Tear breakup time

The aqueous phase of the tears comprises more than 95% of the total volmne and covers the cornea with a layer that averages approximately 7 mm thick. This layer is inherently unstable, however, and begins to thin centrally at the end of each blink.The tear film in healthy subjects has a breakup time that averages between 25 and 30 seconds. 

A single drop of 0.01% BAG can break the superficial lipid layer of the tear film into numerous oil droplets because it can interface with the lipid monolayer of the tear surface and disrupt it by detergent action. BAG reduces the breakup time of the tear film by one-half. Repeated blinking does not restore the lipid layer for some time.The inclusion of BAG in artificial tear formulations is questionable. It neither protects the corneal epithelium nor promotes a stable oily tear surface. 

Topical anesthetics can cause instability of the tear film and diminish reflex aqueous tear production. Because they disrupt the surface microvilli of the corneal epithelium, anesthetics decrease mucous adherence and can contribute to a reduced tear breakup time. Preservatives present in topical anesthetics, such as benzalkonium... 

Allergic blepharoconjunctivitis Dry eye/decreased tear breakup time Corneal anesthesia Macular edema (aphakics)... 

The ideal artificial tear would reproduce the metabolic, optical, and physical characteristics of natmal tears. Additionally, it would have a long ocular residence time and would contain therapeutic additives to treat primary and secondary damage to the eye. Supplementation of natural tears with a substance that prolongs residence time generally improves tear film breakup time (TBUT) and is superior to tear replacement fluids of low retention time. 

Lemp MA, Goldberg M, Roddy MR. Effect of tear substitutes on tear film breakup time. Invest Ophthalmol 1975 14 225-258. 

Tear breakup time (TBUT) is used clinically as a diagnostic aid in dry eye syndromes and for testing the efficacy of therapeutic approaches. Assessment of TBUT, typically defined as the interval between the last complete blink and the development of the first randomly distributed dark spot in the tear film, is commonly used to estimate tear film stability. Fluorescein can be instilled into the eye with either a pipette or wetted fluorescein strip and observed with cobalt blue excitation and with or without a yellow barrier filter for observation. Unfortimately, there is still no global standard as to how TBUT should be determined and no consensus as to appropriate cut-off values. 

Corneal integrity should be assessed before instillation of a topical anesthetic because of the epithelial disruption that may occur. For this same reason, tear breakup time should be measured before topical anesthesia. 

Signs of meibomitis include inspissated orifices of the meibomian glands (Figure 23-8A), cloudy or thickened yellow-white meibomian secretions on gland expression, frothy tear film (Figure 23-85), hyperemia, mild papillary conjunctivitis, and thickened rounded eyelid margins. SPK of the cornea and conjunctiva in the inter-palpebral space is associated with an unstable tear film evidenced by a markedly reduced tear breakup time. 

Measurement of the tear breakup time (TBUT) is one of the more common tests to evaluate the tear film (Figure 24-7).This test is most frequently done using NaFl and the cobalt filter on the slit lamp. Instability of the tear... 

Figure 24-7 Tear breakup time test. (A) Immediately after several complete bUnks, there is homogeneous tear film stained with sodium fluorescein. (B) Randomly formed dry spot signals conclusion of the test and indicates instability of the tear film.

Eyelid retraction can produce findings in several associated tests that may correlate with the onset of the ophthalmopathy. Marginal reflex distance can be used to assess upper eyelid retraction. A light sonrce is placed in front of a patient in primary gaze to prodnce a corneal reflex. This distance between the corneal reflex and the upper eyelid margin is measured. The normal measurement is 4 to 5 mm. Another possible finding is a rednction in the tear breakup time of one or both eyes. EyeUd retraction causes an increase in the ocular surfece area that must be covered by the tear film, and there is an associated decrease in blink frequency in Graves patients. An increase in tear osmolarity also affects the mechanics of tear stability in these patients. The combination of these factors affects stability of the tear film. 

Ousler GW III, Michaelson C, Christensen MT (2007) An evaluation of tear film breakup time extension and ocular protection index scores among three marketed lubricant eye drops. Cornea 26 949-52... 

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