The nerves of the extremities run through bottlenecks or even canal-like tunnels at certain locations.
Carpal tunnel syndrome is well known. It causes severe pain in the forearm, wrist and the three and a half fingers on the thumb side, particularly at night.
Tarsal tunnel syndrome in the foot is much rarer. Even doctors often do not recognize this condition. As a result, those affected often have a long period of suffering behind them.
What is tarsal tunnel?
The tarsal tunnel is a "canal system" behind the inner ankle. It is bordered on the outside by coarse ligament systems. These ligaments run from the medial malleolus down to the heel.
Important tendons also run through the tarsal tunnel. Furthermore, one of the two foot arteries, the posterior tibial artery, for the arterial blood supply to the sole of the foot lies in the tarsal tunnel. The corresponding vein for the return flow of blood also runs through the tarsal tunnel.
Tarsal tunnel syndrome: bottleneck of the nerves in the foot
We will first look at the anatomical conditions of the foot. Thetibial nerve runs from the back of the lower leg towards behind the inner ankle. There it divides into two end branches, the ramus plantaris medialis and ramus plantaris lateralis. The term "plantaris" refers to the sole of the foot.
The nerve conducts electrical impulses
- to the periphery to the muscles and nerve fibers, and
- on the other hand to the central nervous system following irritation of the skin on the sole of the foot.
The tibial nerve initially contains all the nerve fibers responsible for the skin and muscles of the sole of the foot. This makes these functions susceptible to injuries and diseases of the tibial nerve.
Anatomy of the structures around the tarsal tunnel © Aksana | AdobeStock
Impairment of the tibial nerve can have several causes, for example
- Scarring as a result of bruising after injuries or contusions in the region of the ankle,
- chronic tendon sheath inflammation with tendon sheath swelling of the tendon systems running through the tarsal tunnel at the same time and
- unusual strain during long walks, especially with increased body weight.
However, attempts to explain the cause of tarsal tunnel syndrome often fail.
The main symptom of tarsal tunnel syndrome is pain in the sole of the foot. The skin on the sole of the toes is also affected. This pain is not necessarily dependent on the strain that has just been applied.
At the same time, the skin on the sole of the foot feels "furry" and those affected have difficulty bending and stretching their toes. Tarsal tunnel syndrome therefore causes sensory disturbances and motor impairments at the same time.
For the doctor, this points to the tibial nerve, which is made up of sensory and motor nerve fibers.
However, motor impairments in the foot are not as noticeable as in the hand, for example. Those affected therefore notice pain in the sole of the foot, electrifying sensations and a furry feeling in the sole of the foot much earlier.
The transmission of electrical impulses via the tibial nerve serves, among other things, to signal pain after an injury to the sole of the foot.
These nerve fibers can be electrically stimulated from the outside. The neurological specialist uses this to diagnose tarsal tunnel syndrome.
Electrical stimulation for the diagnosis of tarsal tunnel syndrome
The doctor applies an electrical stimulus to a specific point on the lower leg, which the patient finds quite unpleasant. After a few fractions of a second, it triggers a reaction in the muscles of the foot. The connected computer system calculates the speed at which the stimulus travels through the tarsal tunnel.
It is advisable to carry out the same examination on the other leg in order to make a side-by-side comparison. Slowing of nerve conduction signals a functional impairment, for which there are also microanatomical explanatory changes to the nerve fibers.
Carrying out such nerve conduction velocity measurements is the most important investigative tool for providing evidence of tarsal tunnel syndrome.
Imaging procedures for the diagnosis of tarsal tunnel syndrome
Imaging procedures, such as magnetic resonance imaging(MRI), cannot detect a pure bottleneck syndrome such as tarsal tunnel syndrome.
Tarsal tunnel syndrome surgery must achieve two goals:
- the severing of the ligaments that delimit the tarsal tunnel to the outside between the inner ankle and the heel,
- to open the passage of the medial and lateral plantar ramus through the plantar tendon plate to the inside of the sole of the foot.
The latter target is particularly important if foot strain is a possible cause of tarsal tunnel syndrome. Repeated local pressure in the sole of the foot, for example during frequent walking, can trigger the bottleneck syndrome.
