Why does Positive Relative Vergence (PRV) not show the full convergent potential of a patient with a phoria?

Why does Positive Relative Vergence (PRV) not show the full convergent potential of a patient with a phoria?

• A. It does not show the full amount a patient can converge/diverge since it does not account for fusional demand caused by their phoria
• B. It measures the maximum fusional reserve from rest
• C. It is independent of phoria
• D. It represents the fused vergence required for fusion

Answer: (A)

Relating to how much convergence a person can muster beyond their current resting alignment, Positive Relative Vergence shows only the incremental convergence from the rest position and does not include the latent phoria’s fusional demand. In someone with a phoria, the eyes are already relying on fusional vergence to stay single; overcoming that latent deviation requires more convergence than PRV captures. So PRV underestimates the total convergent potential because it starts from rest and ignores the additional convergence needed to neutralize the phoric deviation. The other statements don’t fit: PRV is not the measure of the maximum fusional reserve from rest, it is not independent of phoria, and it does not represent the exact fused vergence required for fusion.

Relating to how much convergence a person can muster beyond their current resting alignment, Positive Relative Vergence shows only the incremental convergence from the rest position and does not include the latent phoria’s fusional demand. In someone with a phoria, the eyes are already relying on fusional vergence to stay single; overcoming that latent deviation requires more convergence than PRV captures. So PRV underestimates the total convergent potential because it starts from rest and ignores the additional convergence needed to neutralize the phoric deviation.

The other statements don’t fit: PRV is not the measure of the maximum fusional reserve from rest, it is not independent of phoria, and it does not represent the exact fused vergence required for fusion.