Contact Lens Case Reports
Assessing Tear Film Quality with Placido Imaging
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
The integrity of the pre-corneal tear film is assessed by measuring its breakup time at the slit lamp (TF-BUT). Simply apply a drop of fluorescein onto the bulbar conjunctiva and instruct patients to blink twice and then refrain from blinking. The examiner then counts the seconds between the last blink and the appearance of the first dry “black” spot (Figure 1).
Figure 1. Tear film breakup as seen following traditional instillation of fluorescein.
Breakup times usually vary between 15 and 35 seconds. Values ≤10 seconds are often considered abnormal. The technique is plagued by errors due to contamination of the tear film through the application of the fluorescein drop and to reflex tearing from the solution pH and from the bright light of the slit lamp.
Placido Capture of the TF-BUT
Today, TF-BUT can be measured with placido-based corneal topographers, which capture and assess a reflected ring image. This technique provides a non-invasive assessment of tear film smoothness and integrity. The instrument’s video function captures multiple photo-keratoscope images continuously during a 30-second interval and records the time until TF-BUT occurs (Figure 2).
Figure 2. Placido-based capture of the TF-BUT.
The TFSQ Index
The Tear Film Surface Quality (TFSQ) index provides a further analysis of the tear film by assessing finite differences in its relative smoothness over 15,000 data points across the cornea. The TFSQ index provides a numeric reference point and is ideal for assessing the effectiveness of various dry eye and meibomian gland dysfunction therapies as well as the relative on-eye surface wettability of contact lenses. To illustrate this, we evaluated three different soft contact lens brands on a subject’s right eye at 0 seconds and 5 seconds post-blink (Figure 3). The TFSQ index clearly shows that lens 1 had the best wetting characteristics. This was followed by lens 2 and, finally, lens 3.
Figure 3. The TFSQ index provides a numeric description and photo display of the on-eye surface wetability of soft contact lenses.
Today, a number of placido-based corneal topographers have tear film analysis modules that can measure non-invasive TF-BUT and tear film meniscus height and that can quantify tear film smoothness. These modules should prove valuable in assessing the effectiveness of various dry eye therapies over time as well as the relative on-eye surface wettability of contact lenses. CLS
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Contamac. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.