by Noelle La Croix, DVM, Dip. ACVO
The Schirmer Tear Test
An examination of an animal’s eye should include a thorough history, visualization of both anterior and posterior ocular structures (via ophthalmoscopy), tonometry, fluorescein staining, and the Schirmer tear test. This article will review the purpose and relevance of the Schirmer tear test.
Simple tests to estimate lacrimation (tear production) in humans were first described by the German ophthalmologist Otto Schirmer in 1903, and a veterinary modification was first described in 1962. Schirmer’s first described tear test (known as STT I) can be used to measure basal and reflexive tear production in a non-anesthetized canine, feline, or equine eye. Basal tear production is defined as the quantity of tears produced which normally lubricate the corneal surface. Reflexive tear production describes the quantity of tears produced in response to an irritant (including an inserted Schirmer tear test strip).
The eponymic Schirmer tear test strip is currently supplied as a strip of 5 mm x 35 mm Whatman no. 41 filter paper impregnated with a blue dye and marked with 1 mm gradations. A strip within a plastic sleeve is first bent at a notch 5 mm below a rounded tip. This step is done within the packaging to avoid contamination of the strip by oil from an examiner’s hands. Once released from its packaging, the bent strip’s rounded tip is then placed within the lower conjunctival fornix (in contact with the cornea) near the junction of the middle and temporal third of the eyelid (Figure 1). The test strips remain in place for exactly 1 minute. Tear production is visualized by dye migration down the strip (distance in mm) and recorded as mm/min.
Schirmer tear testing is commonly performed with minimal restraint, and without the prior application of ocular medications or stains. Sedation and/or topical anesthesia can lower tear production values and should therefore be avoided, if possible, prior to testing. The test strips should be read immediately after removal from the eye, as fluid may continue to migrate over time falsely elevating measurements. Conversely, strips can dry out after removal leading to an underestimation of tear production. Schirmer tear tests for each eye can be performed concurrently, or sequentially, without affecting their accuracy.
The normal rate of canine lacrimation established by Schirmer tear testing is in the range of 18.64 +/- 4.47 mm/min to 23.90 +/- 5.73 mm/min. It has recently been determined that these adult values are not attained until 9-10 weeks of age. Other research indicates that tear production may vary with a circadian rhythm. Tear production is maximal in dark periods and minimal in light periods. However, the maximal difference of 2.3 mm/min is most likely not of clinical significance.
A “normal” tear production is only relevant with respect to other clinical signs that an animal may manifest. Canine tear production measured as less than 10 mm/min is associated with keratoconjunctivitis (KCS), but only in the context of other signs of quantitative tear deficiency (such as conjunctival hyperemia, mucopurulent discharge, corneal vascularization, corneal fibrosis, corneal pigmentation, and/or blepharospasm). Conversely, a tear production rate of 15 mm/min cannot be assumed normal based upon this sole criterion. The Schirmer tear is only describing the quantitative production of tears. Additional testing (tear film breakup time and conjunctival goblet cell density) may reveal qualitative differences in the tears produced.
It is important to note that many pharmacologic medications and some ocular medications can affect tear production. For example, atropine lowers parasympathetic activity of all muscles and glands. Atropine can therefore decrease tear production dramatically, since tear production is dependant on parasympathetic activity. This effect of atropine should be considered whenever treating a patient with a dry eye.
Schirmer tear testing can also be used to diagnose other ophthalmic issues beyond abnormal lacrimation since it measures reflexive tear production. For example, canine tear production that exceeds 30 mm/min can be a sign of ocular irritation caused by ectopic cilia, a corneal ulceration, or the presence of a foreign body.
The Schirmer tear test is a simple but highly diagnostic tool of routine ocular examination. If you have any further questions concerning Schirmer tear testing, please feel free to consult with a veterinary ophthalmologist.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: The Schirmer tear test (STT I) performed in the left eye of a female Labrador retriever.