Ocular Motility and Binocular Vision
Ocular motility and binocular vision
This domain tests extraocular muscle actions, innervation, laws of motor control, strabismus, and sensory adaptation. Expect questions that combine a motility finding with a calculation or a binocular test.
Core rules
- LR abducts, MR adducts, SR elevates, IR depresses, SO intorts, IO extorts. Superior and inferior recti act best when the eye is abducted; obliques act best when adducted.
- CN VI supplies LR, CN IV supplies SO, CN III supplies the rest, including levator and pupil.
- Sherrington's law: increased innervation to an agonist is matched by decreased innervation to its antagonist. Hering's law: equal innervation to yoked muscles.
- AC/A ratio quantifies convergence induced per dioptre of accommodation. Normal is about 4-6 Δ/D.
- Sensory adaptations include suppression, anomalous retinal correspondence, and amblyopia.
Worked example
A patient has 2 Δ esotropia at distance and 10 Δ esotropia at near with a 6 cm PD and 2.5 D near demand. Calculate the AC/A. AC/A = PD + (deviation near - deviation distance)/accommodative demand = 6 + (10 - 2)/2.5 = 6 + 3.2 = 9.2 Δ/D. This is a high AC/A, consistent with convergence excess. The trap is to subtract in the wrong order or to omit the PD term.
Traps
- Reversing the order of deviation near minus distance.
- Forgetting that the heterophoria method uses prism dioptres and PD in cm.
- Confusing comitant with incomitant strabismus.
- Assuming suppression equals amblyopia.