Types of Vision Screening Devices

This text covers two types of devices: Photoscreeners and Automated Refractors

Photo refraction screening utilizes a flash of light and the observation of the reflection of that light from the blur circle of the fundus to detect ocular misalignment and refractive blur. Of the commercially available photoscreening instruments available, the most prominent are the MTI Photoscreener, the iScreen Photoscreener and the Plus OptixSO8 Photoscreener.  The MTI Photoscreener and the iScreen Photoscreener require manual interpretation of the flash reflections and difficulties with interpretation currently limit the acceptance of photoscreening technology for widespread screening.  The MTI Photoscreener is perhaps the most well evaluated automated visual screening instrument.  It is an off-axis photoscreener that utilizes two photographs taken with an eccentric flash unit that rotates 90° for each pair of images.  These two images are captured on Polaroid film for interpretation by trained personnel.  The instrument is easy to use, portable, and although it has a dated analog output, the Polaroid film is available for immediate interpretation.  Other limitation of this instrument include high costs ranging from $6-7 per child, lack of support for current instruments (old cameras need to be retrofitted to handle currently available film), and the lack of a company providing support or new investment (the company is no longer in business).  Despite these limitations, there have been several large validation studies.

Field studies generally report positive predictive value (since normal children are not referred) while clinic-based studies report sensitivity and specificity (since predictive value depends upon disease prevalence).  One large field-based vision screening program using the MTI Photoscreener began in Tennessee.  Referred children were evaluated by local optometrists and ophthalmologists and the results are collected centrally.  Over 200,000 children were screened using the MTI Photoscreener with a referral rate of 4%, a screenability rate of 96% and a positive predictive value of over 75%.  This program has been expanded by the Lions’ Club International Foundation to include multiple other states and foreign countries.  The results from the 17 programs that have screened over 400,000 preschool children show similar overall results with a referral rate averaging 5%, a screenability rate of 97% and a positive predictive value averaging 80%. 

The VIP (Vision in Preschool) studies evaluated photoscreening with the MTI Photoscreener as one methodology in a prospective trial of over 1000 children with an enriched proportion of ocular pathology.  In that study, photoscreening had a lower sensitivity than autorefraction (see below) when specificity was fixed retrospectively at 94% reporting an overall sensitivity of 37% and a 63% sensitivity for amblyopia.  However, the relatively high sensitivity for the autorefraction was likely a result of a retrospective reanalysis of the data using a revised set of referral criteria for the autorefractors (but not for the photoscreeners) that was determined by re-evaluating the study results.  Other issues with respect to the methodology of the VIP study which biased against photoscreening technology have been discussed extensively in the literature.