What is squint surgery?
"Squint", "strabismus" and "heterotropia" refer to a range of conditions in which the eyes are not pointing in the same direction or are otherwise misaligned. (Squints are also sometimes called 'lazy eye', to refer to a turned eye, but this is not an accurate description.)
Squints have an obvious effect on the cosmetic appearance of the individual. Moreover, because perfect binocular vision (where a single combined image is produced in the brain) depends on both eyes being properly aligned, squints may also result in double vision (diplopia), difficulties in hand-eye co-ordination and impaired sense of balance. A range of treatments for 'lazy eye', surgical and non-surgical, are available.
Squints can be classified according to the direction of the turn of the eye: esotropia (convergent) refers to an eye that turns inwards towards the nose; exotropia (divergent) refers to an eye that points outwards; hypertropia is when eye is upwards. The classification of squint is based on a number of features and is established by a detailed history and examination.
The squint, or 'lazy eye', may be present all or only part of the time, in only one eye or alternating between the two eyes. It can occur for a number of reasons:
- Refractive (focusing) abnormality
- Eye muscle imbalance
- It can run in families
- Illness - can make it obvious
- Fatigue, stress or extended work load
- Rarely, it can be due to other diseases or illness.
The aims of squint eye surgery are re-alignment of the eye muscles where necessary to achieve satisfactory function and cosmetic appearance. This can mean that the non-squinting eye may be operated on. Since some patients may recover function over time, squint surgery may not be considered initially. 'Lazy eye' patients need to be advised that, whilst accuracy in measuring and operating upon strabismus is essential, the response to squint surgery is variable and cannot be guaranteed. Detailed examination and consideration of the patient history are therefore essential as well as discussion of all the different options available.
Prisms, refractive lenses, vision therapy, eye patching and pharmacologic measures have all been used to help patients achieve fusion (alignment of the eyes) and alleviate diplopia, in addition to surgery. Some patients will adapt by suppressing the vision of one eye to eliminate their diplopia. An additional technique is the use of Botulinum toxin (BOTOX®).
Botulinum Toxin (BOTOX®)
Botulinum toxin refers to seven toxins all of which cause muscle paralysis. One of these, Botulinum Neurotoxin Type A, is now available for medical use under the trade name BOTOX®.
BOTOX® can be used to alleviate muscle spasm or to weaken a muscle for therapeutic purposes including the treatment of squints and spasm of the eyelid (blepharospasm). Its use should be reserved for patients in whom an ophthalmic diagnosis has been established with reasonable certainty. BOTOX® treatment requires proper identification of the affected eye muscles and should be conducted by a skilled interdisciplinary team with sophisticated instrumentation and electromyography. Since 1990, success rates of over 90% have been reported in medical literature and BOTOX® is now the treatment choice for muscular spasms affecting the face.
BOTOX® is effective as an alternative to surgery to realign the eyes of selected patients with congenital or acquired strabismus, and appears to be more effective in esotropia (in-turning of the eyes) of small to moderate angles than in exotropia (out-turning), vertical deviations, or large angle deviations.
How is BOTOX® used?
Injections into the target muscle are carried out by a very fine needle. Benefits begin in 1-14 days after the treatment and last on average three to four months, after which it can be repeated. All effects of the toxin ultimately reverse with time.
What are the side effects and complications of BOTOX®?
Many studies have confirmed side effects to be minor in the vast majority of applications. Side effects are generally short-lived, well tolerated, and amenable to treatment. They are related to the dose of BOTOX® administered. Persistent complications are distinctly rare, and serious side effects are uncommon. Some of the reported side effects include flu like symptoms, pain at the site of injection, mild headaches, muscle weakness. Allergy is a theoretical risk but is virtually unknown. Antibodies can be produced and if this happens the toxin becomes ineffective, but there are no other effects. The antibodies often disappear with time and treatment may be successfully recommenced after an interval. Other very low incidence of complications include droopy eye (ptosis), dry eye and photosensitivity.
BOTOX® treatment is not advised in cases of:
- Pregnancy and lactation.
- Infection in the vicinity of the injection site.
- Current therapy with a certain class of uncommonly used antibiotics, known as the amino glycosides.
- Blood clotting disorders or current therapy with drugs inhibiting blood clotting, e.g. Warfarin.
- Diseases affecting the muscles, particularly myaesthenia gravis.
- Allergy to tetanus toxoid is a potential, but not absolute contraindication.