Muscle weakness of the tibialis anterior muscle can cause the foot to drag when walking. The tibialis anterior picks the ankle up and pulls the foot toward the face. Degenerative conditions of the lumbar spine such as disc herniation or spinal stenosis (narrowing of the spinal canal around the spinal cord) have been linked with foot drop.
It’s a fairly rare condition. Some studies have been done but no one has really explained what happens and why. Could it be prevented? Is surgery needed? How soon should surgery be done? These are the questions the authors of this study attempt to answer.
They offer us information gleaned from a review of the charts of 28 patients with foot drop from disc herniation or spinal stenosis seen in their clinic over a period of 10 years’ time. Everyone in the study had spine surgery to correct the problem. What did they learn from looking back at these patients after the fact?
The chart review recorded information on each case about a variety of possible predictive or prognostic factors. Such things as affected spinal level, duration of symptoms before surgery, presence of other health conditions such as diabetes, and type of operation done were looked at. They also paid attention to muscle strength tested before surgery and any weakness observed after surgery. As with most studies, they also collected data about the patient including age and gender.
Patients with herniated discs had MRIs done before surgery to determine the level of disc affected and the amount of damage done. The surgeon also recorded observations made during the operation. The number of nerve roots (and which ones) compressed by the herniated disc was reported. For patients with stenosis, the same process was done to discover how much the canal size was reduced by and at what levels.
For all patients, before surgery, the strength of the tibialis anterior was graded (on a scale from zero to five) between zero and three. Zero means there isn’t even so much as a twitch in the muscle. One means it’s possible to feel or see a muscle contraction but there’s no movement. A grade of two tells us the patient had full motion so long as the muscle so long as there wasn’t any gravity and or resistance. A grade of three means there was full motion against gravity but only if there was no resistance to the movement.
The reason the mechanism behind a loss of tibialis anterior strength is difficult to determine is because there isn’t just one nerve that goes to this muscle. It appears from other studies that most patients have footdrop when the L5 nerve root is affected. But there are a fair number of people with L4 nerve root irritation or compression who also develop foot drop. And sometimes the S-1 nerve root is affected, too.
So, here’s what they found about each group in this study. Most of the patients with herniated discs were affected at the L5-S1 level. Some (but not as many) patients had disc herniation at the L3-4 or L4-5 levels. More than half of the disc group had compression of multiple nerve roots (not just one). They also had a free floating piece of disc called a sequestrated fragment pressing on the nerves contributing to this multi-level phenomenon.
Most of the disc patients recovered strength of the tibialis anterior after surgery. They scored a four or five on the manual muscle test, indicating near normal or normal function. For those who still had a three or less on the manual muscle test, there was no apparent predictive factor before surgery. In other words, there was no way to tell before surgery who would recover and who wouldn’t.
As far as the group with stenosis goes, the upper lumbar levels were affected most often (L2-3, L3-4). Most often, multiple levels were involved. Far fewer patients in this group recovered their tibialis anterior strength after surgery. The strength of the tibialis anterior (and the extensor hallucis longus — the big toe muscle) was correlated with recovery. Patients with a higher muscle grade before surgery for stenosis were more likely to get full recovery compared with stenosis patients who had lower muscle test scores.
What does this all mean? The authors sorted through all the data and concluded that foot drop in patients with a herniated disc occurs most often when a piece of the disc breaks off and ends up putting pressure on more than one nerve root. Tibialis strength before surgery was a better predictor of recovery in stenosis patients. But disc patients were much more likely to recover muscle strength after surgery than stenosis patients.
This information gives surgeons a better idea of how to plan treatment for patients with foot drop from either of these two degenerative spinal conditions. Preventing permanent foot drop and restoring full function requires careful attention and early intervention.
More study is needed to determine whether earlier intervention with stenosis patients would make a difference. Since there was a significant correlation between preoperative strength and prognosis for stenotic patients, it’s possible that surgeons could use preoperative tibialis anterior strength as a guide to planning surgery.