Unilateral dangling foot: causes, symptoms, diagnosis

Unilateral foot drop may be of peripheral or central origin, and from this circumstance it is necessary to consider the various causes of this condition. The basic question - peripheral or central - is not always easy to resolve. Many patients have undergone conservative or even surgical treatment for intervertebral disc herniation, although in reality there was central monoparesis due to ischemic stroke or paralysis of the crossed legs.

I. Peripheral:

1. Compression neuropathy (cross-leg paralysis).
   1. Inflammatory or neoplastic lesions of the outer surface of the leg and Baker's cyst of the knee joint.
      1. Traumatic injury to the peroneal nerve.
   2. Iatrogenic paralysis due to incorrect intramuscular injection.
      1. Herniated disc (L5 radiculopathy).
2. Diabetic and alcoholic neuropathy.
3. Anterior tibial artery syndrome.

II. Central:

1. Ischemic infarction and brain tumor.
2. Post-ictal paresis.

The following symptoms will help differentiate between central and peripheral lesions:

Circumduction (circular movement of the leg) due to increased extensor tone indicates central paresis, which can be observed already when the patient enters the office. Excessive lifting of the leg indicates peripheral paresis.

Reflex level: a high Achilles reflex is observed with damage to the central motor pathways, a decrease or absence of the reflex indicates a disturbance in the peripheral reflex arc. When the peroneal nerve is affected or the lesion is limited to the L5 root, it is not necessary to expect a change in reflexes. The plantar extension response may be absent or unclear with central foot drop.

More difficult to assess are:

Muscle tone that is often normal and does not follow the expected pattern, with an increase suggesting central and a decrease suggesting peripheral levels of involvement. Muscle atrophy that would not be expected in acute foot drop.

Distribution of sensory disturbances, if any. The basic rule is that unilateral "stocking" type disturbances are more characteristic of a central lesion, as opposed to the well-known peripheral segmental types of disturbances.

Of course, electromyography and nerve conduction velocity testing are extremely helpful. However, in many cases, a solution can be found or suggested without additional testing.

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I. Drop foot of peripheral origin

If the peripheral nature of the lesion is established, then to determine its level it is necessary to assess whether the foot and toe drop is isolated or whether there is weakness in other muscles. The same question can be formulated in another way: whether the lesion is limited to the peroneal nerve or extends to the tibial nerve. Thus, the lesion of muscles innervated by one lumbar root or two adjacent roots can be established even before EMG, but this requires a detailed examination and anatomical knowledge. Evaluation of the onset of the disease - acute or gradual - is also very useful (see below).

Differential diagnosis includes the following conditions:

Compressive neuropathy

"Cross-legged palsy." This is a compression neuropathy of the peroneal nerve, including the superficial and deep branches, which is accompanied by sensory disturbances such as tingling paresthesia and hypoesthesia. Although the cause is repetitive pressure on the peroneal nerve just below the knee in people who have a habit of sitting with their legs crossed, the onset of weakness is usually acute. A detailed history is necessary. The same syndrome develops with prolonged forced squatting. Nerve conduction velocity testing confirms the diagnosis by revealing a conduction block at the site of injury.

There are patients who are susceptible to compression palsies, and this condition may be familial ("compression palsies"). It is necessary to ask about similar cases of acute transient weakness, for example, occurring with damage to the ulnar nerve. In order not to miss these truly rare cases, it is necessary to clarify the family history, it is advisable to study the conduction velocities of other nerves to detect a general slowing of the conduction velocity. If possible, examine the patient's relatives.

Inflammatory or neoplastic lesions of the lateral aspect of the leg and Baker's cyst of the knee joint. The peroneal nerve may be affected by an inflammatory or neoplastic process on the lateral aspect of the leg (compression-ischemic neuropathy of the common peroneal nerve of Guillain de Seza-Blondin-Walter; professional paralysis of tulip bulb diggers). The syndrome usually manifests itself with pain along the lateral aspect of the leg and foot, hypoesthesia in the area of innervation of the nerve and weakness of the peroneal muscle group. Neuroma or Baker's cyst of the knee joint is another rare cause of damage to this nerve. The first diagnostic step is to establish the level of the lesion close to the head of the fibula by neurological examination and nerve conduction velocity testing. X-ray and ultrasound examination are usually mandatory, but these additional methods can be correctly applied only when the localization is established clinically.

