Ocular Motility, Binocular Vision, and Strabismus
This domain tests measurement and interpretation of eye alignment: which muscle is underacting, whether the deviation is comitant, and how accommodation drives convergence.
Rules
- Extraocular muscles: SR, IR, MR, LR, SO, IO. Elevation in adduction tests the obliques; depression in adduction also tests the obliques. SO intorts, IO extorts.
- Sherrington's law: increased innervation to an agonist is matched by reduced innervation to its antagonist. Hering's law: yoked muscles receive equal innervation.
- Cover–uncover detects a tropia; the alternate cover test reveals the total deviation including phoria; prisms neutralise it.
- AC/A ratio: gradient method (deviation change per 1.00 D of added lens) or heterophoria method: AC/A = PD(cm) + (ΔN − ΔD)/D.
- Normal NPC break is within about 5–7 cm; reduced fusional reserves support a decompensating phoria.
Worked example
PD 60 mm; 2 Δ exophoria at 6 m and 8 Δ exophoria at 33 cm. The heterophoria method gives AC/A = 6 + (8 − 2)/3 = 6 + 2 = 8 Δ/D. That is above the typical 4–6 range and points to a convergence-excess pattern, which responds to plus lenses at near rather than to prism.
Traps
Sign errors when entering esophoria and exophoria into the formula; using the cover–uncover test and reporting the result as a phoria; testing versions when ductions are needed to localise a palsy; and diagnosing suppression without checking stereopsis and Worth lights.