Determining Prism

Sheard's criterion

• Prism needed = 2/3 (phoria) - 1/3 (compinsating fusional vergence)
• eg: pt has 6 XP and BO to blur is 6, the prism needed is 2/3 (6) - 1/3 (6) = 2 BI

Percival's criterion

• Prism needed = 1/3 (greater limit of BI or BO range) - 2/3 (lesser limit of BI or BO range)
• eg: pt has 6 XP and BO ranges of 6/10/8 and BI range of 21/26/22, prism needed = 1/3 (21) - 2/3 (6) = 3 BI

Parks-Bielschowsky Three Step Test (for head tilts "torticollis")

• Which eye is hyper in primay gaze?
• In which horizontal gaze does hyperdeviation increase?
• In which direction of head tilt does the hyperdeviation increase?
• right hyper --> right gaze --> right tilt = LIO
• right hyper --> right gaze --> left tilt = RIR
• right hyper --> left gaze --> right tilt = RSO (most common cause of vertical dipl.)
• right hyper --> left gaze --> left tilt = LSR
• right hyper --> right gaze --> right tilt = RSR
• right hyper --> right gaze --> left tilt = LSO (most common cause of vertical dipl.)
• right hyper --> left gaze --> right tilt = RIR
• right hyper --> left gaze --> left tilt = RIO

Postures and Treatments

• Left head turn --> Right gaze preferred --> LLR or RMR paresis --> give yoked prism base left
• Right head turn --> Left gaze preferred --> RLR or LMR paresis --> give yoked prism base right
• Left head tilt --> Right hyperdeviation --> RSO paresis (most common cause of vertical diplopia)--> BD over right eye if longstanding
• Right head tilt --> Left hyperdeviation --> LSO paresis (most common cause of vertical diplopia)--> BU over left eye if longstanding
• Head tip back --> Downgaze preferred --> V-pattern exotropia or A-pattern esotropia --> BU yoked prism
• Chin depressed --> Upgaze preferred --> A-pattern exotropia or V-pattern esotropia --> BD yoked prism