Refraction or Prescription
Refraction is the “clearer 1 or clearer 2” game that optometrists have played with patients for decades. It’s the art of determining the prescription for each eye based on the information we have available.
We are able to determine the refraction in numerous ways and it often depends on the age, maturity and willingness of the child.
Some of the testing is objective, meaning it doesn’t rely on any answers from your child. Some are subjective, meaning we make decisions based on responses and answers to our questions.
The best refractions are done by a combination of objective and subjective testing. Parents are often surprised at how reliable kids can be when getting their eyes tested.
Of course, achieving an accurate prescription with an experienced practitioner who enjoys testing kids is the most likely recipe for success. You will often hear plenty of giggles and smiling faces emerge after a children’s consultation with both Wes and Heidi at the Myopia Clinic Newcastle within Custom Eyecare.
Historically, it was believed that becoming more than -6.00D short sighted increased the risk of developing retinal pathology. It was then discovered that even low levels of shortsightedness have an increased risk of eye conditions affecting the retinal health. This turned the attention towards axial length as the predictor of retinal risk.
The length of the eyeball (or axial length) is the gold standard of monitoring myopia control. We know the average eye length is 23.3mm. The average length of a -6.00D eye is 26.5mm long. Although the extra 3mm in length does not sound like much, an retina that has stretched another 3mm has a much higher risk of eye disease in the future.
Axial length of 26mm is considered a defining number in the literature, whereby risk of visual impairment is considerably increased. If the axial length is more than 26mm, there is a 25% chance of permanent vision loss. The risk of permanent vision loss in an eye with an axial length of more than 30mm is 90% and 3.8% if the eye is shorter than 26mm.
It is now understood that prescription (the refraction and power of your glasses) and eyeball length (axial length) are not necessarily proportional; for a low myopic refraction may have a high axial length, and vice versa. This is why measuring the length of the eye, in addition to determining the refraction during the consultation is so important. Measuring the axial length, and monitoring for elongation is not something that is routinely done by optometrists and requires specialised equipment.
Measurement of axial length
This specialised piece of equipment is called an optical biometer, commonly referred to as biometry. The testing is quick, non invasive, and doesn’t require eyedrops. It can be easily performed on young children.
An ocular biometer was traditionally only used by an ophthalmologist prior to cataract surgery. Biometry assists eye surgeons accurately measure the eye, a vital measurement for calculating the lens power of the implant used in cataract surgery.
In 2020 we invested in a Zeiss IOL Master, the same ocular biometer used by many of the eye surgeons in Newcastle. We are the only optometrist in Newcastle to implement this technology and now use it routinely in practice. We measure axial length, or eyeball length, on all children who visit us as we know childhood and adolescence is the window for effective intervention. Establishing an accurate baseline is valuable to prevent future blindness.
Measuring axial length is a vital component of myopia control. Measuring the eye length in clinical practice can greatly assist with establishing a risk profile. It also allows us to carefully monitor progression of shortsightedness in young myopes and to customise our management strategies and treatment options for each child.