A retrospective review of medical records was completed, identifying consecutive patients prescribed Fresnel prisms for diplopia, assessed using both simultaneous prism and cover test (SPCT) and prism and alternate cover test (PACT) by a single orthoptist over a 36-month period. The following data was extracted: reason for referral, visual acuity, refractive correction, orthoptic assessment and strength of Fresnel prism prescribed. The definition of successful prism treatment was stable prism strength for over 12 months. The method of identifying which prism to prescribe is described. A total of 244 patients met the inclusion criteria, however only 32 had both SPCT and PACT performed. The types of deviations included were horizontal, vertical and mixed. The mean SPCT and PACT measurements had a 6Δ discrepancy which was statistically significantly different, measuring means 7Δ and 13Δ respectively. The mean strength of Fresnel prism of 8Δ prescribed was closer to the mean SPCT. A total of 66% of patients achieved success with prism therapy. The author acknowledges limitations of the study including small sample size, retrospective design and examiner bias. The definition of success of prism treatment is flawed due to the range of aetiology of small angle strabismus causing diplopia in adults, which include conditions that are progressive or can recover. This study supports using the SPCT measurement as a starting point for identifying the Fresnel prism to prescribe.
Which cover test method is the best starting point for prescribing temporary prisms?
Reviewed by Lauren Hepworth
Comparison of simultaneous prism and cover test (SPCT) and prism and alternate cover test (PACT) measurements with final power of Press-On prism power dispensed.
CONTRIBUTOR
Lauren R Hepworth
University of Liverpool; Honorary Stroke Specialist Clinical Orthoptist, Northern Care Alliance NHS Foundation Trust; St Helen’s and Knowsley NHS Foundation Trust, UK.
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