In terms of comfort, increased efficiency and the number of people who benefit, the prescription of glasses is among the most valuable procedures in medicine. So do what you can to supply them. The refractive errors are: (1) Myopia (short sight). (2) Hypermetropia (long sight). (3) Astigmatism (the refractive mechanism is aspherical). (4) Presbyopia (the common long- sightedness of old age) (5) Aphakia (''no lens') after cataract extraction.
Presbyopia is part of the normal process of ageing: it appears earlier in the tropics than in higher latitudes, and is easily diagnosed and treated. It is responsible for 85% of the need for glasses.
Whereas a myopic child has to be specially fitted, because his eyes may not be the same, a presbyope can, if necessary, be left to choose the glasses which best suit him from a pile of second-hand ones. Astigmatism is more difficult to correct, but it is usually so mild that it needs no correction. In the developed world, its correction is often overemphasized.
The visual acuity of all patients with refractive errors improves when they look through a pin-hole, which uses only their central vision. This is the basis of the pin-hole test (24.1).
REFRACTIVE ERRORS If a patient has astigmatism or hypermetropia, he needs retinoscopy, which you will not have been trained to do, so refer him.
If he cannot see at a distance but can read, he is myopic. Test each eye alone with minus spherical glasses starting with [mi]0.5 dioptres, and increasing in 0.5 dioptre steps. The smallest minus number that gives him the best acuity is the prescription he needs.
If he is a child with reduced visual acuity due to refractive errors or squint, refer him.
If he is aphakic after a cataract extraction, try him with [+]10, 11 or 12 lenses for distance, and [+]13, 14 or 15 lenses for reading.
DIFFICULTY READING [s7]PRESBYOPIA If a patient complains of difficulty reading books or newspapers, (1) Ask him his age. If he is under 40 he does not need reading glasses for presbyopia, so explain this to him. (2) Measure his visual acuity. If it is not good, see Section 24.4, otherwise proceed. (3) Examine his eyes to make sure they are white, and that he has no serious problems. If they are otherwise normal, you can give him glasses.
TO PRESCRIBE READING GLASSSES give him those with the lowest number (the weakest pair) which will let him read easily. A useful guide is: age 45 [+]1.00, 50 [+]1.50, 55 [+]2.00, 60 [+]2.50, 65 [+]3.00. If he is choosing from a pile, he needs the weakest satisfactory pair (usually [+]2.00). Explain that they are only to be worn for reading and other close work, not for seeing in the distance.