Ocular Anatomy, Physiology, and Basic Optics
Domain 2 tests structure-function relationships and the optics behind refraction. Expect questions on corneal layers, aqueous outflow, lens accommodation, retinal organization, extraocular muscle innervation, refractive error, and diopter calculations.
Anatomy and physiology essentials
- Cornea: epithelium, Bowman layer, stroma, Descemet membrane, and endothelium. The endothelium pumps fluid out; failure causes corneal edema and blurred vision.
- Aqueous humor: produced by the ciliary epithelium, flows from the posterior chamber through the pupil into the anterior chamber, then through the trabecular meshwork into Schlemm canal and episcleral veins. Outflow resistance raises IOP.
- Lens: capsule, cortex, and nucleus. Accommodation occurs when the ciliary muscle contracts, zonules relax, and the lens becomes more convex.
- Retina: rods mediate dim-light and peripheral vision; cones mediate color and central acuity. The macula provides detailed vision, and ganglion cell axons form the optic nerve.
- Motility: LR6, SO4, and the rest are innervated by CN III. The lacrimal system drains through puncta, canaliculi, lacrimal sac, and nasolacrimal duct.
Basic optics
- Diopter equals 1 divided by focal length in meters. Plus lenses converge light; minus lenses diverge it.
- Myopia focuses light anterior to the retina; hyperopia focuses behind it; astigmatism has unequal meridians; presbyopia is age-related loss of accommodation.
- Snellen notation compares the patient's letter size with a standard: a 20/40 letter subtends 5 minutes of arc at 40 feet.
- Keratometry identifies the steeper corneal meridian. With-the-rule astigmatism is steeper vertically; against-the-rule is steeper horizontally.
Worked example
Keratometry reads 42.00 D at 180 and 44.00 D at 090. Corneal astigmatism is 2.00 D, and the vertical meridian is steeper, so this is with-the-rule astigmatism. Average K is 43.00 D. A minus cylinder correction would use axis 180, the flatter meridian, because the correcting minus cylinder is placed along the meridian that needs no reduction in power.
Traps that cost marks
- Confusing with-the-rule and against-the-rule axes.
- Using centimeters instead of meters in the diopter formula.
- Mixing corneal astigmatism with lenticular astigmatism.
- Reversing the aqueous outflow pathway or the role of the ciliary muscle.