Managing refractive errors in primary care
Refractive errors are ocular disorders that occur when the optical power of the eye produces an object image that is not focused on the retina. Although easily correctable in developed countries, the global burden of uncorrected refractive errors is enormous, with an estimated 145 million visually impaired and 8 million blind individuals.
Audrey Abella spoke with Dr Inez Wong, senior consultant ophthalmologist and director of paediatric ophthalmology and strabismus service at the Eagle Eye Centre in Singapore, on how GPs can manage refractive errors in the primary care setting.
Refractive error types
The most common types of refractive errors are myopia, hyperopia, astigmatism, and presbyopia.
Spherical refractive errors occur when the anteroposterior diameter of the eye is too long (myopia) or too short (hyperopia) relative to the refractive power of the cornea and lens. In myopia, the focal point of an object image occurs anterior to the retina whereas in hyperopia, the focal point occurs behind the retina.
In astigmatism, there is more than one focal point due to the variability in the optical powers of the eye in various meridians or axes. Presbyopia occurs due to loss of accommodative power that comes with ageing, usually manifesting around the age of 40.
Myopia is the most common refractive error, and high myopia more than -5D is associated with significantly increased risks of potentially blinding diseases such as myopic maculopathy, retinal detachment, retinal degeneration, cataracts, and open-angle glaucoma.
Refractive errors are highly prevalent in Singapore and other urbanized Asian countries. In one local study, nearly 40 percent of Singaporean adults above 40 years were diagnosed with myopia, while almost 60 percent had astigmatism. Hyperopia was the least prevalent, occurring in 31 percent of the study population.
There has been, however, a tremendous increase in the prevalence of myopia in the last few decades, and the current prevalence in young adults in Singapore has reached 80 percent. This is likely due to higher levels of near-work exposure and lower levels of outdoor activity. Of great concern is that more Singaporean children are getting myopic at an earlier age, with a high likelihood of progression to very high levels in adulthood. Therefore, early detection is essential to allow for early implementation of control measures.
Diagnosing refractive errors
It is important to detect and treat refractive errors, as uncorrected or improperly corrected cases can lead to degradation of stereopsis and contrast sensitivity, and induce generalized ocular pain and fatigue (asthenopia).
Additionally, frequent changes in refraction may be a sign of other conditions such as cataract or keratoconus, which need to be ruled out.
The primary symptom to look out for is blurred vision, which could be for distant and/or near objects. Headaches may also signal refractive errors, which could be attributed to excessive ciliary muscle tone or prolonged squinting and frowning. Occasionally, excessive staring can lead to ocular surface desiccation causing eye irritation, itching, visual fatigue, foreign body sensation, and redness.
Diagnosis may be carried out through visual acuity testing, refraction, and a comprehensive eye examination. In primary care, improvement of visual acuity testing with a pinhole will usually indicate the presence of a refractive error.
In children, it is important to take note of frowning or ‘squinting’ when reading, or excessive blinking or rubbing of eyes as these may signal refractive errors. Young children with inappropriate visual acuity should be referred for a full examination to rule out underlying causes. They may also need instillation of cycloplegic drops (cycloplegic refraction) for accurate assessment of their refractive error. Once confirmed, the ophthalmologist can then decide whether correction is appropriate.
There are no clinical guidelines for refractive errors in the primary care setting. However, eye and vision screening is effective for identifying abnormalities in visual development, and is most effective when performed periodically. Any visual impairment detected at any age should prompt referral to an ophthalmologist for a comprehensive eye examination.
Managing refractive errors
Spectacles are the gold standard for correcting refractive errors. Spherical lenses are used to treat myopia (concave lenses) and hyperopia (convex lenses). Cylindrical lenses are used for astigmatism, while progressive or multifocal lenses are used for presbyopia.
Despite its potential to correct refractive errors, contact lenses may not be suitable for most patients. Aside from issues such as dry eyes or allergies, it requires compliance with specific instructions to avoid complications such as infectious keratitis. Furthermore, specially designed contact lenses, including orthokeratology and peripheral defocus modifying contact lenses, carry a high risk of infection.
Among the surgical interventions available for correcting refractive errors, laser-assisted in situ keratomileusis (LASIK) is the most popular. Using a femtosecond laser, a flap is created, lifted, and replaced after reshaping the cornea with an excimer laser.
Another common type of refractive surgery is epiLASIK which, unlike LASIK, does not require a corneal flap. Consequently, this saves on corneal tissue utilisation and is ideal for patients with very thin corneas. Furthermore, epiLASIK does not run the risk of flap-related complications seen in LASIK. However, the downside is slower visual recovery.
The latest technique is SMall Incision Lenticule Extraction (SMILE), a bladeless and flapless laser vision correction using only one femtosecond laser for the entire procedure. There is less corneal nerve transection during surgery, hence the reduced risk of dry eyes. Furthermore, it offers more biomechanical stability for the cornea. However, SMILE may only be performed for myopia and astigmatism, and any enhancement would require conversion to LASIK.
For patients with high myopia or thin corneas wherein laser treatment is not suitable, implantable contact lens (ICL) can be inserted behind the iris to correct the refractive status and afford the patient good vision without reshaping the cornea.
A detailed preoperative evaluation is mandatory to rule out any comorbidities and enable the surgeon to formulate a treatment plan that best suits each patient. If a patient has cataracts, the refractive error is usually treated at the time of surgery with implantation of the appropriate intraocular lenses.
Treating childhood myopia in Singapore
With regards to paediatric cases, focus is on delaying the onset of myopia. Staying outdoors at least 2–3 hours a day may be beneficial as the higher outdoor light levels appear to be linked to dopamine release in the eyes which inhibits myopia.
In terms of limiting its progression in children once diagnosed with myopia, the most effective treatment is pharmacologic intervention using muscarinic antagonists such as atropine. Daily application of low-concentration (0.01 percent) atropine eyedrops appear to have the most favourable risk-benefit ratio given its minimal side effects, and is currently the method of choice for slowing the progression of myopia in Singapore.
Refractive errors may influence the development of other ocular conditions. Myopia increases the risks of early cataracts, open-angle glaucoma, retinal detachment, and myopic maculopathy, while hyperopia is associated with a higher likelihood of developing narrow-angle glaucoma and age-related macular degeneration.
Refractive errors may be easily corrected once detected. Starting treatment among young children is imperative to prevent amblyopia and curb the progression of myopia. In adults, frequent degree change may be a sign of cataract or other pathologies that should be ruled out.
American Academy of Ophthalmology
Eagle Eye Centre
Efficacy Comparison in Interventions for Myopia Control in Children