Dry eye symptoms are common but a thorough examination should include measuring the amount of tear production and quality of tears, as well as a clinical examination to evaluate the condition of the tissues.
Measurement of tear film stability
The normally functioning eye has a tear film that is continuously available and blinking maintains the tear film. In patients with dry eyes, the tear film is unstable and it breaks up faster. Therefore, the tear break-up time is shorter. You can demonstrate the symptoms by forcing your eye to be open and you will have a feeling of dryness in your eyes, forcing you to blink or close your eyes.
Tear Break Up Time (BUT)
This measures the interval between the individual’s last complete blink and the break-up of his or her tear film.
This simple test involves the use of fluorescein and a slit lamp.
A BUT lower than 10 sec suggest a dry eye
BUT can be measured with a keratograph now, removing the need for fluorescein, and it is called NIBUT. A value of less than 15 seconds suggest of a dry eye.
Schirmer is a tear volume test that measures the quantity of dry eyes but does not reveal the quality of the tears. In rare cases the amount of tears in insufficient, but in most cases the question is about the quality of tear production. A value below 5 or 6 suggests lack of tears, 6-10 is borderline and above is normal.
Height of Tear Menisci
this can be measured simply with a slit lamp by adjusting the height of the light to match the height of the tear film on the lower lid
A height of less than 0.2mm suggests dry eyes.
Staining serves as an indicator of the health of the ocular surface.
Fluorescein is the most widely used method for staining the cornea and conjunctiva and the result is evaluated with a slit lamp.
A Rose Bengal test identifies devitalized tissue. It is best done 3-5 minutes after installing the drops. A pattern of interpalbebral cornea and bulbar conjunctiva is typically seen with a deficiency of aqueous tear secretion.
Lissamine green stains the dying cells of ocular surface and the result is observed with a slit lamp.
In dry eye conditions the strength or osmolarity of tears is increased and is called hyperosmolarity. It can be tested with a device that collects a nanoliter of tears and analyzes the osmolarity of tears. Normal values lie around 304mOsm/kg whilst values over 320mOsm/kg suggest a dry eye condition.