- An essential part of the clinical, neurologic [Neurology: examination - adult] and ophthalmologic examination [Neurology: vision testing - overview] of the horse.
- Afferent sensory pathway:
- Optic nerve → optic chiasm → lateral geniculate nucleus → ipsi- and contralateral midbrain occulomotor nuclei.
- Efferent pathway:
- Occulomotor nuclei → oculomotor nerve → ciliary ganglion → pupillary constrictor muscles.
- A useful neuro-ophthalmologic test of integrity of retina, optic nerve, peripheral visual pathway, central pathway to level of thalamus and pretectum, oculomotor nucleus/nerve and iridal sphincter muscle.
- Not a test of vision as pupillary light reflexes are subcortical.
- Easy to perform.
- Minimal equipment.
- A blind eye lacks a direct PLR.
- A cataract will speed up the PLR.
- A horse with a cerebral cortical lesion may be blind with a normal pupillary reflex.
- Absence of a PLR may occur due to parasympathetic pathway damage without causing blindness.
- The size and symmetry of the resting pupil is related to the amount of ambient light, emotional state of the patient (fear) and effect of any lesion of the iris, eg uveitis or drugs, eg atropine .
- Bright portable penlight, or direct ophthalmoscope.
- Focal beam illuminator.
- Darkened loose box.
The horse should be examined in dim light.
- Dim lighting.