Felipedia

The upper and lower eyelids have many functions. They protect the cornea [clear portion out front of the eye] and the eye itself from drying out, from insults and trauma from the outside. They spread the tears (tear film) across the cornea. They produce portions of the tear film from glands along the eyelid margin and from cells in the folds of the eyelids. They determine the shape and size of the eyelid openings. They keep out the light. Finally they pump the tears out to the tear duct. Meanwhile, the third eyelid helps to spread the tear film and produces from 30-to-60 % of the watery portion of the tears. The eyelids of dogs and cats open between 10 and 14 days of age. If the eyelids open too early, tear production is not present and signs of dry eye (keratoconjunctivitis sicca) will occur unless you apply antibiotic ointment 3-to-4 times daily until tear production begins. A condition known as conjunctivitis neonatorum occurs when bacteria or viruses enter the eye through the eyelids that are closed following birth in dogs and cats. Corneal rupture and chronic scarring of the cornea and the white of the eye (conjunctiva) will occur unless the eyelids are opened by a veterinarian and the eyes are treated with appropriate medication.

Lesions of the Central Visual Pathway

Pre-chiasmal lesions
Vision	Visual deficit: either partial or total blindness results. One or both eyes can be affected
PLR	Unilateral lesions: static anisocoria in normal light with pupil of affected eye slightly more dilated. Both pupils evenly dilated in darkness. An abnormal swinging flashlight test is demonstrable. The direct and consensual PLR from the affected eye are absent
	Bilateral lesions; fixed, dilated pupils
Aetiology	Retrobulbar optic neuritis, neoplasia, trauma and optic nerve compression
Chiasmal lesions
Vision	Total lesions cause total blindness in both eyes. In complete lesions may lead to partial bilateral deficits (e.g. loss of lateral visual fields - bitemporal hemianopsia)
PLR	Complete lesions lead to bilateral fixed dilated pupils. Partial lesions produce variable effects
Aetiology	Cerebral vascular infarction (resulting in ischaemic necrosis of the optic chiasm), neoplasia (rare), inflammation and abscessation
Optic tract lesions
Vision	Vision is not always affected and it may be difficult to demonstrate visual defects in other cases. Bilateral involvement is rare
	Unilateral lesions; loss of medial visual field in eye ipsilateral to lesion, loss of lateral field in contralateral eye (incongruous homonymous hemianopsia). Field loss is greatest in the eye contralateral to the tract lesion. Hemisensory and hemimotor defects affecting the side of the body contralateral to the lesion may also be present
PLR	Proximal (rostral) two-thirds; unilateral lesions, static anisocoria in normal light. An afferent pupillary defect may be seen in the eye contralateral to the tract lesion. Distal (caudal) one-third: The afferent pupillary fibres leave the optic tract, so lesions of the distal one-third produce no defect of PLR.
Aetiology	Space-occupying lesions, inflammation and abscessation, vascular infarction and ischemia. Most cases are unilateral but severe inflammation may produce bilateral involvement.
Lesions of the lateral geniculate nucleus, optic radiation and occipital cortex
Vision	Homonymous hemianopsia as described for unilateral optic tract lesions
PLR	Unaffected, as described for unilateral optic tract lesions affecting the distal one-third of the tract
Aetiology	Unilateral lesions; space-occupying lesions (e.g. neoplasia, haemorrhage), inflammation (encephalitis, meningoencephalitis) and abscessation, trauma, vascular infarction. Bilateral lesions; space-occupying lesions, inflammation (encephalitis) and abscessation, trauma, toxins (e.g. lead poisoning, hepatoencephalopathy)