Affections of retina and refraction of eye
Affections of retina and refraction of eye in animals are Amaurosis, Ametropia, Hypermetropia, Myopia, Astigmatism, Liquified vitreous or Synchysis scintillans, Retinal haemorrhage, Retinal detachment, Collie eye anomaly, Optic disc edema or papilledema, Optic disc atrophy etc.
Emmetropia (Normal sight)
When the refraction of the eye is normal, parallel rays coming into the eye in a condition of rest, are focused exactly on the retina. This condition is known as emmetropia.
Note
The amount of divergence of light rays falling on a given area is inversely proportionate to the distance from the source of light. When the distance is 20 feet or more, the divergence is so slight that the rays can be considered as parallel.
Surgical affections of retina and refraction of eye in animals are-
- Amaurosis
- Ametropia
- Hypermetropia
- Myopia
- Astigmatism
- Liquified vitreous or Synchysis scintillans
- Retinal haemorrhage
- Retinal detachment
- Collie eye anomaly
- Optic disc edema or papilledema
- Optic disc atrophy
Amaurosis
Amaurosis is blindness without any apparent lesion in the eye. It may be temporary or permanent.
Possible causes are toxaemia, lesions in the brain, etc. (A temporary form of amaurosis is sometimes seen in cattle due to deficiency of vitamin A which can be corrected by administration of vitamin A)
Ametropia
Ametropia is a term used to denote a condition of abnormal refraction of the eye due to hypermetropia, myopia, or astigmatism, in which parallel rays are focused either in front or behind the retina.
Hypermetropia
Hypermetropia is a condition of abnormal refraction of the eye in which parallel rays come to a focus behind the retina. This type of ametropia is caused if the axis of the eyeball is too short or if the refractive power of the eye is too weak.
Myopia
Myopia is a condition of abnormal refraction of the eye in which parallel rays get focused in front of the retina. This may happen either due to the axis of the eyeball being too long or due to the refractive power of the eye being too strong. (In this condition the eye is able to see clearly only objects very close to it)
Astigmatism
When the refraction through several meridians of the eye is different, the condition is called astigmatism.
Agtigmatism may be caused by irregularities in the cornea or the lens. Astigmatism causes blurred vision. (A certain degree of astigmatism is normally present in the horse)
Liquified vitreous or Synchysis scintillans
Liquified vitreous or Synchysis scintillans is usually seen in aged patients or as a sequela to inflammation.
Vitreous
About 3/4th of the volume of the eye is occupied by a gelatinous structure called vitreous, which also gives, its shape to the eye. 98% of the vitreous is water and the rest 2% consists of collagen fibrils, Hyalocyte, and mucopolysacharides and Persistent Hyaloid Artery.
When the head is moved the freely floating bodies tends to move and settle. this condition may predispose to retinal detachment.
When the suspended particles consists of calcium lipid complex, the condition is called Asteroid Hyalosis.
Retinal haemorrhage
Retinal haemorrhage causes anaemia, thrombocytopenia, hypertension, neoplasia etc., Haemorrhage may occur at any layers of the retina.
Retinal detachment
Causes of Retinal detachment are Subretinal fluid accumulation, vitreous traction, liquefied vitreous, etc. Because of the anatomical proximity of the choroid and retina, pathologies are usually interlinked.
Collie eye anomaly
Collie eye anomaly is a congenital anomaly seen in collie breeds of dogs, characterised by choroidal hypoplasia, coloboma, retinal detachment and intraocular haemorrhage.
Optic disc edema or papilledema
Swollen optic disc with hazy margins usually caused due to vitamin A deficiency in steers or due to space occupying lesions.
Optic disc atrophy
Optic papilla will be small with attenuated blood vessels. The pupil will be dilated with the eye blind.
Cause of Optic disc atrophy are sequela to inflammatory conditions.