Retina

Summary

• location: innermost layer of the eyeball. 
• blood supply: 
  • choroidal capillaries & central retina vessels 
• innervation: Optic nerve 
• relations: Outer surface of retina in contact with choroid (Bruch's membrane); Inner surface of retina in contact with vitreous; terminates anteriorly at ora serrata 

Arterial supply

The blood supply of the retina is from 2 sources, supplying different portions of the organ. The integrity of the retina depends on both of these circulations, neither of which alone is sufficient. 

The outer laminae, including rods and cones and outer nuclear layer, are supplied by choroidal capillaries. 

The inner laminae is supplied by the central retina artery. These are anatomic end arteries. The central retina artery is the first branch of the ophthalmic artery, it divides into 2 equal superior and inferior branches, and these branches subsequently divide dichotomously into superior and inferior nasal and temporal branches. The retina capillary network formed are most concentrated at the macula, but absent at the fovea centralis. 

Blood-Retina Barrier

Venous drainage

The outer laminae is drained by the choroidal circulation which drains into the ophthalmic veins via vorticose veins. 

The inner laminae is drained by the central vein of the retina. The central vein of the retina leaves the eyeball through the lamina cribrosa, draining directly into the cavernous sinus or through the superior ophthalmic vein. 

Lymphatic drainage

The retina is an alymphatic structure, as with the optic nerve and cornea. 

Innervation

Optic nerve (Cranial nerve II) is made up of the neural axons in the ganglionic layer of the retina, converging at the optic disc. The disc lies nasally to the macula lutea. There is a complete absence of rods and cones atthe disc, making this part of the retina insensitive to light, termed as the "blind spot". The axons are arranged in bundles, and leave exit the eyeball by passing backward through the lamina cribrosa. As they pass the lamina cribrosa, they acquire myelina sheaths made of oligodendrocytes (not Schwann cells). The intra-cranial course of the optic nerve terminates at the optic chiasma in the floor of the third ventricle. 

Histology

Radiographic features

Ultrasound

CT

MRI

Nuclear medicineOcular Coherence Tomography (OCT)

OCT utilizes light waves and a concept known as inferometry to create a cross-sectional map of the retina that is accurate to within at least 10-15 microns. 

Highly utilised in clinical practice to produce detailed cross-section images of the retina, including macula imaging. 

However, limited utility if path of light wave is obscured, e.g. by cornea opacities or vitreous haemorhage.

Others:

Fundus Imaging

• color fundus photography
• stereo fundus photography
• hyperspectral imaging
• scanning laser ophthalmoscopy (SLO)
• adaptive optics SLO
• fluorescein angiography (FFA) and indocyanine angiography (ICG)

Development

Embryologically, the retina develops from the optic cup, neuro-ectodermal in origin, in 2 layers. The outer pigment layer contains melanosomes, while the inner neural layer containing photoreceptor cells. 

Related pathology

• Retina detachment - pathological separation of neural retina from pigment epithelium 
• Central retina artery occlusion - complete occlusion leading to permanent monocular loss of vision
• Branch retina artery occlusion - occulsion of any of 4 branches of the central retina artery, leading to permanent loss of a quadrant of vision. No overlap between blood supply of vessels; end-vessel. 
• Albinism - absence of pigment in retina is common 
• Cone-rod dystrophy - Genetic mutations in any of the genes leading to degeneration of cones, then rods, in the neural retina