2.3 Uvea, Retina, Vitreous, Optic Nerve, Ocular Circulation & the Visual Pathway

Uvea

The uveal tract is the pigmented vascular middle coat, in three parts:

• Iris — the aperture. Sphincter pupillae (parasympathetic, CN III) constricts; dilator pupillae (sympathetic) dilates. That dual supply is the anatomy behind every pharmacologic pupil test.
• Ciliary body — pars plicata (70 or so ciliary processes secreting aqueous) and pars plana (the 3 to 4 mm safe zone for intravitreal injection and vitrectomy sclerotomies). The ciliary muscle drives accommodation.
• Choroid — the most vascular tissue in the body per unit weight, supplying the outer retina and providing thermal regulation.

Inflammation is classified by site: anterior uveitis (iritis/iridocyclitis), intermediate uveitis (pars planitis), posterior uveitis (choroiditis, retinitis) and panuveitis.

Retina: ten layers

From the vitreous inward toward the choroid:

1. Internal limiting membrane
2. Nerve fibre layer (ganglion cell axons)
3. Ganglion cell layer
4. Inner plexiform layer
5. Inner nuclear layer (bipolar, horizontal, amacrine, Müller nuclei)
6. Outer plexiform layer
7. Outer nuclear layer (photoreceptor nuclei)
8. External limiting membrane
9. Photoreceptor inner and outer segments
10. Retinal pigment epithelium

Approximately 120 million rods dominate the periphery and mediate scotopic vision; roughly 6 million cones concentrate centrally and mediate photopic vision and colour. Light must traverse the whole neural retina to reach the photoreceptors — except at the foveola, where the inner layers are displaced aside, producing the highest acuity in the eye.

Macular landmarks: the macula is about 5.5 mm across, the fovea about 1.5 mm, the foveal avascular zone about 0.5 mm and the foveola about 0.35 mm. The foveal avascular zone is the anatomical reason the early arteriovenous phase of a fluorescein angiogram shows a dark central window, and its enlargement is the hallmark of macular ischaemia.

Vitreous

A clear gel, 98 to 99% water with type II collagen and hyaluronan, filling about 4 mL. It is firmly adherent at the vitreous base (straddling the ora serrata), and less firmly at the optic disc margin, the macula and along retinal vessels. Age-related syneresis leads to posterior vitreous detachment, whose traction at those adherent points produces flashes, a Weiss ring, and — where a vessel or a strong adhesion tears — vitreous haemorrhage or a retinal break.

Optic nerve

About 1.2 million ganglion cell axons leave through the lamina cribrosa. The nerve has four segments: intraocular (1 mm), intraorbital (about 25 mm, deliberately redundant so the globe can rotate), intracanalicular (about 9 mm, the segment vulnerable in canal fractures) and intracranial (about 16 mm). Axons are myelinated only posterior to the lamina cribrosa; myelin anterior to it is the anomaly of medullated nerve fibres.

The physiologic blind spot sits about 15 degrees temporal to fixation because the disc lies nasal to the fovea. Every automated field plot uses it as an internal check of fixation.

Ocular circulation

The ophthalmic artery, the first branch of the internal carotid, supplies the eye through two separate systems:

The cherry-red spot of central retinal artery occlusion exists because the foveola has no inner retina to become oedematous and its underlying choroid is intact, so the normal choroidal red shows through pale surrounding retina.

Venous drainage runs through the central retinal vein and vortex veins into the superior and inferior ophthalmic veins and then the cavernous sinus.

Visual pathway and localisation

Retina → optic nerve → chiasm (nasal fibres decussate) → optic tract → lateral geniculate nucleus → optic radiations (Meyer's loop inferiorly through the temporal lobe; parietal fibres superiorly) → primary visual cortex in the occipital lobe.

Three localising rules carry most of the exam value:

1. Anterior to the chiasm → the defect is monocular (an optic nerve lesion affects one eye).
2. At or behind the chiasm → the defect is binocular and respects the vertical midline. Chiasmal compression from a pituitary adenoma gives bitemporal hemianopia; a tract or radiation lesion gives a homonymous defect on the side opposite the lesion.
3. Congruity increases posteriorly. An optic tract lesion gives an incongruous homonymous hemianopia; an occipital lesion gives an exquisitely congruous one, often with macular sparing from dual middle and posterior cerebral artery supply.

Two classic named patterns: Meyer's loop damage in the temporal lobe produces a superior homonymous quadrantanopia ("pie in the sky"), and a parietal lesion produces an inferior quadrantanopia ("pie on the floor").

Cranial nerves III to VII at a glance