The Dystonia Society

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The role of muscles in neck dystonia

The purpose of this article is to give a brief explanation of how some of the muscles in the neck can cause the abnormal movements and positions seen in neck dystonia. For a more simple introduction to neck dystonia, click here.

The anatomy of the neck is very complex and so the explanation will be extremely simplified. It will focus only on a few simple movements and some of the key muscles involved in them – there are other muscles that can also play a part in neck dystonia that are not mentioned here. Also, this article will discuss only muscle movement and will not cover how the abnormal messages causing the dystonia get to the muscles from the brain.

The types of neck movement involved in dystonia

When a doctor assesses neck dystonia, they use a standard definition of normal posture to compare to the abnormal posture. This is called “The standard anatomical position”. In this position, a person is standing upright and facing forwards, feet together, and arms down by the side with palms facing forward. The midline of the body is an imaginary vertical line running from the top of the head to the floor. If the head moves sideways away from the midline so that the ear approaches the shoulder, it is termed a lateral movement; hence dystonia involving this movement is called laterocollis (ear to shoulder movement). If the head twists either left or right away from the midline this is termed rotation. Dystonia relating to rotation of the neck is called torticollis (twisting of chin to shoulder) from the Latin term for “twisted neck”. If the head tilts forwards from the midline it is termed anterocollis (chin to chest), or if the head tilts backwards retrocollis (chin in the air). Neck dystonia can be seen in any of these forms or in a combination of them.

How neck muscles create neck movement

To understand how the muscles in the neck produce abnormal dystonic movements, it is necessary first to explain how muscles cause the body to move. Skeletal muscle (which is the type of muscle responsible for moving bones and joints) produces movement through its ability to contract, shorten and relax in a coordinated fashion. Think how the springs in an anglepoise lamp counteract each other and hold the light in a constant position.

At each end of the muscle it is attached to one or more bones by ligaments – the bone(s) at one end of the muscle stay still during a muscle contraction, whilst those at the other end are pulled towards them.  The end of the muscle that remains stationary is called the point of origin – while the end that moves is termed the point of insertion. Upon stimulation by the nerve cells, the muscle will attempt to bring these two ends (origin and insertion) closer together.

Neck muscles 1An example in the neck is a muscle called the sternocleidomastoid (SCM) (number 6 in the pictures); its origin is at the top of the breast bone and on the end of the collar bone. Its insertion is just behind the ear. When the SCM on the right side of the body contracts, it pulls the right ear forwards and sideways closer to the chest bone and the head starts to rotate to the left and bend forwards a little.

Muscles usually work in pairs

Skeletal muscles can only pull in one direction, so muscles usually work in pairs or paired groups. For instance, once one or more muscles has contracted and pulled the head forward, another group of muscles can then counteract that action and contract to pull the head back. This is termed agonist and antagonistic movement. The antagonist muscles work against or opposite to the agonist muscles and so counter-balance them. For instance, when the SCMs on both the left and right act together, they pull the chin to the chest. In this case, the SCMs are agonist muscles and the tilt forward of the head is the agonist movement. The antagonist muscles that reverse this movement are the trapezius (number 4 in the pictures) on the left and right working together. They pull the head back towards the standard anatomical position.

Rotation of the neck

Neck muscles 2  Neck muscles 3

The trapezius does not always work in opposition to the SCM. For instance, the trapezius and the SCM on the right side of the body can work together to cause the head to rotate to the left. These are not the only muscles that can cause head rotation to the left – for instance, the levator scapulae (number 2) on the left assists the left splenius (number 3) to complete rotation of the head leftward. Conversely, the left SCM and trapezius and/or the right levator and splenius can cause the head to rotate to the right.

Resting tone and the equilibrium of balance

The symmetry and balance of the human body usually requires a limited quantity of muscular contraction (termed resting tone) to maintain an erect posture and this is achieved by the agonist and antagonist muscles working together in harmony. An analogy is balancing a broom, upside down, in one’s hand. Once in place, relatively little action of the hand is required to keep it balanced and stable.

