Strabismus is a misalignment of the eyes. The condition occurs in 4% of the population for both children and adults. There are several types of strabismus:
- Esotropia - eye turns in toward the nose
- Exotropia - eye turns out laterally
- Hypertropia - eye turns up
- Hypotropia - eye turns down
Strabismus is often referred to in general terms as a "lazy eye", but this term is also applied to eyes with poor visual development (amblyopia).
While esotropia can occur due to a variety of factors, a moderate to large degree of uncorrected hyperopia (far-sightedness) is a common cause. This type of strabismus is referred to as accomodative esotropia or refractive esotropia. If far-sighted, a child's eyes must accomodate in order to focus and see clearly. A large amount of accomodative effort can cause a child's eye to "cross." As a general rule, the more farsighted a child is, the greater the accomodative effort and the likelihood of crossing.
Treatment of accomodative esotropia requires a glasses prescription to correct the refractive error. When glasses are worn full-time, the eyes can relax from doing the extra work that had been required to make images clear. As a result of less accomodation, many children will stop crossing as long as they wear their glasses. Some children require bifocals in addition to the initial glasses prescription. These children have particularly high accomodative amplitudes and cross more when looking at objects near them. In such cases bifocals may fully correct a child's alignment for both distance and near. Surgery is only indicated if glasses do not adequately align the eyes. Even in surgical cases, glasses are often still required after surgery to maintain eye alignment, although possibly at a reduced strength.
Non-accomodative esotropia is defined as eye crossing unrelated to refractive error. For non-accomodative esotropia, prism glasses or surgery will often improve the overall alignment of the binocular fields.
Exotropia refers to the eyes drifting apart from each other. Many children with exotropia initially have an intermittent type. Intermittent exotropia begins with occassional or infrequent drifting of the eyes apart with fairly good control. Children with intermittent exotropia may be able to volutionally return the eye to the correct position. The eye misalignment often occurs between the ages of 2-4 years old, but 75-80% of cases worsen as the child becomes older. Symptoms of exotropia include eyes drifting outward, light sensititivity, and decreased vision. Glasses to correct a child's refractive error and/or glasses with prism may be helpful in early, mild cases. However, many children eventually decompensate to the point that surgery becomes the only effective option for improving ocular alignment. Surgery may also benefit the development of depth perception and binocular vision, and it may reduce the chance of strabismic amblyopia.
Hypertropia and Hypotropia
Hypertropia or hypotropia refers to the vertical misalignment of the eyes. Hypertropia occurs when one eye is elevated too high. Hypotropia occurs when the affected eye deviates too low compared to the other eye. There are various causes for vertical imbalances including but not limited to medications, systemic diseases, and trauma. In mild cases, prism glasses may alleviate the visual problems associated with vertical misalignment. In more advanced cases, surgery may be required to improve the overall ocular alignment.
For some patients, surgery is the best option for improving their eye alignment and improving their vision. Strabismus surgery involves adjusting the position or strength of one or more of the six muscles that attach to each eye. Adjustments to the muscles that move the eyes can help the eyes move together with better alignment. The impact of strabismus surgery for any ocular misalignment may take several weeks to become clear and may not be evident in the immediate postoperative period due to the effects of swelling and healing. Strabismus surgery may result in overcorrection or undercorrection and may require further surgery in the future.
For information on strabismus in adults, please select below: