Optometrists tend to think of myopia as a simple concept, but it is much more complex, as there are multiple forms, etiologies and sequelae, which result from the presence of myopia. While this may be a bold statement, allow me to explain in the next few pages.
A knowledge of the four categories of myopia, along with a collection of related patient data, aid in determining a patient’s diagnosis of myopia and, thus, guides recommendations for immediate treatment and long-term planning. (For more on therapies, read “Myopia Management: What Are The Options?” p.26.)
FORMS OF MYOPIA
There are four basic categories of myopia:
- Congenital myopia. This is present at birth, commonly caused by premature birth or birth defects, including cataracts microphthalmos, aniridia and megalocornea. The amount of myopia at birth remains relatively constant throughout life.
- Simple myopia. Often called “school myopia,” this is the most common form of myopia. It occurs during the natural growth of the eye. The shape and structures of the eye determine the degree of myopia, but it is due predominantly to excessive axial elongation; even low amounts can increase the risk of disease or damage to the eye.
- Acquired myopia. This is caused by a condition, such as cataracts, lenticular changes due to diabetes, corneal ectasias, subluxation of the lens and refractive outcomes post-corneal surgery, cataract surgery and refractive surgeries. Pseudomyopia may also be considered a form of acquired myopia and occurs when the accommodative system is over stimulated.
- Pathological myopia. Also known as high myopia, this results from early onset and rapid progression over the first decade of life. Genetics may play a greater role in this form. Growth of the eye causes the retina and sclera to stretch and the axial length to increase. Ocular complications impacting the sclera, choroid, retina, vitreous, optic disc and macula occur with this aggressive form of myopia because of the rapid changes. These patients have the greatest potential to benefit from myopia management.
Before utilizing any diagnostic testing, the patient history and the conversations the optometrist has with the patient and the patient’s loved ones can aid in the diagnosis. For example, demographic factors, such as ethnicity, age and a family history of myopia can provide clues. (For more on this, see “Consider Demographics,” p.30.) This information can be captured on the patient history form. Additionally, a diagnosis of any of the following systemic conditions are often associated with myopia:
- Down syndrome
- Stickler syndrome
- Marfan syndrome
- Noonan syndrome
- Ehlers-Danlos syndrome
- Pierre-Robin syndrome
Further, O.D.s may want to include questions on the patient history form surrounding the patient’s hobbies, as the amount of near work being done per day can also be a predictor for myopia, based on Figure 1.1 The more visually intensive a job or hobby, the greater the impact on myopia development.2 The same has been found for children and schoolwork, near work, computer and device use. All excessive near work is connected with a higher risk of myopia development, but the exact relationship between reading and near work activity and myopia susceptibility is complex and still poorly understood.2,3
The daily activities of a child are also important to evaluate. We know through research that one hour of time spent outdoors can help protect a child from developing myopia. Understanding daily habits and including more time outdoors should be a part of your treatment plan. However, if a child is myopic, time outdoors, unfortunately, will not slow the progression of myopia. It is hypothesized that environmental influences may affect ocular growth patterns, but more studies are needed to understand the correlation.4
Finally, if the patient complains of or the parent notes symptoms, such as squinting, sitting close to the TV, having trouble seeing far away, blurred vision, headaches caused by eyestrain, rubbing eyes or blinking a lot, these can all be indicators of the presence of myopia.
A key part of determining the myopia diagnosis is evaluating and monitoring the eye’s structure, specifically axial length and corneal radius.
Measuring axial length is more valuable today than years before because we now have more data on age-related norms, rates of change and patterns of progression. We know the rate of axial length elongation is fastest before myopia onset, in particular the year before and then slows after myopia onset.5,6
I have started measuring axial length on all my pediatric patients at their annual exams. I look for patterns, compare their number to normative data and their current prescription to assist me in determining their risk of progression and needed management strategy. These numbers provide more data that I can share with patients to help me with my case to initiate management protocols. To measure axial length, O.D.s can use A-scan optical low-coherence interferometry, A-scan partial coherence interferometry, A-scan ultrasound biometry or B-scan swept-source optical coherence tomography. If you don’t have one of these devices, consider co-managing your patients with a fellow optometrist or cataract ophthalmologist who does. For current normative data for axial length, based on averages of data in various studies, see Table 1, below.7
|GIRLS AXIAL LENGTH||BOYS AXIAL LENGTH|
Sometimes, it’s not easy to obtain cycloplegic refractions on our young patients. This is needed to understand their refractive error and risk of myopia. However the axial length and corneal radius of curvature ratio (AL/CR) is highly correlated to refractive error in myopes, more so than axial length or corneal radius alone.8 It can be used as a predictor of myopia. The corneal radius can be measured with a topographer. The AL/CR ratio is higher in myopes than it is in emmetropes or hyperopes. For an emmetrope, an AL/CR ratio of greater than 3 mm is a better predictor of myopia than axial length or refractive error.9
This is comprised of measurements of refractive error, binocular vision and accommodation. With regard to refractive error, clearly the patient’s prescription tells us about our patient’s myopia, whether the optometrist uses an auto-refractor, a phoropter to manually refract or performs retinoscopy. Additionally, I have found that testing monocular VA with and without correction is important to validate and prove consistency with the prescription. Using the guide of a -0.25 D of prescription for each line on a Snellen chart can give the O.D. a rough estimate of what the prescription should be.
Further, retinoscopy, in addition to providing a measure of the refractive error, helps in revealing ocular media opacities and pupil size fluctuations; orthokeratology as a myopia therapy has had greater effects on those who have larger pupil sizes. (See also “Myopia So Far,” by Dr. Thomas Aller, at bit.ly/myopiaaller .) Retinoscopy in a completely darkened room may be useful in the diagnosis of nocturnal myopia, although there is no proven procedure for the correction of nocturnal myopia.10
The key to diagnosing myopia in assessing the refractive error is over accommodation. For this, I have found a cycloplegic refraction is helpful to control the accommodative system. I will cycloplege my low myopes (below -1.00 D) to make sure they are not over accommodating, so I can rule out pseudomyopia.11 When refracting, I always balance and do a plus building at the end. These simple checks have made such a difference in my prescription quality over the years.
For binocular vision testing, O.D.s can employ the cover test to determine phorias; dynamic retinoscopy to look for a lag of accommodation, amplitude of accommodation and accommodative convergence/accommodation (AC/A) ratio.
Binocular vision studies provide clues as to which therapies may be best for each patient, based on our diagnosis. Some examples of patients:
- Children who have esophoria and children who have accommodate lags may respond better to myopia management with progressive addition lenses (PAL) than those who have normal binocular vision.12
- We should be cautious when putting an exophoric patient into contact lenses from glasses because their exophoria can get worse. Their glasses provide a base-in prism. Contact lenses can show an exophoric shift because this prism is no longer present.13
- Patients who have esophoria and accommodative lag are likely to improve their binocular vision status with multifocal or ortho-k contact lens wear.14
- When fitting an esophoric patient with multifocal contact lenses, neutralizing the phoria with enough add power can improve the myopic management impact by 70% reduction in axial length elongation over one year compared to traditional methods, which offer 30% to 50% reduction.15
- Those children who have lower than average accommodative amplitudes will have more success with ortho-k myopia management control than those who have normal accommodative amplitudes.16
With all the information we have gathered so far through our case history, structural and functional vision testing, optometrists can start to look for patterns that can help to predict the future of the patient’s myopic refractive error.
Note, if a patient is emmetropic or hyperopic, that doesn’t mean they are clear from developing myopia. A decrease in hyperopia by 0.75 D to 1.00 D can occur up to two years before myopia sets in.17 At certain ages, the most significant risk factor for future myopia can be the level of hyperopia present.1 See Table 2.
|AGE||LEVEL OF HYPEROPIA|
|Age 6||Less than +0.75 D|
|Age 7 & 8||Less than +0.50 D|
|Age 9 & 10||Less than +0.25 D|
As mentioned above factors, such as genetics, and time spent outdoors, among others, can all affect the risk of myopia development.
Some binocular vision predictors of myopia to develop include these cases:
- A child who has a higher AC/A ratios, typically seen with esophoria, has an increased risk of myopia development of more than 20 times within one year.18
- Higher than average accommodative lag in children may be an indication of future myopia. Accommodative lag improves as myopia onsets and may suggest that it is a feature and side effect of myopia.19
- Intermittent exotropia also has been associated with onset of myopia.20
MORE LEARNING TO COME
Myopia is complex, but there is a lot we can do to better understand the form we are dealing with, such as the role that genetics and environmental factors play in the progression of myopia, the role binocular vision plays to help predict the success of treatments, and how to better recognize the signs of future myopia in our patients.
