INJECTION TREATMENT OF STRABISMUS

Dr. S. Kraft

Since 1978 botulinum toxin has been used an an alternative to surgery for strabismus (misaligned eyes).  The drug preparation approved for use in Canada is known as Botox^TM (manufactured and licensed by Allergan, Inc.). 

Botulinum toxin has been known to cause food poisoning and temporary weakness in humans.  However, the dose that in injected into an eye muscle is several hundred times smaller than the dose that causes poisoning.  No patient has ever had poisoning or weakness when Botox has been used to treat strabismus.

The small dose of Botox that is injected into the eye muscle causes a temporary weakness of the muscle.  The muscle recovers its strength over a period of several weeks, and the recovery is complete by 3 or 4 months after injection.  During this interval the muscle on the opposite side of the eye becomes stronger since it can pull the eye over more easily against the weakened muscle.  When the injected muscle recovers its power it is hoped that its final strength will be matched by that of the now stronger opposite muscle.  This balance of muscle power leads to a straighter eye.

To treat the eye muscle, anaesthetic drops are first placed into the eye to numb it.  A tiny amount of Botox is injected into the eye muscle through a needle connected to a small machine that helps the surgeon locate the position of the muscle.  The injection itself is usually painless, although some patients feel a slight “pressure”.  Occasionally there is a feeling of irritation (“like a hair in the eye”) at the injection site for a few hours afterward, and this subsides within 24 hours.

There have not been any side effects on the body from this treatment.  However, there are possible effects on the structures around the eye:
 

1.  The most common one is spreading of the drug within the eye socket to affect other muscles around the eye in addition to the target muscle.  If some drug enters the eyelid muscle this can cause a drooping of the eyelid (risk 1 in 7).  It can also “drip” into an adjacent eye muscle and weaken it, leading to an undesired horizontal or vertical misalignment of the eye (risk 1 in 7).  These effects wear off over periods lasting from 2 to 10 weeks and do not affect the potential result of weakening the intended muscle.

2.  In some cases the eye may become red over the injected muscle.  This redness resolves within a few days.

3.  Many patients complain of double vision for a few weeks, due to the temporary realignment of the eye while the injected muscle is still weak.  This almost always disappears once the drug wears off and the eye is straighter.

4.  There is a very slight possibility of bleeding behind the eye (1 in 250) or injury to the eyeball itself (risk 1 in 500), either of which could lead to loss of vision in the eye.

One treatment with Botox straightens the eye for at least 6 months in approximately 40 % of patients.  About 50-60 % of patients can be straightened with 2 injections.  By comparison, eye muscle surgery aligns the eye on the first attempt in about 80 % of cases.

Once the eye is straightened with Botox there is no guarantee the eye will remain straight in the long-term (over many years).  Some patients require periodic treatments every few months or years.