Eye examinations – 5 special diagnostic procedures

Most of us will see a variety of eye complaints in all species almost every week. It therefore helps to have a reliable tool kit available to help you check the eye in order to make an accurate diagnosis. The good news is that most of the diagnostic equipment required for the basic ophthalmic examination is inexpensive and readily available.

Posted: 15 December 2018

Eye examinations – 5 special diagnostic procedures


In this article, we take a quick look at 5 of the most commonly used special diagnostic procedures we should all be using.

1. Schirmer tear test

The pre-corneal tear film is essential in maintaining normal corneal health. Insufficient tear formation can lead to painful and debilitating complications, so it’s essential we are able to identify abnormalities early before any long-term damage can occur.

We can all make a qualitative assessment of the tear film by looking at the luster of the corneal surface, but in order to make a quantitative assessment we need the Schirmer tear test. The diagnosis of "dry eye" can easily be missed if this test is not routinely used.

The Schirmer tear test measures only the aqueous aspect of tears, and should therefore be carried out before you place any drops (fluorescein/local/atropine) into the eye. It’s easy to perform and produces clinically relevant results within seconds. The test strips have a numbered ruler on the strip, which is impregnated with a blue dye that travels up the strip as the tears are absorbed. 

What are the normal values?

  • Dog: 22mm +/- 4 mm/minute
  • Cat: 20mm +/- 4 mm/minute
  • Rabbit: 6mm +/- 3 mm/minute

Errors can occur

  • Lower lid only - avoid the mistake of placing the tear strip in the upper eyelid. These strips are calibrated for when the strip is placed in the lachrymal lake of the lower eyelid. By incorrectly placing the strip in the upper eyelid, you will get an abnormally low result, as there is no lacrimal lake in the upper eyelid. This could potentially lead you to make a misdiagnosis of Keratoconjunctivitis sicca.
  • Don’t extrapolate - if by accident the test strip falls out, don’t be tempted to infer a 60 second reading from a 15 or 30 second result. If you do, you’ll almost certainly get an incorrect result. The 60-second test should take a full minute to perform.
  • Beware of drugs and handling - excessive manipulation of the eyelids, applying topical anaesthaesia and adding other topical and systemic drugs (such as tranquilizers and atropine) are best avoided before the test. Interpreting the results can be made unreliable.

2. Intraocular pressure - using tonometry

Reading and understanding intraocular pressure measurements is an important part of eye examinations in many patients. Subtle elevations in the intraocular pressure can be significant, and the repeated measurement of glaucomatous eyes under medical treatment is essential to ensure the treatment you presctibe is effective. The most effective way to do this in practice is by using instrument tonometry.

Applanation tonometers, such as the ‘tonopen’, calculate the intraocular pressure from the force required to flatten (applanate) a constant area of the cornea. They tend to be accurate and easy to use. They are affordable and readily available from most veterinary equipment suppliers, and have made it much easier to diagnose and treat animals with glaucoma. Make sure that you keep the patient relaxed, as if they are stressed, the results can read high. When restraining the animals, be gentle as you open the eyelids. If you are suspicious that there was excessive eyelid pressure or patient restraint, retest.

What are the normal intra-ocular pressures?

16.8 ± 4.0 mm Hg in dogs

20.2 ± 5.5 mm Hg in cats

3. Direct ophthalmoscopy

Direct ophthalmoscopy is used more frequently in general practice than indirect ophthalmoscopy. It produces a direct optical image of the patient's eye, with the fundus image being upright and magnified.

There are however some limitations with direct ophthalmoscopy, such as;

  • Limited penetration – this is particularly true if the eye is cloudy or partially crystalised
  • Limited field of view – the peripheral fundus can be difficult to see
  • Refractive errors – these can make interpretation difficult
  • Eye movement – the animal may not keep the eye fixed in one place, limiting the time you have to check small features
  • Short focal depth – this can make it difficult to accurately check some of the smaller details
  • Risk of being bitten - the short working distance between the clinician and the patient will inevitably place the operator at risk if the animal is unpredictable or nervous.

4. Indirect ophthalmoscopy

Indirect ophthalmoscopy complements direct ophthalmoscopy. To perform indirect ophthalmoscopy a fairly bright light source is directed into the eye. A condensing lens is introduced between the light source and the eye. Incident light is condensed to illuminate the fundus. The reflected light then is condensed by the same lens to form a virtual, inverted, and reversed image between the lens and the light source.

The advantages of indirect ophthalmoscopy are;

  • Good penetration – especially if the eye is slightly cloudy
  • Large field of view – allowing most of the eye to be seen at the same time, including the peripheral fundus
  • Useful even with eye movements – the wide field of view also helps with visualising even when the eye is moving
  • Safer to use – the greater distance between examiner and patient keeps you away from nervous teeth
  • Minimal equipment - only a light source and a lens are required

There are a few disadvantages, the main one being that since is provides less magnification than the direct ophthalmoscopy, you may miss some of the finer detail.

When the indirect ophthalmoscope is used, the lens is usually held close to the cornea initially to permit observation of the ocular fundus and then moved away from the eye until the image is maximum size.

5. Fluorescein

Examination of the cornea is incomplete without using topical ophthalmic stains. Fluorescein is probably the most popular, and is used to highlight the existence or absence of corneal ulcers. For topical use, fluorescein impregnated paper strips are sometimes preferred to fluorescein solution to ensure sterility.