For anatomical reasons, a longer incision is required for tarsal tunnel syndrome surgery.
The surgeon first identifies the tibial nerve and the posterior tibial artery above the tarsal tunnel . He then cuts through the rough ligament structure(flexor retinaculum) covering the tarsal tunnel in the direction of the periphery or sole of the foot.
This initial exposure just above the tarsal tunnel also protects against getting into the accompanying tendon canal. This would entail the risk of the tendons slipping around the medial malleolus and losing their effect.
The doctor must not injure the vital arterial accompanying vessel and the lateral vascular branches. The one or two veins with cross-connections that accompany the nerve must also be preserved during the tarsal tunnel syndrome operation.
This type of tarsal tunnel syndrome surgery is time-consuming and must therefore be performed under general anesthesia for the patient.
Currently, only open surgery is possible. It remains to be seen whether endoscopic, minimally invasive surgical methods will prevail in the future. These would also have to
- guarantee the patency of the accompanying arterial and venous vessels in the tarsal tunnel and
- ensure the opening of the nerve entry points through the plantar tendon plate.
The open procedure does not require a microscope. Opening the inside of the nerve under the microscope with fine micro-instruments (microsurgical neurolysis) can have an aggravating effect on pain.
Occasionally, doctors treat tarsal tunnel syndrome by infiltrating it with crystalloid cortisone suspensions. According to the new interdisciplinary guidelines of the individual specialist societies, this is an approved treatment for carpal tunnel.
In the case of tarsal tunnel syndrome, the important vessels accompanying the nerve must be taken into consideration. Bleeding with subsequent hematoma-induced scarring of the tibial nerve must not occur. Such injections should therefore be regarded as problematic.
Pain medication is largely ineffective in tarsal tunnel syndrome. This also applies to a new generation of antiepileptic drugs (gabapentin).
Specific follow-up treatment is not absolutely necessary. However, the patient must rest the sole of their foot. This includes walking on the affected foot as little as possible for a few days and rolling it over the forefoot. The patient is given a walking aid for this.
However, this rest is not possible for long because the use of the muscle press in the lower leg is the most important prevention against venous thrombosis. The patient should therefore be given subcutaneous heparin until normal weight-bearing.
There is no specific postoperative physiotherapy after a tarsal tunnel syndrome operation.
Complications mainly occur if the structures to be protected within the tarsal tunnel are injured. Injuries to one of these parts of the system inevitably lead to impairments or even malfunctions.
Wound closure should be secured by drainage for at least 24 hours in order to avoid the risk of a new hematoma in the event of unwanted secondary bleeding.
The main problem after tarsal tunnel syndrome surgery is that the patient strains the surgical area with every step:
- When stepping, they squeeze the two nerve branches radiating into the sole of the foot
- When rolling over the forefoot, the entire surgical region is stretched longitudinally each time.
The very soft, sensitive nerve fibers are repeatedly impaired by the weight load with each step. As a result, the chances of nerve recovery are worse than with surgery on the hand, despite the most careful tarsal tunnel syndrome surgery.
Another risk worth mentioning is the difficulty in making a diagnosis. There are numerous problems in the sole of the foot that can cause pain and have nothing at all to do with tarsal tunnel syndrome. Only nerve conduction velocity measurements by a neurologist can provide the decisive clue.
A patient will also be dissatisfied if a pain-producing heel spur remains despite an accurate diagnosis and careful surgery. It is therefore one of the risks that other pain-inducing changes may also be present in combination.
Tarsal tunnel syndrome is a rare condition that not every doctor is aware of. Neurological impairments affect the skin and muscles throughout the sole of the foot. The diagnosis is based on nerve conduction velocity measurements by the neurological specialist. Treatment is surgical. The tunnel system to be opened is more extensive than comparable systems in the hand.
The localization of the operation is problematic due to the subsequent strain on the standing or walking patient. Therefore, the chances of improvement are always impaired by the subsequent crushing of the nerves with every step.
Despite these limitations, only careful opening of the entire tarsal tunnel can help - there are no effective alternatives.
