When Myopia Is Not Myopia
A 13-year-old schoolboy was brought to me because he had noticed the whiteboard becoming progressively more blurred during the past six months.
His attempts to read the letter chart confirmed that his distance vision was blurred. The retinoscope, an instrument which determines the lens power needed to correct vision without the patient having to compare the clarity of different lenses, confirmed that he was in fact myopic (shortsighted).
However, it also revealed that the optical system was distorted. The effect of this is similar to ripples on the surface of a swimming pool. Although the bottom is clearly visible when the water is clear and the surface is smooth, visibility is reduced if the surface is rippled.
This appearance is indicative of a condition called keratoconus in which the cornea, the transparent membrane in front of the iris (coloured portion of the eye), becomes pear shaped instead of its usual round (soccer ball) shape.
As the patient had a mild form of the condition it was possible to correct the vision with spectacles.
However, in more advanced stages this may not be adequate because of the distortion of the cornea. In such cases a rigid contact lens, which effectively eliminates this optical distortion, usually restores clear vision. In very advanced stages or cases where the patient cannot tolerate a contact lens, a corneal transplant may be necessary.