Collapsing arch in foot causes nerve compression discomfort

Conway McLEAN, DPM

I have a new case for you, and it’s one that some of you may be able to relate to. Have you ever experienced pain from the bottom of your foot? Suzie, our protagonist, did and had for years. She considered herself to be fairly normal: brought up in an average, middle-class family, worked in an office. Medically, nothing remarkable, being moderately over-weight, with mild, controlled hypertension, and that’s about it. Certainly, she was not able to be as active as she used to be, but she attributed that to the pain from her foot.

A critical part of our story is the insidious nature of her condition. This sensation, which wasn’t a “real pain” early on, was just something she lived with. Initially, it was only a mild numbness from her toes, noticed only after an extremely long day. Over the following years, it progressed in both location and intensity. The numbness eventually came to involve the whole bottom of her foot, but it wasn’t for some years that the burning started. If she spent several days relaxing, with no housework, by Monday morning, the burning had subsided considerably. The numbness remained, but that seemed trivial compared to the burning pain.

Possibly, some of the more astute or educated readers will be able to hazard a guess as to what was occurring. Most of the doctors she talked to told her she was experiencing the symptoms of neuropathy, a systemic nerve condition typically associated with diabetes. She was correctly informed there are many causes for this often-debilitating condition, and frequently no specific reason can be discovered. Her primary care doctor ran a very specific test for diabetes, but it came back negative. Another generalized test for nerve function showed no systemic condition present, meaning that it really could not be neuropathy, which naturally frustrated her immensely.

When pains began shooting up her leg with activity, she became understandably upset. Her primary care physician had no further recommendations, and so referred her to podiatry. After an extremely thorough exam and x-rays, a diagnostic injection of a local anesthetic was performed along the course of the primary nerve supplying sensation to the bottom of her foot. Shockingly, for the next 6 hours, she had complete relief of all her symptoms! It was a miracle. Suzie could walk and stand comfortably, without a shred of burning, shooting, or the pins and needles which she had experienced for years. She had forgotten what it was like to be able to stand without pain!

Although everyone seems to have heard of carpal tunnel syndrome, few seemed to have any knowledge of its corollary in the lower extremity. This discussion hinges on the effect of recurrent, chronic compression of a nerve. Nerves are an interesting type of tissue, and function via a very special electro-chemical process. Mechanical irritation of a nerve, secondary to recurrent stretching, or trauma of some kind, perhaps a tumorous growth, all these causes can lead to swelling of the nerve. When the swollen nerve passes through a narrow tunnel, such as occurs in the wrist, as well as the inside of the ankle, problems occur. They don’t function well, and will cause the individual so afflicted to experience a variety of strange and troubling sensations. Just ask Suzie what it felt like.

The condition is called tarsal tunnel syndrome, and can pose a diagnostic dilemma to the treating physician. Various studies are used, and some are more helpful than others. Everyone is familiar with simple x-rays, although the general public often is unaware that they are primarily used for bone problems, and are not particularly enlightening for soft tissue pathology. CT scans are basically a computerized 3-D x-ray, so, again, are primarily for bone problems. Everyone has heard about the wonders of the amazing MRI, which can look at most any tissue type, and it is a wonderful technology. But because tarsal tunnel syndrome is so often rooted in mechanical issues, such as a collapsing arch (aka excess pronation), no significant findings will be seen with this study. The changes that occur are on the cellular level, and cannot be appreciated with this test.

The gold standard for diagnosing tarsal tunnel syndrome is the nerve conduction velocity test, a rather unpleasant affair, where small electrical shocks are administered, and the speed at which these signals travel down the nerve is measured. Unfortunately, often this test is administered with the foot in a non-weight bearing, relaxed position, which is not when the problem occurs. As is often the case, a physician familiar with the anatomy, and, just as importantly, familiar with the mechanical functioning of this region of the body will have a high index of suspicion, aiding in making a definitive diagnosis.

Treatments are many for tarsal tunnel syndrome. Unlike carpal tunnel syndrome, we try to avoid surgery, as the rate at which complications occur with the procedure is uncomfortably high. Fortunately, conservative measures are successful a majority of the time. If excess pronation is the cause, reducing this motion will be very helpful. This can be achieved in the short term with a soft cast or walking boot, while lasting control of foot mechanics is best obtained through prescription foot orthoses. If severe enough, control of abnormal lower extremity motion requires a foot-ankle brace (which is not as cumbersome as you might envision).

Therapies to reduce inflammation can be helpful, such as therapeutic ultrasound, infra-red light treatments and electrical stimulation. Photo-biomodulation (aka the cold laser) seems to be very helpful with this condition, although several applications are usually required. Steroid injections are also beneficial in reducing inflammation, similar to oral anti-inflammatory medication. Obviously (I hope), if the problem is a physical stretching and compression of the nerve, less activity means less stress, as in resting the part!

Your symptoms may not be this dramatic, but simply consist of recurrent numbness of the toes. But nerve problems are difficult to treat: nerves do not have a great blood supply, and don’t heal particularly quickly. Letting this kind of problem go can lead to permanent damage to the nerve. We can’t work on nerves like we can bone, which can be moved around and re-routed like play-doh. If nerve symptoms start to develop on a predictable basis, it’s wise to have it “checked out”. Treatment may be as simple as a foot support, or some type of ankle brace. Living with it is not a good option. No pain, no gain? Not in this case!

Editor’s note: Dr. Conway McLean is a podiatric physician now practicing foot and ankle medicine in the Upper Peninsula, having assumed the practice of Dr. Ken Tabor. McLean has lectured internationally on surgery and wound care, and is board certified in both, with a sub-specialty in foot orthotic therapy. Dr. McLean welcomes questions, comments and suggestions at drcmclean@penmed.com.