Traumatic injury of the peroneal nerve

Any type of knee injury or proximal fibular fracture can result in damage to the peroneal nerve, and in these cases the diagnosis is easy to make. In contrast, compression injury to the nerve from a plaster cast is often missed by the physician who does not pay attention to the patient's complaints of paresthesia and pain on the dorsum of the foot between the first and second toes, or weakness of extension of the first toe (peroneal neuropathy).

Iatrogenic paralysis due to incorrect intramuscular injection. Another example of iatrogenic injury is incorrect intramuscular injection into the gluteal region. The division of the sciatic nerve into its main branches, the peroneal and tibial nerves, sometimes occurs high enough so that only the peroneal nerve is affected. About 10% of patients experience no paresthesia or pain during or immediately after the injection, and the onset of weakness may be delayed. There is a simple way to differentiate an injury at the level of the lumbar roots from a disorder along the course of the sciatic nerve. The lumbar roots do not carry sympathetic fibers to innervate the sweat glands. They leave the spinal cord no lower than the level of L-2, and join the sciatic nerve only in the pelvis, where they go to the periphery. The absence of sweating in the area innervated by the sciatic nerve or its branches clearly indicates a peripheral injury.

Herniated disc

Unilateral foot drop may be a consequence of a herniated disc. The onset of the disease is not always sudden and painful, and the presence of tension in the back muscles and a positive Lasegue sign are not mandatory. If only the fifth lumbar root is affected (L5 radiculopathy), the knee reflex may be preserved, although all the above symptoms are present. The muscles innervated by the fifth root, however, are not identical to those supplied by the peroneal nerve. These conditions can be distinguished based on a thorough examination and knowledge of anatomy.

Diabetic and alcoholic neuropathy

Finally, it should be mentioned that there are cases of polyneuropathy, when the patient has only unilateral foot drop, while the damage to other nerves is subclinical. This is observed in diabetes mellitus and chronic alcoholism. In this case, there is, at least, a bilateral decrease in Achilles reflexes.

Muscle box syndrome (anterior tibial artery syndrome)

The name of the syndrome refers to ischemic damage to the muscles of the long extensors of the foot and toes (anterior tibial and common digital extensor muscles). They lie in a narrow channel formed dorsally by the anterior surface of the tibia and ventrally by the taut fascia. Overloading these muscles can lead to their edematous swelling. Since the fascia limits the space, the swelling leads to compression of the capillaries and, finally, to ischemic necrosis of the muscles together with ischemic damage to the anterior tibial nerve. A similar mechanism (swelling and ischemia of muscle tissue) is observed with excessive muscle tension, for example, during a game of football or during prolonged walking.

On examination, painful swelling of the pretibial region is revealed, followed by weakness of extension, which increases to complete weakness within a few hours. As a rule, there is no pulsation in the dorsal artery of the foot. The diagnosis must be established before the onset of muscle paralysis, since only surgical treatment is effective - extensive dissection of the fascia for decompression.

Lumbar plexopathy can also lead to foot drop.

II. Drop foot of central origin

Several of the described cortical and subcortical lesions may present with foot drop.

Ischemic infarction and brain tumor

Acute onset implies the development of an ischemic infarction, whereas chronic development is typical for a brain tumor. Blood pressure levels can be misleading, since hypertensive patients can also develop primary or metastatic brain tumors. On the other hand, headache and cognitive impairment may only occur at a late stage of brain tumor growth. Therefore, both alternatives should always be considered and, if possible, neuroimaging should be performed. Considering the treatment options, this measure is entirely justified.

Postictal paresis

Any transient weakness may be a postparoxysmal phenomenon in cases where an epileptic seizure (partial or generalized) has not been recognized. In these cases, serum creatine kinase levels are often elevated. Focal signs during or after the seizure should prompt a careful search for a space-occupying or vascular lesion in the brain. A search for epileptic activity on the EEG is warranted.