Anatomy, Physiology, and Etiology of the Visual System
Anatomy, Physiology, and Etiology of the Visual System
Items here ask you to trace light through the eye and visual pathway, link a condition to its functional classroom effect, and distinguish eye-based from brain-based vision loss. Memorizing labels is not enough; the exam usually asks what the student will need.
The pathway, in order
- Cornea, aqueous humor, pupil and iris, lens, vitreous, retina, optic nerve, optic chiasm, optic tract, lateral geniculate nucleus, optic radiations, occipital cortex.
- Rods support dim-light and peripheral vision; cones cluster in the macula and fovea and support acuity and color. Macular damage costs central detail; peripheral damage costs field and mobility.
- At the chiasm, fibers from the nasal retina cross. A chiasmal lesion produces bitemporal hemianopsia; post-chiasmal lesions produce homonymous field loss.
- The normal field is about 180 degrees horizontally. Loss is described as hemianopsia (half) or quadrantanopia (quarter).
Refractive error and measurement
- Myopia: long eye, distance blur. Hyperopia: short eye, effortful near work. Astigmatism: uneven corneal curvature. Presbyopia: lost accommodation with age.
- Snellen compares performance to the typical eye: 20/200 means the student sees at 20 feet what a typical eye sees at 200. Teller cards and preferential looking serve preverbal children; Lea symbols and HOTV suit young verbal children.
Conditions and functional implications
- CVI (cerebral visual impairment): brain-based loss with a normal or near-normal eye exam; variable performance, attraction to movement, difficulty with clutter, color preference, light gazing. A leading cause of pediatric visual impairment in developed countries.
- ROP: occurs in premature infants; stage and zone guide treatment (laser, anti-VEGF); risk of myopia, field loss, and retinal detachment.
- Albinism: foveal hypoplasia, nystagmus, photophobia, reduced acuity; seat away from glare.
- Glaucoma causes early peripheral loss; cataracts cause clouding and glare; diabetic retinopathy causes hemorrhages and macular edema.
- Retinitis pigmentosa causes night blindness and progressive tunnel vision; Usher syndrome adds hearing loss. LCA, optic nerve hypoplasia (watch for endocrine and midline brain anomalies), coloboma, and aniridia (glaucoma, WAGR) appear in vignettes.
Worked example
A student bumps into people on the right and misses words at the right end of a line, but acuity is 20/25. Reasoning: acuity is normal, so think field, not clarity. A right homonymous hemianopsia from a post-chiasmal lesion fits. The response is environmental and instructional (seat so the field faces the class, teach systematic scanning), not stronger glasses.
Traps that cost marks
Letting acuity alone define vision; calling CVI ocular; treating nystagmus or photophobia as behavior or cognition; forgetting ROP's link to prematurity and myopia; mixing the effects of macular versus peripheral disease.