What is Strabismus?
Strabismus, also known as a squint, is the condition where the two eyes are not properly aligned. There are different types of strabismus:
- Esotropia (eye/s turned inward).
- Exotropia (eye/s turned outward).
- Hypertropia (one eye deviated upward).
- Hypotropia (one eye deviated downward).
It may present as a constant deviation, or may manifest intermittently.
As the eyes are not aligned, two different images are perceived by the two eyes, which leads to double vision and confusion. In some patients, the brain adapts by suppressing the image seen by the weaker/non-dominant eye. If left untreated early in life, the visual pathway and cortical connections in the brain for normal visual function will not develop optimally, and a condition named amblyopiaAmblyopia is commonly known as 'lazy eye'. The eye appears healthy, but visual images are not properly recognized. It is the consequence of a developmental abnormality in the visual cortex of the brain, caused by suboptimal stimulation of the retina due to poor image projection onto it. Causes for this include a high uncorrected refractive error, a large difference in refractive powers of the eyes, an opacity in the optical system of the eye, or even a crossed-eye. Amblyopia should be treated before the age of seven to avoid permanent vision development problems. (a ‘lazy eye’) sets in. This occurs because the brain will usually suppress the ‘misaligned’ image as seen by the deviating eye.
Once an individual is older than 8 years of age, the visual pathway is mature, and development errors (like amblyopia) will remain for the rest of that person’s life, even if the squint is fixed at a later stage. It is therefore of the utmost importance to treat strabismus as early as possible (preferably before the age of 6 years). If strabismus is treated in time, it is possible to avoid amblyopia by intensive patching of the ‘good eye’, allowing the ‘lazy eye’ to be stimulated to function at a normal or near normal level.
Any condition that may lead to an eye not perceiving a well focussed image on the retinaThe retina is the receptor of light at the back of the eye. It fulfils the same function as the film in a film camera or the image sensor in a digital camera. The retina translates the images into electrical signals that are sent via the optic nerve to the visual cortex in the brain, where it is interpreted as the images we see. See Info on Eyes – Anatomy. early in life may lead to amblyopia. It is therefore important to note that there are also other causes of amblyopia, which includes a unilateral high refractive error, or a droopy eyelid (ptosis).
Causes of Strabismus
- Inadequate development of the central nervous system.
- Brain injuries/tumours.
- Problems with the extra-ocular muscles.
- Intra-ocular lesions, like cataractA cataract forms when the natural lens in the eye is, or is starting to become, opaque. If not treated, it can lead to blindness, which in most cases can be treated. See Cataract Centre – Understanding Cataracts. or retinal tumours.
- Damage to the nerves innervating the eye muscles.
- Systemic diseases like Cerebral Palsy, Down syndrome, Hydrocephalus, Graves disease (thyroid disease).
- High hypermetropic refractive error.
The management of strabismus may be divided into two broad categories:
- Correction of the squint.
- Prevention/treatment of amblyopia.
The correction of the squint may be addressed with spectacles, surgery, or a combination thereof.
Spectacles are provided to correct the concomitant refractive error, should this be the principle reason for the squint. In some cases, the squint is partly the result of a refractive error, and partly due to a muscle imbalance. In these patients it is necessary to provide spectacles for the correction of the refractive error, as well as to perform a surgical alignment of the eyes.
It is often required to perform surgery to re-align the eyes. The best outcome following surgery is achieved by first treating co-existing amblyopia, thereby ensuring that the operated eye will be forced into alignment by the clear image it perceives after being treated for amblyopia.
During surgery, the eye muscle(s) are either shortened or their attachment to the globe is moved by the required amount to re-align the eyes. In complex cases, it is sometimes required to perform a secondary procedure to correct a residual squint after the initial surgery.
Treatment of Amblyopia
The successful treatment of amblyopia require a positive attitude and perseverance from both the patient as well as the parents. As noted above, the better eye is patched, forcing the ‘lazy eye’ to become functional. In the early stages of treatment, this is frequently unacceptable to both patient and the parents, as the vision in the affected eye is often far below functional levels, leading to difficulty in everyday tasks.
However, with disciplined patching, the vision usually recovers within a month to levels that would allow normal functioning. This is often an invaluable service offered to the child by the parents, as it is possible to restore the vision in a non-functional, legally blind eye to normal levels.