Tear break-up time (TBUT) also known as tear film break-up time (TFBUT) is the time taken for the first dry spot to appear on the

cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power ...

after a complete

blink Blinking is a bodily function; it is a semi-autonomic rapid closing of the eyelid. A single blink is determined by the forceful closing of the eyelid or inactivation of the levator palpebrae superioris and the activation of the palpebral portio ...

. TFBUT measurement is an easy and fast method used to assess the stability of

tear film Tears are a clear liquid secreted by the lacrimal glands (tear gland) found in the eyes of all Mammal, land mammals. Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes. The different ty ...

. It is a standard diagnostic procedure in the

dry eye Dry or dryness most often refers to: * Lack of rainfall, which may refer to **Arid regions **Drought * Dry or dry area, relating to legal prohibition of selling, serving, or imbibing alcoholic beverages * Dry humor, deadpan * Dryness (medical) * ...

clinics. The volume of tear in the eye depends on two factors, drainage through the lacrimal passages and evaporation. Factors like decreased tear production, increased evaporation rate, tearfilm instability, tear hyperosmolarity, inflammations, ocular surface damages etc. can cause dryness to the eyes. TBUT test was first described by Norn MS, and then revised by Lemp and Holly. The conventional and most common TBUT measurement method is using

slit lamp A slit lamp is an instrument consisting of a high-intensity light source that can be focused to shine a thin sheet of light into the eye. It is used in conjunction with a biomicroscope. The lamp facilitates an examination of the anterior segm ...

and

sodium fluorescein Fluorescein is an organic compound and dye based on the xanthene tricyclic structural motif, formally belonging to triarylmethine dyes family. It is available as a dark orange/red powder slightly soluble in water and alcohol. It is widely used ...

. Noninvasive instruments (Oculus Keratograph 5M, Germany; K5) are used for automatic non-invasive tear breakup time (NIBUT) measurements. In non-invasive procedure, a grid or concentric ring pattern is projected onto the cornea and the patient is asked to blink. The rings will appear distorted when the cornea becomes dry. Tearscope is a commercially available instrument used to measure NIBUT.

Clinical significance

Tear film break-up time is abnormal in several conditions like aqueous tear deficiency,

keratoconjunctivitis sicca Dry eye syndrome (DES), also known as keratoconjunctivitis sicca (KCS), is the condition of having dry eyes. Other associated symptoms include irritation, redness, discharge, and easily fatigued eyes. Blurred vision may also occur. Symptoms range ...

, mucin deficiency, and

meibomian gland Meibomian glands (also called tarsal glands, palpebral glands, and tarsoconjunctival glands) are sebaceous glands along the rims of the eyelid inside the tarsal plate. They produce meibum, an oily substance that prevents evaporation of the eye ...

disorders. Evaporative dryness can also be seen in conditions like

lagophthalmos Lagophthalmos is the inability to close the eyelids completely. Cline D; Hofstetter HW; Griffin JR. ''Dictionary of Visual Science''. 4th ed. Butterworth-Heinemann, Boston 1997. Blinking covers the eye with a thin layer of tear fluid, thereby p ...

proptosis Exophthalmos (also called exophthalmus, exophthalmia, proptosis, or exorbitism) is a bulging of the eye anteriorly out of the orbit. Exophthalmos can be either bilateral (as is often seen in Graves' disease) or unilateral (as is often seen in ...

Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...

computer vision syndrome Computer vision syndrome (CVS) is a condition resulting from focusing the eyes on a computer or other display device for protracted, uninterrupted periods of time and the eye's muscles being unable to recover from the constant tension required to ...

contact lens Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic ...

use, drugs (

antihistamines Antihistamines are drugs which treat allergic rhinitis, common cold, influenza, and other allergies. Typically, people take antihistamines as an inexpensive, generic (not patented) drug that can be bought without a prescription and provides re ...

beta blockers Beta blockers, also spelled β-blockers, are a class of medications that are predominantly used to manage cardiac arrhythmia, abnormal heart rhythms, and to protect the heart from a second myocardial infarction, heart attack after a first heart ...

