Friday, April 11, 2008

Tight Spaces and Tunnels

Ever been to a rock concert where everyone is packed in tight?

I have, and the feeling can be disconcerting. The feeling of having your exterior slowly squeezed like a tube of cheap toothpaste, kneaded like a loaf of dough by a series of sharp elbows... not the highlight of the concert experience.

Nerves in the body sometimes pass through tight passages, generally fascial, fibrous, or fibroosseous tunnels such as the cubital tunnel of the elbow and the carpal tunnel of the wrist. Nerves can be compressed in these tunnels, and they don't enjoy the compression, either. The patient can experience symptoms such as pain, tingling, and motor weakness.

The tarsal tunnel is a well-known structure in the medial aspect of the ankle. The tarsus is the skeletal region between the distal tibia and fibula, and the metatarsals. Some imaginative Greek thought that this region resembled an inverted wicker basket ("tarsus" in Greek). A Dionysian romp with a bottle or two of wine might put you in the frame of mind to see an inverted wicker basket here:

Tarsal tunnel syndrome is due to compression of the tibial nerve, as it swings through the tarsal tunnel, meandering its way into the foot. Tarsal tunnel syndrome can manifest as foot pain, paresthesias, and numbness. In some cases, there can be atrophy of the foot intrinsic muscles.

There are various causes, including bony prominences, varicose veins in the tunnel, and soft tissue masses. Patients with suspected tarsal tunnel syndrome are often referred for MRI scans. Here is a 54 year-old female with bilateral tarsal tunnel syndrome.

Axial image of the right ankle reveals an accessory muscle (red arrow) in the tarsal tunnel. Note the normal posterior tibial neurovascular bundle (green arrow).

For comparison, here is a normal ankle:

The posterior tibial tendon (yellow arrow), FDL (black arrow), and FHL (orange arrow) are well-seen, as is the posterior tibial neurovascular bundle (green arrow). There is no accessory muscle tissue.

Sagittal images from our symptomatic patient:

Note the accessory muscle tissue (red arrows) blending in with the quadratus plantae muscle (yellow arrow). This accessory muscle tissue is the flexor digitorum accessorius longus (FDAL). There are several accessory muscles that can occur in the region of the tarsal tunnel, including the FDAL, peroneocalcaneus internus and tibiocalcaneus internus (Sammarco and Stephens JBJS 1990; 72:453-454). Only the FDAL is reported to be associated with tarsal tunnel syndrome.

The FDAL has a variable origin, including the fibula, tibia, soleus, FHL, FDL and peroneus brevis. The muscle may have a single or double head. Within the tarsal tunnel, it is typically posterior to FHL, close to the neurovascular bundle (NVB). It may occasionally cross the NVB superficially. Once it exits tunnel, it inserts into the flexor digitorum longus prior to the latter's division into four tendinous slips. Alternatively, it may also insert onto the quadratus plantae, as in this case.

It's easy to miss this accessory muscle, as it is the same signal as the muscle tissue that is normally found in this area. Thus, it is prudent to specifically evaluate for the presence of accessory muscle tissue in this area in every patient.

This patient has a FDAL in the contralateral ankle as well:

The FDAL in this ankle is smaller, and the patient was less symptomatic in this foot.

Vic David MD

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