Posterior Segment, Retina, and Vitreous
Core concepts
Build a framework around the retina's vascular supply: the central retinal artery supplies the inner retina, the choroid supplies the outer retina and photoreceptors. This explains why CRAO causes inner retinal whitening with a cherry-red fovea (spared choroidal supply), while CRVO causes diffuse hemorrhages and disc edema. Know the diabetic retinopathy staging (nonproliferative vs proliferative, with neovascularization as the dividing line) and the indications for panretinal photocoagulation versus anti-VEGF. Understand retinal detachment types-rhegmatogenous (break), tractional (fibrovascular), exudative (no break)-and their management differences. Recognize macular degeneration subtypes and the role of OCT in guiding anti-VEGF therapy.
Traps
Questions frequently test timing in CRAO: treatment is a time-critical emergency and the window is short, but the key teaching is that most cases are due to emboli and workup for embolic source matters. Another trap is confusing vitreous hemorrhage with retinal detachment on ultrasound-both are hyperechoic, but a detachment is a membrane tethered at the optic nerve and ora serrata. Also beware of attributing vision loss to cataract when a relative afferent pupillary defect suggests optic nerve or extensive retinal disease.
Worked example
A patient with sudden painless monocular vision loss has a pale retina with a cherry-red spot. The diagnosis is CRAO. Rule of thumb: the cherry-red spot is normal foveal choroidal perfusion seen against surrounding ischemic inner retina-it is a sign of arterial occlusion, not a lesion itself.