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David S Bardenstein; Occult Prism Displacement (OPD) an Unrecognized Source of Error in Exophthalmometry. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4085.
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Exophthalmometry (XO) is a widely used technique for assessing position of the eye in the orbit. In some conditions, abnormal readings are sufficient criteria for recommending surgery or initiating expensive diagnostic evaluations. As such, obtaining accurate XO is of great import. Unexpected XO results identified by an experienced examiner led to examination of the instrument being used. OPD was identified in the instrument. When a comprehensive search of available data showed no information on this issue, we studied 2 questions: How often does OPD occur and what is its quantitative significance.
All XOMs in our institution were studied with respect to the stability of the reflective or refractive mechanism converting the side assessment to a front view. We assessed whether the mechanism was displaced or could be moved mechanically from its resting position. Findings were analyzed by XOM type. Age of the instrument was documented when known. Quantitative analysis of the effect of OPD on XOM accuracy was measured using a standardized head model. XO measurement were made with the prism in its most anterior, standard and posterior positions. The maximal deviation between anterior and posterior positions was calculated.
Several XOM types were examined including prisms with single point fixation, prisms with two point corner fixation, as well as double mirror mechanisms. OPD was identified only in single point fixation (SPF) XOMs. It was identified in 4/7 SPF instruments with 5/14 prisms affected. Bilateral OPD occurred in 1 of those 4. OPD occurred in relatively new instruments (< 2 years old) as well as older ones. Prisms were displaced in some XOMs and movable in others. Maximal prism deviation resulted in XO measurement variation of 3mm (range 1-3mm) in the most mobile prisms. The variation paralleled the prism deviation in degree.
Occult Prism Displacement is an unappreciated source of error in exophthalmometry. It occurs in a surprisingly high percentage of devices of certain design. It can occur in newer and older devices. The 3mm of variation caused by OPD alone is enough to alter clinical care decisions. Since the impact of inaccurate exophthalmometry can have serious consequences for patients, prism stability and device calibration should be regularly verified in susceptible exophthalmometers.
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