Ametropias (vision issues)
Definition of vision disorders
Ametropias can be combined. It is possible to be myopic and astigmatic and presbyopic, or hyperopic and astigmatic and presbyopic. However, it is not possible to be both myopic and hyperopic at the same time, as the eye cannot be both too long and too short.
Vision assessment can be performed by an ophthalmologist or an orthoptist. The examination takes place in several steps.
Refractive errors, or “ametropias,” are all due to different anatomical abnormalities of the eye:
- A myopic eye is too long.
- A hyperopic eye is too short.
- Astigmatism is due to an abnormality of the cornea, which is not spherical but deformed along one axis, oval like a rugby ball.
- Presbyopia is due to a loss of the ability to accommodate (focus) for near vision.
- An eye without any refractive error is called emmetropic.
Step 1 – Assessment of ametropia
We begin by performing various measurements using the TONOREF3 autorefractor:
- This device approximately measures the patient’s refractive error (myopia, astigmatism, hyperopia).
- The machine also measures the shape of the cornea (keratometry), which allows analysis of astigmatism.
- We measure intraocular pressure.
- Finally, we measure corneal thickness (pachymetry).
Step 2 – Diagnosis of ametropia
The examination continues with measurement of distance and near vision, with and without correction.
- We record vision using a measurement scale. In France, we most often use the Monoyer chart for distance vision, which measures visual acuity in tenths of a minute of arc: a visual acuity of 10/10 corresponds to the ability to distinguish two points separated by one minute of arc at 10 meters from the patient. A vision of 10/10 is considered normal.
- For near visual acuity, we most often use the Parinaud scale, with normal vision at Parinaud 2 (P2), and sometimes the Rossano scale.
- We assess the patient’s vision by correcting the ametropia approximately measured by the autorefractor. We refine this measurement using vision tests and trial lenses, thus determining the optimal and comfortable optical correction required to achieve the best visual acuity.
- When the patient is examined with the autorefractor, they accommodate, which can distort the measurement of the refractive error. Children accommodate a lot (so the measurements are often unreliable), while adults over 60 accommodate little (they are presbyopic and can no longer accommodate), making the measurements more reliable.
- When the patient accommodates excessively, or when the various measurements do not match the symptoms, we may instill drops that dilate the pupil and temporarily prevent accommodation. This examination, called “under cycloplegia” (using mydriaticum, skiacol, or atropine drops), allows for an accurate measurement with the autorefractor, unaffected by accommodation. Cycloplegia is frequently used in children.
- The vision exam is subjective: it is possible that the correction for the refractive error is comfortable for the patient in the office but not in daily life when trying glasses at the optician. In such cases, we perform a new measurement and may need to modify the prescription.
Types of ametropia
Myopia
Myopic vision is like looking through a magnifying glass: without glasses, the myopic eye sees distant objects as blurry and near objects as clear.
Astigmatism
The cornea has two axes, one shorter and more curved than the other, which leads to the formation of two focal points for the image. An astigmatic eye therefore sees blurry both at distance and up close, since at all distances the image is split into two different focal points.
Presbyopia
Presbyopia is the progressive loss of the ability to accommodate for near vision. It appears around the age of 40 and progresses until about age 60, when it stabilizes.
Hyperopia
Hyperopia is due to an eye that is too short: light rays enter the eye and come to a focus behind the retina, because the eye is too short.
Have a question? Ask Dr. Rambaud
This page was written by Dr. Camille Rambaud, an ophthalmologist based in Paris and a specialist in refractive surgery.
0 Comments