Tibial muscular dystrophy is a form of distal muscular dystrophy in which there is weakness and atrophy (wasting) in the ankle muscles, progressing to the muscles of the shinbone or tibia in the front of the lower leg. Also called Udd distal myopathy, this type of muscular dystrophy is particularly prevalent in Finland, affecting at least one in every 10,000 people.

Causes and inheritance

Tibial muscular dystrophy is caused by a mutation in the TTN gene that carries the instructions to make a protein called titin. The TTN gene is located on chromosome 2; inheriting one mutated copy of the TTN gene from the mother or the father is sufficient to have tibial muscular dystrophy. Therefore, an affected individual has a 50 percent risk of passing on the condition to his or her children.

The titin protein plays a vital role in both skeletal and heart muscles. It provides stability and flexibility to sarcomeres — specialized structures in the cells that are responsible for muscle contraction. Mutations in the TTN gene cause the production of faulty titin protein, which is altered in structure and function. The defective titin protein interferes with the function of sarcomeres, disrupting normal muscle contraction.

Chemical signaling is an essential process that governs cell communication and coordinates various activities inside cells. Titin protein is also known to play an important role in chemical signaling between cells. Thus, mutations in the TTN gene can also affect this function.

It is not clear why the effect of mutations in the TTN gene is limited to muscles in the lower leg.

Symptoms

The severity of symptoms of tibial muscular dystrophy is governed by the type of TTN mutation and varies from patient to patient.

The onset of symptoms is usually around age 35, and the condition progresses very slowly. In patients with the mildest form of tibial muscular dystrophy, symptoms can go unnoticed.

The strength of the muscles just below the ankle can be affected by ankle muscle weakness. Tibial muscle weakness can affect the up-and-down movement of the foot. Therefore, patients with tibial muscular dystrophy may find it difficult to walk on their heels, although walking may not be severely impaired in earlier stages of the condition.

As the condition progresses, the long-toe extensor muscles, which help toes extend, can become weak. These muscles lift the toes upward. Weakness and wasting of long-toe extensors make it difficult to pick up the foot while walking, resulting in a condition called foot drop. This can cause clumsiness and difficulty walking. But despite the difficulties, most patients retain the ability to walk.

Although tibial muscular dystrophy usually affects only the muscles in the lower leg, some patients may experience muscle weakness in the thighs and other leg muscles, making walking moderately difficult. In rare instances, the condition may weaken the arm muscles.

Diagnosis

The doctor will review the patient’s medical history and family health history, assessing the symptoms to diagnose tibial muscular dystrophy. The doctor will perform a physical exam and test muscle strength.

The doctor can determine the extent of muscle involvement using tests such as electromyography (EMG), magnetic resonance imaging (MRI), computed tomography (CT), and biopsy.

In EMG, a needle electrode is inserted into the muscle being assessed to determine its response to electrical stimulation. EMG helps determine whether muscle weakness and atrophy are due to the muscle itself or an underlying nerve disorder.

Imaging techniques such as MRI or CT can be used to visualize the extent of muscle damage. Large patchy lesions on the muscles and degeneration of tibial and ankle muscles are characteristic of tibial muscular dystrophy and can be confirmed with an MRI or CT scan.

A muscle biopsy may also be taken and analyzed to reveal tibial muscular dystrophy-specific changes in the muscles.

Mutation in the TTN gene is the only known cause of this type of distal muscular dystrophy. Therefore, the doctors may recommend DNA testing to identify mutations in the TTN gene to confirm the diagnosis.

Treatment

Exercise to strengthen the ankle and tibial muscles can help slow muscle weakness. However, it should only be performed in consultation with a trained physiotherapist or occupational therapist.

Patients may be prescribed braces or ankle-foot orthosis to prevent foot drop and assist in walking. An occupational therapist should be consulted before using such devices.

In cases when foot drop is severe and corrective measures have not helped restore function, the doctors may recommend a surgical procedure called posterior tibial tendon transfer to restore muscle function.

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