Table 2.

Modifying Factors for Asymmetric DR Prevalence and Progression

Modifying factors	Clinical conditions	Major mechanisms	Relationship with ipsilateral DR
Vascular	Carotid obstructive disease	Carotid stenosis occurring after DR onset causes increased ischemia	Progression
		Carotid stenosis developing before DR with coexistent hypertension	Protection
	Ocular ischemic syndrome	Added ischemia	Progression
	Retinal vascular diseases  Unilateral BRVO  Unilateral CRAO	Thinning of inner retina Reduced retinal oxygen demand	Protection
Inflammatory	Uveitis  Acute retinal necrosis in  Herpes Zoster infection  Toxoplasmosis  Other causes	Blood-ocular barrier breakdown Vasculitis-induced ocular ischemia Free-radical induced damage Increased VEGF and intraocular cellular fibronectin Efflux of albumin and lymphocytes into aqueous and vitreous Juxtapapillary retinitis and vitritis in toxoplasmosis	Progression
	Endophthalmitis	As in Uveitis Effect of cataract surgery and vitrectomy	Progression
	Fuch’s heterochromic iridocyclitis	Sympathetic denervation hypersensitivity, reflex vasoconstriction, reduced perfusion	Protection
Degenerative	Posterior vitreous detachment	Removal of scaffold for neovascularization Release of subtle tractions on retina	Protection
	Axial refractive errors  Unilateral, High or  Anisomyopia  Anisometropia>1D	Reduced retinal metabolic demand Stretched vessel walls, increased pressure attenuation Effect of uveal coloboma and PVD Thinning of veins Dilution of VEGF	Protection
	Uveal coloboma	Lack of nutrition from the choroid in colobomatous area Outer retinal atrophy Relative decrease in oxygen demand	Protection*
	Retinitis pigmentosa	Greater oxygen and nutrient flux from choroid to inner retina Secondary attenuation of retinal blood vessels Early PVD Release of a cytokine that inhibits neovascularization from the retina Effect of Optic atrophy	Protection
	Healed choroiditis	Chorioretinal atrophy and scarring, reduced metabolic demand	Protection
	Optic atrophy	Reduced metabolic demand	Protection
Surgery	Cataract surgery	Breakdown of blood-retinal barrier Increased inflammation including prostaglandin production	Progression
	Vitrectomy	Increased diffusion and clearing of sequestered growth factors PVD Increased retinal oxygen saturation	Protection
Miscellaneous	Elevated intraocular pressure and glaucoma	Decreased metabolic demand Decreased perfusion	Protection
	Amblyopia, visual deprivation, and retinal detachment	Decreased metabolic demand Decreased perfusion	Protection
	Idiopathic

DR=diabetic retinopathy, PVD=posterior vitreous detachment, VEGF=vascular endothelial growth factor. *Cannot be conclusively opined