PERRLA

Measuring of pupil size with PERRLA
PERRLA: Pupils Equal and Round; Reactive to Light and Accommodation
Pupils Equal and Round:




The pupil size is measured in scoptopic condition. The pupils are dilated and diameters of 4.8±1.0mm are observed for both eyes under low light condition.

The pupil size is measured in photopic condition. The pupils are constricted and diameters of 3.3±1.0mm are observed for both eyes under bright light condition. Both of the pupils are round and of the same diameter.

Reactive to Light:



The pentorch is shone onto the subject's eye and the constriction of the pupil is observed through the magnifier. This test is used to assess the reactivity of the pupil towards light.

The near triad test is used in observing the accomodation of the subject's eyes and two other aspects, namely convergence and miosis.

Accommodation - this is the focussing of the eye, which comes about through the change in shape of the lens inside the eye.

Convergence - this is the 'turning inwards' of the two eyes when looking at the object of regard, and it happens so that its image falls onto the fovea of each eye

Miosis - this is the decrease in size of the pupil that accompanies accommodation and convergence. No-one knows why this happens (although the mechanism is clear enough - there are neural links between the control centres of the three responses). It has the effect of increasing the depth of field of the eye, thereby reducing the amount of accommodation needed to see the object clearly, but whether this is by design or is just fortuitous is an unanswered question.