As we all become more comfortable with asking the right questions and doing the correct tests, we will become better clinicians to treat and plan for the future success of our patients. OM
- Zadnik K, Sinnott LT, Cotter SA, et al. Prediction of Juvenile-Onset Myopia. JAMA Ophthalmol. 2015;133(6):683–689. doi:10.1001/jamaophthalmol.2015.0471
- Tokoro T. Effect of visual display terminal (VDT) work on myopia progression. Acta Ophthalmologica Supplement. 1988;185: 172–174.
- Mutti DO, Mitchell GL, Moeschberger ML, et al. Parental myopia, near work, school achievement, and children’s refractive error. Invest Ophthalmol Vis Sci. 2002;43: 3633–3640.
- Xiong S, Sankaridurg P, Naduvilath T, et al. Time spent in outdoor activities in relation to myopia prevention and control: a meta-analysis and systematic review. Acta Ophthalmol. 2017;95(6):551-566. doi:10.1111/aos.13403.
- Rozema, Jos et al. Axial growth and Lens Power Loss at Myopia Onset in Singaporean Children. Invest Opthalmol Vis Sci. 2019;60: 3091-3099.
- Tideman JWL, Polling JR, Vingerling JR, Jaddoe VW V., Williams C, Guggenheim JA, et al. Axial length growth and the risk of developing myopia in European children. Acta Ophthalmol. 2018 May;96(3):301–9.
- Fuensanta A. Vera-Diaz, Fuensanta. The Importance of Measuring Axial Length. Review of Myopia Management. http://reviewofmm.com/the-importance-of-measuring-axial-length-when-managing-childhood-myopia/ . Accessed May 2020.
- He X, Zou H, Lu L, Zhao R, Zhao H, Li Q, Zhu J. Axial length/corneal radius ratio: association with refractive state and role on myopia detection combined with visual acuity in Chinese schoolchildren. PLoS One. 2015 Feb 18;10:e0111766.
- Xian, Valencia, Kumar Pavan, et al. Axial Length/Corneal Radius of Curvature Ratio and Myopia in 3-Year-Old Children. Vision Science & Technology. 2016;5:5
- Optometric Clinical Practice Guidelines. “Care of the Patient with Myopia.” American Optometric Association, 1997, Reviewed 2006. https://www.aoa.org/Documents/optometrists/CPG-15.pdf Accessed April 2020.
- Sankaridurg P, He X, Naduvilath T, et al. Comparison of noncycloplegic and cycloplegic autorefraction in categorizing refractive error data in children. Acta Ophthalmol. 2017;95(7):e633–e640. doi:10.1111/aos.13569.
- Berntsen DA, Sinnott LT, Mutti DO, Zadnik K. A randomized trial using progressive addition lenses to evaluate theories of myopia progression in children with a high lag of accommodation. Invest Ophthalmol Vis Sci. 2012;53(2): 640–649. Published 2012 Feb 13. doi:10.1167/iovs.11-7769.
- Hunt OA, Wolffsohn JS, García-Resúa C. Ocular motor triad with single vision contact lenses compared to spectacle lenses. Cont Lens Anterior Eye. 2006 Dec;29: 239-245.
- Gifford K, Gifford P, Hendicott PL, Schmid KL. Binocular visual function in orthokeratology contact lens wear for myopia. Invest Ophthalmol Vis Sci. 2017;58:ARVO E-Abstract 2683878.
- Huang J, Wen D, Wang Q, et al. Efficacy Comparison of 16 Interventions for Myopia Control in Children: A Network Meta-analysis. Ophthalmology. 2016 Apr;123: 697-708.
- Zhu M, Feng H, Zhu J, Qu X. [The impact of amplitude of accommodation on controlling the development of myopia in orthokeratology]. Zhonghua Yan Ke Za Zhi. 2014 Jan;50: 14-19.
- Xiang, F, He M, Morgan IG. The impact of parental myopia on myopia in Chinese children: population-based evidence. Optom Vis Sci. 2012 Oct;89(10): 1487-96.
- Mutti DO, Jones LA, Moeschberger ML, Zadnik K. AC/A ratio, age, and refractive error in children. Invest Ophthalmol Vis Sci. 2000 Aug;41: 2469-2478.
- Mutti DO, Mitchell GL, Hayes JR, et al; the CLEERE Study Group. Accommodative lag before and after the onset of myopia. Invest Ophthalmol Vis Sci. 2006 Mar;47: 837-846.
- Ekdawi NS, Nusz KJ, Diehl NN, Mohney BG. The development of myopia among children with intermittent exotropia. Am J Ophthalmol. 2010 Mar;149:503-507.