallucis longus, and extensor digitorum longus. These muscles help the body clear the foot during swing phase of walking and on heel strike. Such weakness often results in a steppage gait because the patient tends to walk with an exaggerated flexion of the hip and knee to prevent the toes from catching on the ground during swing phase of walking, resulting in the foot slapping the ground. As stated, foot drop can result if there is injury to the dorsiflexors or to any point along the neural pathways that supply them, from the motor neuron to the nerve-muscle junction. Foot drop can be observed with direct injury to the dorsiflexors. A few cases of rupture of the tibialis anterior tendon leading to foot drop have been reported. This subcutaneous tendon rupture usually follows a minor trauma with the foot in plantar flexion.

Compartment syndromes can also lead to foot drop as a result of progressive edema, or hemorrhage in the muscles of the anterior compartment of the lower leg, which is often associated with strenuous activity in unconditioned individuals. Sciatic nerve lesions, lumbosacral plexopathy, lumbar radiculopathy, motor neuron disease, myopathy, or parasagittal cortical or subcortical cerebral lesions also can manifest as foot drop. The latter lesions are often differentiated by involvement of other muscle groups or extremities through clinical and electrodiagnostic examinations.

Peroneal neuropathy is caused by compression at the fibular head and is the most common compressive neuropathy in the lower extremity, likely because it is susceptible to injury along its course. The sciatic nerve is made up of nerve fiber bundles that make up the common peroneal and tibial nerve. These nerves branch off right above the posterior knee to innervate the anterior/lateral and posterior compartments of the lower leg and foot, respectively. As part of the sciatic nerve, the common peroneal nerve is relatively isolated from the tibial nerve. Therefore, trauma to the sciatic nerve might only affect the peroneal or tibial divisions. The common peroneal nerve is more susceptible to injury while on its course as part of the sciatic nerve. Also, the peroneal nerve is larger and has less protective connective tissue, making the peroneal nerve more susceptible to trauma. In addition, the peroneal nerve has fewer autonomic fibers, so in any injury, motor and sensory fibers bear the brunt of the trauma.

The peroneal nerve runs a more superficial course, especially at the fibular neck, making it vulnerable to direct insult, such as during surgical procedures or compression due to boots or knee braces. With peroneal neuropathy, ankle dorsiflexors (tibialis anterior, extensor hallucis longus, and extensor digitorum longus) and ankle evertors (peroneus longus and peroneus brevis), will be affected. Ankle inversion will be spared in a peroneal neuropathy, as this is mediated by the tibialis posterior muscle, which is innervated by the tibial nerve. The most common differential diagnosis for peroneal neuropathy is an L5 radiculopathy. With an L5 radiculopathy, the patient will have foot drop and will be unable to evert or invert the ankle. With a peroneal neuropathy, the patient will have foot drop and will be unable to evert the ankle; however, ankle inversion will be spared (since this is an action dependent on the tibial nerve).

DIAGNOSIS

Workup of foot drop proceeds according to the suspected cause. In instances where a cause is readily identified, such as trauma, no specific diagnostic laboratory studies are required. A spontaneous unilateral foot drop in a previously healthy patient as presented in this case requires further investigation into neuropathic causes or metabolic causes, including diabetes, alcohol abuse, and exposure to toxins.

Imaging Studies

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 a tumor or a compressive mass lesion to the peroneal nerve is being investigated, magnetic resonance neurography (MRN) can be used. MRN has made it possible to produce high-resolution images of peripheral nerves as well as associated intraneural and extraneural lesions when compared to standard magnetic resonance imaging (MRI).

Electrodiagnostic Tests

EMG and NCSs are useful in differentiating among the various causes. This study can confirm the type of neuropathy, establish the site of the lesion, estimate the extent of injury, and predict a prognosis. Sequential studies are useful to monitor recovery of acute lesions.

TREATMENT

The treatment of foot drop is directed to its etiology. If foot drop is not amenable to surgery, an ankle-foot orthosis (AFO) often is used. The specific purpose of an AFO is to provide toe dorsiflexion during the swing phase and medial and/or lateral stability at the ankle during stance. Foot drop caused by direct trauma to the dorsiflexors generally requires surgical repair. When nerve insult is the cause of foot drop, treatment is directed at restoring nerve continuity, either by direct repair or by removal of the insult. If foot drop is secondary to lumbar disc herniation (a finding in 1.2%–4% of patients with this condition), lumbar disc surgery should be considered.

