Today i want to discuss the subject of hypermetropia, especially axial hypermetropia and its correction because i noticed that the subject is not very clear and it is more of an art than the correction of myopia.
A hypermetropic eye is a small eye, that is why we are all born hypermetropes and this hypermetropia decreases gradually in the first 7 years of life (especially first 3) then stabilizes so practically from 7-55 the hypermetropia is constant.
The uniqueness of hypermetropia is that it can be corrected by the accommodative mechanism of the eye.This correction decreases with age which makes hypermetropia less and less corrected and most hypermetropes start to experience drop of distant vision by age 30 which is a surprise to them having experienced very sharp vision throughout their 30 yrs but they do remember how near work was always troublesome. They generally hate to study.
Back to accommodation, its power in the first decade is in the range of 10 D (units) which can correct up to 8 D of hypermetropia easily and reaches 6D at 20 yrs and 3D at 30 yrs (approx). This means practically speaking that a hypermetrope of +1.0 or 2 or 3 will see 6/6 at the age of 30.
The problem of correction of hypermetropia with accommodation is that the effort of accommodation is tiring and can cause many symptoms some are acute as headache, nausea and ocular pain or more chronic as blepharitis, loss of lashes and repeated chalazia. So a hypermetrope can be practically correcting all his Error and seeing 6/6 and suffering which make it difficult to convince him of wearing glasses “Why do i need glasses if i see 6/6?”.
Scientists found that for comfortable correction of hypermetropia you should have approx 1/2 your accommodation IN RESERVE, ie you only use half of your age-determined accommodation in correcting hypermetropia.
So lets work out some examples
*a hypermetrope +5.0 at 20 years: accommodative power approx 6.0, he can use 3.0 comfortably so he should wear 5.0-3.0= +2.0 glasses.
Sounds easy? No
Because it is always difficult to know how much is the TRUE hypermetropia which can only be revealed by Repeated atropine drops which is very impractical because atropine stays for 10days but it is the Only sure way to know ALL the hypermetropia, all other ways involve a degree of guessing.
Hypermetropia has COMPONENTS
1)absolute part: is the part NOT corrected by accommodation, and is usually the part you get on the autoref without cycloplegia
2)facultative part: is the part corrected by accommodation and is the difference between what you get on the autoref before and after cycloplegia.
Eg before cyclo autoref reads +1.5= approx absolute hypermetropia. After cyclo reads +4.0, facultative is approx 4-1.5= 2.5.
Refraction scientists studying hypermetropia gave the formula of correction as Absolute+ 1/3-1/2 facultative, so in the previous example he should wear 1.5 + 2.5/2 or 3= 2.25-2.75 depending on age and amount of near work.
So practically you have 2 ways to correct hypermetropia
1)you know the total hypermetropia, you know the accommodation for age, you keep 1/2 the accommodation in reserve, use half and correct the rest by glasses.
2)you correct the absolute + 1/3 the facultative.
Lets work another example
A child at 5 yrs has 5D of hyperopia
WAY 1: his accommodation is 10, half in reserve remains 5= all his error corrected comfortably and no glasses unless he squints
WAY 2: autoref before cyclo -1.0 after cyclo +4.0 (1/3 facultative is 5/3= 1.67, remove absolute (-1.0) then glasses are +0.67 prescribed with acute or chronic complaints
Other techniques like “best tolerated plus”, or max tolerated plus mean nothing in practice