Long-Sightedness (Hypermetropia)
Definition
In hypermetropia light rays entering the eye from a distant object are focussed behind the retina (the "light catching" tissue at the back of the eye). When the object is closer to the eye, the image moves further back from the retina and causes even worse blurring of the picture.
In children and younger adults with mild to moderate hypermetropia, the lens inside the eye can change shape to become more powerful (accommodation) and focus distant rays of light onto the retina.
Incidence/Age
Hypermetropia can occur at any age, but it is very common for newborns and infants up until around 7 years of age to have mild hypermetropia. Often people who are hypermetropic only require correction later in life when their ability to accommodate diminishes (presbyopia).
Anatomy and Physiology
There are 3 main types of hypermetropia, each having a different reason why light rays entering the eye are focussed too far back behind the eye:
- Axial hypermetropia - when the eyeball is too short
- Refractive hypermetropia - when the front of the eye (cornea) is too flat
- Aphakia - when the lens has been removed from inside the eye (usually due to cataract) but without a lens implant replacement.
Causes
There is usually no definite cause for hypermetropia, although there may be genetic factors with children occasionally inheriting long-sightedness from their parents. Rarely hypermetropia can be produced by conditions that indent and shorten the eyeball, eg, orbital tumours behind the eye.
Symptoms and Signs
The effect of hypermetropia varies considerably depending on age and severity. Classically people see quite well in the distance but less clearly for near. Mild degrees in young people are not usually associated with any visual disturbance. People who perform a lot of close work may complain of eye-strain or headaches due to an excessive focussing effort.
Complications of Disorder
Accommodation is inherently linked to convergence (ability to turn eyes inwards to look at near objects). A subgroup of children will over-converge and develop a squint (one eye points inwards) as they accommodate excessively to overcome moderate to severe hypermetropia.
Small hypermetropic eyes tend to have narrow drainage channels in the front chamber of the eye. In a small proportion of people these can become blocked later in life with a resulting large rise in eye pressure (acute glaucoma).
Tests
Hypermetropia can be detected by an optometry refraction examination using optical lenses, and children may also require an orthoptic assessment to assess for the presence of a squint. If required the type of hypermetropia can be determined by using keratometry to measure the curvature of the cornea at the front of the eye and an ultrasound test to calculate the length of the eyeball.
Treatment
Optical
Hypermetropia is usually corrected satisfactorily with spectacle lenses or contact lenses.
Surgical
EXCIMER laser treatment on the surface of the eye, initially developed for short sightedness (myopia), is now available for certain levels of hypermetropia. Secondary lens implants can usually be placed in aphakic eyes in order to avoid strong hypermetropic spectacles.
Outcome/Prognosis
The vast majority of people achieve good vision with appropriate optical correction. Older people with reduced focusing ability (presbyopia) may require 2 pairs of spectacles, bifocals or varifocals to obtain clear vision for distance and near vision. Hypermetropic children with a squint need to be monitored carefully to avoid a lazy eye (amblyopic) developing.
