Regional Veterinary Surgery

Visual function tests , Detailed ophthalmic examination

VISUAL FUNCTION TESTS

• A blind animal is nervous and is easily excitable.
• It shows anxious movements of the ears in an attempt to grasp the environment.
• It walks with the head held upwards and takes very cautions steps and has a “feeling gait”.
• During progression it stumbles on account of the inability to see obstacles on an uneven ground; and in order to avoid such obstacles it may lift the limbs unusually high (“high stepping”).
• When driven towards an object like a wall or a post, the animal may go and strike against the object because of the inability to see.
• When light is suddenly flashed into a normal eye, immediate closure of the eyelids is noticed. This is a protective reflex known as palpebral reflex. Palpebral reflex is absent in a blind eye.

Photomotor pupillary reflex (Photomotor pupillary reaction)

• This is the ability of the pupil to react to changes in light. If the eyes are normal, the pupil contracts when exposed to bright light and dilates when there is shade or darkness.
• Absence of this reflex may indicate some abnormality.
• Consensual reflex: If both eyes are visual, the flashing of light into one eye constricts both the pupils. This is called crossed reflex or consensual reflex. If one eye is blind, flashing of light into the blind eye will not induce pupillary reflex of the normal eye.

Detailed ophthalmic examination

• Naked eye examination
  • Gross abnormalities of the anterior segment of the eye can be detected by naked eye examination with the aid of artificial illumination if necessary.
• Using magnifying binocular loupe
  • The binocular loupe consists of two magnifying lenses and its use is therefore preferable to naked eye examination.
• By using ophthalmoscope
  • Ophthalmoscope is mainly used to view the fundus
  • Indirect ophthalmoscope

Indirect ophtalmoscope with condensing lens

• It is necessary to dilate the pupil. This can be brought about by instilling a solution of homatropine (2%) or tropicamide into eye about fifteen to thirty minutes before ophthalmoscopic examination.
• The ophthalmoscope contains lenses of varying powers through which the examination can be conducted. The anterior segment of the eye can be examined by using a lens ranging from +12 to +20. For observing the lens +8 to +12, and for vitreous humour 0 to +8, are required. For the fundus of the eye (retina, optic disc) – 3 or less, may be suitable.

 

Direct ophthalmoscope

 

• By using tonometer (tonometry): The intraocular pressure (IOP) can be measured by using an instrument called tonometer.
• There are two methods of tonometry, indentation tonometry using schiotz tonometer and applanation tonometry using Tonopen - Vet. 

Tonometry

• The normal intraocular pressure in the dog ranges from 16 to 30 mm of mercury. The normal IOP in man is 15 to 20 mm of mercury.

Schirmer tear test

The test can be performed with commercially available Schirmer tear strips. These strips have a notch at one end which is placed into the ventral conjunctival cul de sac. The strip is allowed to remain in the cul de sac for exactly one minute. The strip is removed after a minute and the distance the wetness have traveled down the strip is immediately measured in millimeters from a scale printed directly on the strip. Normal values in the dog are 15 to 25 mm/minute.

Naso lacrimal flush

• Irrigation of the nasolacrimal duct system
• Fluorescein dye is  instilled on the eye
• Partial or complete
• 26G cat 24G dog and 20G cattle 2 – 10 ml saline
• Cannulated through the upper punctum
• Note the passage time