X. Effect of Cycloplegia

Latent Hypermetropia

Normal tone of ciliary muscle is affected by the cycloplegics. When the tone is lost (Cycloplegia) the lens zonules are taut flattening the lens.

This leads to less convergence of light rays by the lens. So there is a shift of refraction of the eye to the hypermetropic side (light rays focused posterior to the previous focus).This value of hypermetropia, made manifest by cycloplegia is the latent hypermetropia.

So an emmetropic eye will become hypermetropic, myopic eye becomes less myopic and hypermetropic eye will become more hypermetropic.

As a rule the latent hypermetropia
amounts to only one Dioptre.

So when we do retinoscopy under cycloplegia the values we get are to be corrected for this hypermetropic factor. I call it ‘Cycloplegic Factor’. This extra power of converging (+) lens (cycloplegic factor) used to reach the point of reversal must also be subtracted from the total power.Now the Correction Factor includes Observer Factor and Cycloplegic Factor

Observer Factor + Cycloplegic Factor = Correction Factor

Total Power – Correction Factor =Patient Factor(Refractive status of the Subject’s eye).

When the observer is at 1 metre from the subject and
the cycloplegia is with atropine then the Observer Factor
is +1.0 D and the Cycloplegic Factor is +1.0 D. So the Correction Factor is +2.0 D.

Total Power – Correction Factor = Patient Factor
Total Power – (+1 + +1) = Patient Factor
Total Power – (+2) = Patient Factor (Ref.Status)

Eg. (+2) - (+2) = 0 (zero) Eye is Emmetropic
(+3) - (+2) = +1.0 Eye is 1D Hypermetropic
(+1) - (+2) = - 1.0 Eye is 1D Myopic
( 0 ) - (+2) = -2.0 Eye is 2D Myopic
(-1) - (+2) = - 3.0 Eye is 3D Myopic

The value of Total Power can be plus or minus. So for the ease of calculation instead of subtracting +2 we can add -2 with the Total Power.

Total Power + (-2) = Refraction of Patient’s eye
Eg. (+2) + (-2) = 0 (zero) Eye is Emmetropic
(+3) + (-2) = +1.0 Eye is 1D Hypermetropic
(+1) + (-2) = - 1.0 Eye is 1D Myopic
( 0 ) + (-2) = -2.0 Eye is 2D Myopic
(-1) + (-2) = - 3.0 Eye is 3D Myopic