, antispasmodics,

diuretics A diuretic () is any substance that promotes diuresis, the increased production of urine. This includes forced diuresis. A diuretic tablet is sometimes colloquially called a water tablet. There are several categories of diuretics. All diuretics in ...

) and

vitamin A deficiency Vitamin A deficiency (VAD) or hypovitaminosis A is a lack of vitamin A in blood and tissues. It is common in poorer countries, especially among children and women of reproductive age, but is rarely seen in more developed countries. Nyctalopia (ni ...

, amongst others.

Procedure

Procedure using fluorescein dye

* Fluorescein 2% is instilled into the lower fornix of patient's eye. Alternatively, impregnated fluorescein strip moistened with preservative (benzalkonium chloride) free saline solution can also be used. * The eye is examined under a slit lamp with a low

magnification Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to a reduction in siz ...

and a broad beam covering the whole cornea. The lamp is switched to a cobalt blue filter. * The patient is asked to blink once and to keep their eyes open. * Due to the fluorescein, the tear film will appear green in color. * A black spot indicating the dry area will appear a few seconds after each blink. * TBUT is the time interval between the last blink and appearance of the first randomly-distributed dry spot. If the patient blinks before 10 seconds have elapsed, the test must be restarted. Taking two or more measurements and calculating its average may give a greater accuracy of measurement of TBUT.

Non-invasive procedure

In annon-invasive tear breakup time measurement procedure, a grid or concentric ring pattern is projected onto the cornea and the patient is asked to blink. The rings will appear distorted when the cornea becomes dry. The time interval between the last blink and the distortion of the ring pattern gives the measurement of NIBUT.

Normal values

Generally, a TBUT value of 10–35 seconds is considered normal. A value of less than 10 seconds is usually suspicious and may indicate tear film instability. Even if TBUT value is under the normal limit, if the ocular protection index is less than 1.0, dryness and discomfort can occur.

Ocular protection index

Ocular protection index (OPI) is used to quantify the interaction between tear film break-up time and blink intervals of a person. It is based on the idea that, even when the TBUT is normal, if the blink interval is too slow it may cause breakage of tear film due to increased evaporation. The ocular Protection Index can be calculated by dividing tear break-up time by inter blink interval (IBI). If the inter blink interval and TBUT are same dividing both give a value of 1.0. If TBUT is more than IBI, value will be more than 1.0, so dryness will not occur. A patient is at risk of developing dry eye and if the OPI is < 1.0. Dry eye may occur in certain environmental conditions or while performing certain visual tasks like computer use movie watching etc. Calculating ocular protection index may be useful in these conditions also. For example, If a person's TBUT is 15 seconds which is thought to be normal and his blink interval is 20 seconds, here TBUT is less than IBI, so ocular protection index will be less than 1.0 and is abnormal. Since computer use affect blink rate, OPI is a clinically relevant in detecting dryness related to

Contraindications

Benzalkonium chloride Benzalkonium chloride (BZK, BKC, BAK, BAC), also known as alkyldimethylbenzylammonium chloride (ADBAC) and by the trade name Zephiran, is a type of cationic surfactant. It is an organic salt classified as a quaternary ammonium compound. ADBACs hav ...

, a preservative used in saline solution can increase tear break up speed, so it is important to choose preservative free saline solutions for moistening the flourescein strip. * Localised corneal surface abnormalities can produce dry spots in that location and can give a falsely low break-up time. To avoid this error, repeat measurements should be taken. Appearance of dry spots in the same location in repeated procedures can indicate a surface abnormality in that area. * Touching the cornea with flourescein strip can cause excessive tearing and will affect the TBUT.

References

{{reflist Diagnostic ophthalmology