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

Purpose

To investigate the association of strabismic angle obtained by simultaneous prism and cover test (SPCT) and prism alternate cover test (PACT) with the final power of Press-On prism used in adults with symptomatic strabismus.

Methods

The medical records of consecutive patients seen by a single orthoptist at an urban, tertiary care hospital over a 36-month period were reviewed retrospectively. All subjects had been prescribed Press-On prism for diplopia after evaluation with SPCT and PACT measurements at distance fixation. Prism dispensed was chosen by the patient using a trial Press-On prism set.

Results

A total of 244 charts were reviewed. Of these, 108 adult patients had prism dispensed, and 32 had both SPCT and PACT measurements. Mean SPCT measurement was 7^Δ (median, 6^Δ; range, 3^Δ-14^Δ); mean PACT, 13^Δ (median, 11^Δ; range, 8^Δ-20^Δ). The mean prism power dispensed was 7^Δ (median, 6^Δ; range, 1^Δ-15^Δ); this was not significantly different from mean SPCT (P = 0.35). There was a large difference (6^Δ) between mean PACT measurements and mean prism dispensed (P < 0.001). At follow-up of 1 year, 21 patients (66%) remained in prism. Nine patients (4%) elected to undergo eye muscle surgery, 7 with horizontal strabismus, 6 of whom had SPCT measurements of >10^Δ and PACT measurements of >15^Δ.

Conclusions

Press-On prism power used was more closely tied to the angle of strabismus obtained by SPCT. Overall success rate of Press-On prism was good when the prism power dispensed was close to the SPCT measurement, suggesting that it is reasonable to initiate treatment with a weaker prism, approximating the SPCT measurement, building up only if there are continuing symptoms.