Pupil- general data. Normal reactions of the pupil. Abnormal reactions of the pupil
Pupillary Reflexes:
Light reflex: When light is shone in one eye, both the pupils constrict. Constriction of the pupil to which light is shone is called direct light reflex and that of the other pupil is called consensual (indirect) light reflex. Rods and cones initiate the light reflex.
Near reflex: Near reflex occurs on looking at a near object. It consists of two components: (a) convergence reflex, i.e., contraction of pupil on convergence; and (b) accommodation reflex, i.e. contraction of pupil associated with accommodation.
Psychosensory reflex: It refers to dilatation of the pupil in response to sensory and psychic stimuli. It is very complex and its mechanism is still not elucidated.
PERRLA: Pupils Equal Round Reactive to Light and Accommodation
This abbreviation is used when testing pupillary reactions- this will be the result of a normal pupillary examination.
Abnormalities of pupillary reactions:
- Amaurotic light reflex. It refers to the absence of direct light reflex on the affected side (say right eye) and absence of consensual light reflex on the normal side (i.e. left eye). This indicates lesions of the optic nerve or retina on the affected side (i.e., right eye), leading to complete blindness. In diffuse illumination both pupils are of equal size.
- Efferent pathway defect. Absence of both direct and consensual light reflex on the affected side (say right eye) and presence of both direct and consensual light reflex on the normal side (i.e. left eye) indicates efferent pathway defect (sphincter paralysis). Near reflex is also absent on the affected side. Its causes include: effect of parasympatholytic drugs (e.g., atropine, homatropine), internal ophthalmoplegia, and third nerve paralysis.
- Wernicke’s hemianopic pupil. It indicates lesion of the optic tract. In this condition light reflex (ipsilateral direct and contralateral consensual) is absent when light is thrown on the temporal half of the retina of the affected side and nasal half of the opposite side; while it is present when the light is thrown on the nasal half of the affected side and temporal half of the opposite side.
- Marcus Gunn pupil. It is the paradoxical response of a pupil of light in the presence of a relative afferent pathway defect (RAPD). Swinging flash light test tests it.