For instance, when agonist muscles contract to turn the head, their corresponding antagonists relax and lengthen. Once the head has turned to the right degree, the antagonistic muscles start contracting to prevent the head being twisted too far. To keep the head still without discomfort, the forces generated by the muscles pulling the neck to the left need to be in balance with the forces generated by the muscles preventing further rotation creating an equilibrium of balance.

In people with dystonia, this equilibrium of balance fails. When one muscle develops a dystonic spasm, it causes an imbalance between the forces generated by the agonist and antagonist muscles and as a result, the abnormal movements seen in neck dystonia appear.

Spasmodic torticollis

Spasmodic torticollis, the most common type of neck dystonia, causes the neck to twist away from the midline, either to the left or right side. As explained above, there are a large number of muscles that can be involved in neck rotation and the equilibrium of balance can be disrupted by activity in any one or more of them. The degree of rotation depends upon the number of muscles misperforming and the strength of the spasms. For this reason, spasmodic torticollis can appear in many different ways. (There can also be abnormal activity in some of the deeper muscles found in the neck area that have not been discussed in this article.)

Laterocollis

Laterocollis causes the head to be pulled sideways and downwards towards the shoulder. In this form of dystonia, there is excessive activity in the agonists pulling the head towards the shoulder and, at the same time, some relaxation and lengthening in the antagonists that are being stretched into this movement. It is usually the result of abnormal muscle activity in the SCM, splenius and/or levator scapulae in the side of the body the head is being pulled towards. The degree of tilt is dependent upon the number of muscles involved and the difference between the strength of the pull of the agonists and the resistance of lengthening antagonists.

Anterocollis

Anterocollis which causes the head to be pulled forward is caused by abnormal activity in muscles at the front of the neck (which can include the SCM) on both the left and right side of the body which results in the chin being pulled towards the chest. The degree of pulling is dependent upon the imbalance between the forces of the agonists in the front of the neck and the resistance of lengthening antagonists (which can include the trapezius) at the back of the neck

Retrocollis

Retrocollis, which causes the head to be pulled backwards, is the reverse of anterocollis and is caused by abnormal activity in the agonists at the back of the neck (trapezius and splenius) which creates an imbalance with the forces produced by the lengthening and resistance in the antagonists at the front of the neck.

Treatment for neck dystonia

Botulinum toxin injections are often the treatment of choice for neck dystonia. By weakening selected muscles the injector can attempt to rebalance the forces generated by the muscle pairs and so diminish the effect of the dystonia. Botulinum toxin injections can sometime have side effects – but these are usually mild and wear off after a few days. If you have any concerns about side effects, you should consult the person doing the injections.  

By observing the head movement or position, the injector can generally determine the muscles that need to be injected in each type of neck dystonia. The muscles mentioned in this piece are often injected as they are close to the surface of the body. The injector is able to feel them to determine the location of the dystonia - as muscles affected often feel more bulky than non affected muscles. Alternatively they can use an Electromyography (EMG) machine to measure muscle activity.

As the pictures above illustrate, some of the muscles affected are long so injections need to be done at selected points along the length of the muscle. There are often also deeper muscles contributing to the abnormal positions and it is more difficult to target and inject these, which is one reason the response to the botulinum toxin injections can be varied.

 

Last reviewed August 2012

Disclaimer
The Dystonia Society provides the information on this page as general information only. It is not intended to provide instruction and you should not rely on this information to determine diagnosis, prognosis or a course of treatment. It should not be used in place of a professional consultation with a doctor.

The Dystonia Society is not responsible for the consequences of your decisions resulting from the use of this information, including, but not limited to, your choosing to seek or not to seek professional medical care, or from choosing or not choosing specific treatment based on the information. You should not disregard the advice of your physician or other qualified health care provider because of any information you receive from us. If you have any health care questions, please consult the relevant medical practitioner.

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