By Peter D. Donofrio, M.D.
Professor of Neurology, Vanderbilt University Medical Center
Member, GBS|CIDP Foundation Medical Advisory Board
Foot drop (FD) is a term used by physicians and patients to describe weakness of the foot such that the patient cannot bend the ankle back toward the knee. Foot drop can affect one foot or both feet. When foot drop is mild, patients may notice weakness only when walking up stairs or stepping onto a curb or stool. If foot drop is more severe, the foot may flop down whenever they try to use the foot or ankle; it interferes with any use of the lower limb.
Because of the anatomy of the nerves in the lower limbs, foot drop can result from several conditions including a pinched nerve at the knee region (peroneal or fibular nerve palsy), a sciatic nerve lesion high or low in the posterior thigh, an abnormality of the lumbosacral plexus where the nerves cluster together in the region of the groin, or a compressed nerve in the lower back, usually the L5 root. Some patients with spinal cord disease or stroke can also have foot drop. Foot drop can develop slowly over weeks to months or may occur acutely. Foot drop is common in patients with underlying diffuse neuropathies, diabetes being the most common. Other causes of diffuse neuropathies where foot drop may arise include alcohol abuse, vitamin deficiencies, HIV infections, connective tissue disorders (such as a lupus, rheumatoid arthritis, Sjogren’s syndrome), amyloidosis, vasculitis and other causes.
In GBS and CIDP, foot drop is often bilateral as the underlying illnesses affect both sides of the body and are relatively symmetric. In Guillain-Barré syndrome (GBS), foot drop may be seen within the first few days of the illness and may persist for months if the GBS is severe and protracted. In chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), foot drop may not appear for several months to years, but again tends to be symmetric. Foot drop may or may not respond to treatments with IVIG, plasma exchange, cortical steroids, or other immune suppressants.
In addition to the foot and ankle weakness, patients with foot drop may have neuropathic pain, numbness, and tingling. If only one nerve or root is the cause of the foot drop, the numbness and tingling may be present only over the anterior and lateral portion of the shin and top of the foot. If the foot drop is part of a diffuse and symmetric process, the numbness, tingling, and pain may involve the entire leg from the knee downward or even higher up the leg if the underlying process is severe and chronic.
The evaluation of foot drop should begin with a thorough neurologic examination, preferably provided by a neurologist or another physician skilled in neuroanatomy of the leg, such as a neurosurgeon, orthopedic surgeon or physical medicine doctor. Those physicians will often order nerve conduction studies and electromyography (EMG) to assist in localizing the lesion and determining if the abnormality is in the peroneal nerve only or higher up the leg in the sciatic nerve, lumbosacral plexus, or nerve root. An MRI scan of the lumbosacral spine will often be necessary, particularly if a lumbar root abnormality is suspected. Further imaging studies of the lumbosacral plexus, thigh, or knee region may be needed if the localization by examination, nerve conduction studies, or EMG suggest an abnormality in those regions.
Treatment of foot drop depends upon the cause and localization of the abnormality. Foot drop in one limb may be amenable to surgery at the level of the knee, shin, or thigh. If the foot drop is due to a lumbar radiculopathy (pinched nerve), surgical decompression would be necessary in most patients. An ankle foot orthosis (AFO) is commonly prescribed by physicians or therapists to mechanically bend the foot at the ankle so the patient can walk more easily without the dropped foot interfering with foot clearance. The newer types of AFOs are light and strong and permit the patient to walk with many types of shoes. In some patients, the wearing of lightweight boots that lace up to the mid shin can maintain the foot at close to 90 degrees of ankle flexion and make walking easier, similar to the benefit of an AFO.
If the foot drop affects both legs and is a complication of GBS, CIDP, or diabetes, surgical treatment would probably not be needed. In those conditions, medical treatment might improve the foot drop. Some patients with GBS or CIDP may need an AFO applied to both lower limbs to permit walking. Pain arising from the foot drop would be treated with medications commonly prescribed for neuropathic pain (nerve pain) including gabapentin, pregabalin, amitriptyline, nortriptyline, duloxetine, and other medications.