Appendix 1: Correction of Refractive Errors
Light travels in rays that are reflected from objects into the eyes. Light rays travel in straight lines from a distance of 6 metres or more. At a shorter distance they diverge as they enter the eye. When light rays meet a transparent object at an angle they bend. This is called ‘refraction’. Light rays entering the eye meet the curved cornea and bend inwards or converge. They continue to converge as they pass through each of the refractive media of the eye, the cornea, the aqueous, the lens and the vitreous, so that they are brought to a focal point on the retina (American Academy of Ophthalmology 2003/2004).
The ‘refractive power’ of the eye is the degree to which the eye is able to refract the light rays. This power is expressed in dioptres. One dioptre brings rays of light to a focus over one metre. Ten dioptres bring rays of light to a focus over one-tenth of a metre or 10 cm. The refractive power of the eye is 60 dioptres. (that of the lens is 20 dioptres and of the cornea 40 dioptres).
Long sight or hypermetropia
A long-sighted person has a short eyeball. The light rays therefore come to a focus behind the retina causing blurred vision. A long-sighted person consequently has to accommodate for their distant vision to be clear. No further accommodation is possible for near vision, so this is blurred. If a convex lens is placed in front of the eye, the light rays will converge more sharply and come to a focus on the retina. A convex lens is a spherical lens because its shape is equal in all meridians. It is known as a ‘plus’ lens.
Short sight or myopia
A short-sighted person has a long eyeball. The light rays therefore come to a focus in front of the retina. The vision is usually more blurred for distant vision than near vision as the lens can accommodate for near vision. If a concave lens is placed in front of the eye, the light rays will diverge before converging through the cornea and lens and will come to a focus at the retina. A concave lens is also spherical and is known as a ‘minus’ lens.
240
Appendix 1 |
241 |
Iris claw lenses have been placed in phacic eyes above -6.00 dioptres myopia with successful results (Menezo et al., 1995).
Presbyopia
From the age of about 45 years, the lens in the eye no longer has the ability to accommodate for near vision. The light rays therefore fall behind the retina before coming to a focus. This is known as presbyopia. Convex or plus lenses are needed to bring the image into focus on the retina. An increasingly powerful lens is required until the age of 70 years when no further deterioration in focusing occurs.
Astigmatism
The astigmatic cornea has an uneven curvature so that there is no point of focus of the light rays on the retina. A cylindrical lens placed in front of the eye with its axis corresponding to the abnormal plane on the cornea will focus the light rays. The cylindrical lens can either be concave or convex.
Most spectacles combine both spherical (plus or minus) lenses with cylindrical lenses to provide a compound lens to correct myopia/hypermetropia and astigmatism. Full information on optics is beyond the scope of this book.
Correction techniques
The main principle of correcting refractive errors is to modify the refractive power. Correction of refractive errors can be made in a variety ways listed below.
Spectacles
Spectacles are still the most widely used and are universally the safest devices. However for various reasons, some people find spectacles unacceptable and not necessarily the answer to their vision problems. For example, wearing spectacles in some working environments can be hazardous; others find glasses impractical for some sports like rugby or football; and for others they are cosmetically unacceptable.
Contact lenses
Contact lenses are more popular as they provide convenience, are reasonably safe but not risk free. Usually contact lenses can either be gas permeable or soft lenses. Soft lenses can be daily or monthly disposable or extended wear. Users of contact lenses can encounter problems such as eye infections, abrasions, chemical injury (as a result of inadvertently using contact lens cleaner
242 Ophthalmic Nursing
instead of the wetting solution), corneal ulcers and problems of over wearing their lenses. Contact lenses wearers must visit their opticians for regular eye check if problems are to be avoided. Cleaning guidelines must be strictly adhered to. However, some people cannot tolerate contact lenses either because of dry eyes, allergy to the solution or to the lenses themselves. Certain environments such as very dusty or smoky surroundings make wearing of contact lenses intolerable.
Refractive surgery
In recent years there has been an increased interest in refractive surgery. Initially this was performed for myopia, but recent advances have enabled patients with hypermetropia and astigmatism also to be treated. It is still at an early stage of development and the long-term results are unknown. There is a degree of controversy and caution about the process (Gartry, 1995).
Radial keratotomy
This operation consists of radial incisions involving 90% of the corneal thickness (Vaughan et al., 1999) near towards the limbus. Radial keratotomy is more successful in individuals who are within the myopic range of -2 to -4. Side effects such as fluctuations of vision and glare have been reported. Corneal infections as a result of delayed healing of the corneal incisions is another complication.
Laser photorefractive keratectomy (PRK)
Laser photorefractive keratectomy has largely replaced redial keratotomy. In PRK, the corena is subjected to light energy from the excimer laser to excise tissue from the cornea. A computer estimates the depth and position of the corneal tissue to be removed which will vary depending on the refractive error being treated. The energy from the laser beam is subjected to the central cornea with resultant flattening of the cornea. The Bowman’s memebrane is removed during PRK which can sometimes produce a corneal haze.
Shortcomings of the procedure are:
•Visual results are better predicted in patients whose refractive error is less than -6 dioptres than higher myopes (-6 to -10). However, higher myopes can obtain a reduction in their myopia (Carson & Taylor, 1995).
•Severe pain while the epithelium regenerates.
•Complications include: corneal haze which is significantly more in myopes greater than -10 (Carson & Taylor, 1995), regression, loss of best corrected visual acuity, night halo effects, wound infection, delayed healing and perforation.
Appendix 1 |
243 |
Laser in-situ keratomileusis (LASIK)
This is a surgical procedure to correct myopia, hypermetropia and astigmatism which utilises a microkeratome to create a corneal ‘flap’ of about onethird of the total corneal thickness. The thickness of the flap is dependent on the degree of myopia to be corrected and the individual’s corneal thickness. The flap is reflected towards the hinge. The excimer laser is focused and centered on the exposed middle layer of the cornea. When laser treatment is completed, the flap is swept back into position.
The patient is usually seen the next day following surgery to measure the visual acuity, to inspect flap position and to ensure that no signs of infection or inflammation are present. A broad spectrum antibiotic such as Ofloxacin is prescribed. However the consensus on the use of steroids does not exist.
Patient’s needs
•The patient must have detailed explanations given of the procedure itself and any complications.
•Topography investigations: the curvature of the cornea is measured in detail resulting in a coloured map of the cornea. Areas that are too flat are coloured blue and those too steep red. The ideal curvature is green.
•Preand post-operative care.
•Follow-up information.
Nursing action
•Assist/perform topography.
•Ensure the patient has a full understanding of the procedure and that he does not have unrealistic expectations.
•Give pre-operative care. Local anaesthetic drops will be instilled.
•Assist in the laser surgery.
•Give post-operative care:
a bandage contact lens will be in place (see p. 245)
ensure the patient has adequate analgesia
ensure the patient has follow-up information including understanding that if treated for myopia, he will initially be hypermetropic
Paralytic squint
When a paralytic squint occurs, the image to each eye is not focusing on the same area of each retina. If a prism is placed in front of the squinting eye, the light rays bend towards the base of the triangular-shaped prismatic lens and will cause the image to focus in the area of the retina of that eye corresponding to the area of retina in the other eye. This results in a single image being seen or binocular single vision.