Gait training and stretching through a rehabilitation program should be incorporated regardless of whether surgery is chosen. If foot drop is due to a central nervous system lesion (such as stroke or multiple sclerosis), then physical rehabilitation with the use of a functional electrical stimulation device should be considered. With a functional electrical stimulation device, the patient needs to have an intact peroneal nerve and healthy ankle dorsiflexor muscles. The patient is assessed by a physical therapist or rehabilitation physician while wearing a functional electrical stimulation device to determine if it is effective. The functional electrical stimulation device consists of a sensor pad on the heel of the affected leg and a stimulator over the ipsilateral peroneal nerve. As the patient walks, the sensor pad detects where the foot is in the gait sequence and triggers an electrical impulse to fire from the stimulator, causing activation of the peroneal nerve and subsequent contraction of the ankle dorsiflexors, resulting in dorsiflexion of the foot at the appropriate time in the gait to eliminate tripping on the toes. This mimics natural dorsiflexion that occurs and allows the patient to pick up the foot more normally without the use of a rigid AFO.

Prognosis

In a peripheral compressive neuropathy, recovery can be expected in up to 3 months, as long as further compression is avoided. A partial peroneal nerve palsy following total knee replacement has a uniformly good prognosis. A variable amount of recovery is seen with a complete postoperative palsy. Follow-up EMG and NCSs can be useful to assess recovery. A partial palsy recovers faster because of local sprouting. Complete axon loss reinnervates by proximal-to-distal axonal growth only, usually proceeding at 1 mm per day. Therefore, injuries of a nerve close to its target muscle also have a more favorable outcome. In a nerve root compressive neuropathy, one study concluded that severe motor weakness of longer than 6 months, a negative straight leg raising test, and old age were considered poor prognostic factors for recovery of dorsiflexion. When there is direct injury to the peroneal nerve, a more favorable outcome is noted with sharp versus blunt trauma. A traction or stretch injury to the nerve has an intermediate outcome. When nerve grafting is used, functional recovery depends on the severity of injury and therefore the length of graft used. Good functional recovery in grafts longer than 12 cm is rarely seen.

COMPREHENSION QUESTIONS

43.1 A 14-year-old adolescent boy has joined an ice hockey team as the goalie for the past 3 weeks. Two weeks after a near-daily set of practices and games, he began experiencing bilateral foot weakness resulting in frequent tripping of feet. His examination reveals mild to moderate bilateral foot drop and weakness with eversion of his feet. He also complains of tenderness to the proximal aspect of his lower leg, which he attributes to wearing shin protectors. Which of the following is the likely site of disease?

A. Sciatic nerve as it exits the sciatic notch below the buttocks

B. Peroneal nerve at the fibular head

C. Posterior tibial nerve in the calves

D. Lumbar nerve root in the spinal column

43.2 What nerve roots are likely to be associated with the condition in Question 43.1?

A. L2-L3

B. L3-L4

C. L4-L5

D. L5-S1

43.3 Which of the following is the best method of assessing the damage in the condition of Question 43.1?

A. MRI of the spine

B. Computed tomography (CT) scan of the spine

C. Radiograph of the lower leg

D. EMG/NCS

ANSWERS

43.1 B. The patient has isolated weakness of dorsiflexion and eversion of both feet and a history of wearing sports equipment associated with compression of the peroneal nerve as it wraps around the head of the fibula in the lower leg.

43.2 C. The nerve roots associated with the peroneal nerve are L4-L5. For this reason, differentiating a peroneal neuropathy from an L5 radiculopathy is sometimes a challenge.

43.3 D. EMG/NCS is the best modality to assess damage and recovery (function) of the peripheral nerve. The radiologic studies do not evaluate function.

    CLINICAL PEARLS    

▶ An isolated foot drop can be the initial or predominant manifestation of an inherited demyelinating polyneuropathy or hereditary neuropathy with liability to pressure palsies. ▶ When foot drop is not amenable to surgery, an AFO helps to provide toe dorsiflexion during the swing phase and medial and/or lateral stability at the ankle during stance. ▶ Certain muscular dystrophies affect distal muscles, resulting in foot drop, including myotonic dystrophy and Duchenne and Becker muscular dystrophies. ▶ Peroneal neuropathy is the most common nerve compression condition of the lower extremity near the fibular head. ▶ Amyotrophic lateral sclerosis or Lou Gehrig disease can present with an isolated foot drop caused by motor neuron degeneration.

REFERENCES

Anselmi SJ. Common peroneal nerve compression. J Am Podiatr Med Assoc. 2006;96(5):413-417. 

Garg B, Poage C. Peroneal nerve palsy: evaluation and management. J Am Acad Orthop Surg. 2016;24(5):e49. 

Gilchrist RV, Bhagia SM, Lenrow DA, Chou LH, Chow D, Slipman CW. Painless foot drop: an atypical etiology of a common presentation. Pain Physician. 2002;5(4):419-421. 

Nercessian OA, Ugwonali OF, Park S. Peroneal nerve palsy after total knee arthroplasty. J Arthroplasty. 2005;20(8):1068-1073. 

Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders: Clinical–Electrophysiologic Correlations. 3rd ed. London, UK: Elsevier, Inc; 2013. 

Wilkenfeld AJ. Review of electrical stimulation, botulinum toxin, and their combination for spastic drop foot. J Rehabil Res Dev. 2013;50(3):315-326.

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