Late Eye Reactions

The 'eye' consists of numerous tissues, including the eyelid, conjunctiva, cornea, lens, retina, supporting lacrimal apparatus, optic nerve and optic chiasm. These are all formed by quite different tissues and have unique late responses following radiation exposure, often through an interaction of parenchymal cell loss, endothelial damage, fibroblast activation and involvement of immune cells.

Eyelid

The eyelid is covered by specialised skin and has the eyelids, special skin adnexa. Similar to skin in other regions, telangiectasia is observed with doses over 50 Gy. It would follow that necrosis of the eyelid is unlikely until dose exceeds 60 Gy. Loss of eyelashes occurs with dose over 40 - 60 Gy, and may be permanent. Other observed changes include entropian (infacing) and ectropian (outfacing) eyelids, which can cause discomfort, tearing or cosmetic problems. The tarsal glands, located within the tarsus of each eyelid, produce an oily secretion which prevents the eyelids sticking together and also prevents lacrimal fluid from escaping the eye. These glands are also susceptible to radiation which may therefore interfere with the lacrimal system.

Conjunctiva

The conjunctiva is the specialised epithelium and supporting submucosa that circulates the lacrimal fluid. It is more sensitive to the development of telangiectasia than the skin, showing signs of this condition after doses of 30 Gy. If telangiectasia develops on the tarsal or marginal conjunctiva, it may cause abrasion of the corneal epithelium and blindness.

Cornea

The cornea is a transparent and avascular structure which covers the anterior eyeball. It is coated with a delicate layer of epithelium which is essential to its health. Corneal damage may result from other eye abnormalities induced by radiation, but has been specifically irradiated in some conditions (eg pterygium). Ulceration of the cornea is uncommon until dose climbs over 60 Gy.

Lens

The lens is one of the most sensitive structures in the body to ionising radiation and deserves special consideration. The lens is a special organ in which no cells are lost throughout development.

Cataract

A cataract is an opacity of the lens which prevents light from passing through. This can vary in severity and may cause blindness. The development of cataract following radiation has a very low threshold dose of just 2 Gy. Radiation causes abnormal proliferation in the equator zone (the zone which produces new lens cells), and these cells tend to migrate posteriorly and collect at that aspect. Anterior cataracts in the absence of posterior cataracts are almost certainly not related to radiation. The latent time for development of cataracts is variable and inversely related to dose - at low doses it may take several decades to develop which makes it hard to distinguish from normal cataracts.
With the advent of cataract removal surgery, cataracts are less of a problem in the modern world.

Retina

The retina is an outgrowth of the central nervous system. The vascular supply of the retina may undergo similar effects to that of the white or gray matter of the brain, leading to loss of retinal function (retinopathy). The severity of retinopathy depends on the location; it may have minimal effect if the peripheral vision is affected but can cause blindness if the fovea is involved. Retinopathy develops over weeks to months and has a tolerance dose of about 45 Gy.

Lacrimal system

The lacrimal system consists of the lacrimal gland and ducts on the supero-latearl aspect of the orbit, and the lacrimal canaliculi on the medial sides of the eyelid which drain to the lacrimal sac and nasolacrimal duct.
The lacrimal gland may suffer a permanent loss of function with doses over 40 Gy. It is now considered the most dose limiting structure of the eye now that cataract surgery is easily available. It is characterised by loss of secretory cells in the acini of the lacrimal gland.
The opening of a lacrimal canaliculus into the eyelid is the lacrimal punctum; this may be closed due to oedema of the eyelid during radiation. Chronic changes of the canaliculi, sac and nasolacrimal duct include fibrosis which may obliterate this passage. This can cause tearing of the eye as the lacrmial fluid overflows the eyelid.

Optic Nerve and Optic Chiasm

Optic neuropathy is damage to the neurons that convey visual signal from the retina to the thalamus where further processing occurs. The TD5/5 of the optic nerve and optic chiasm is about 55 Gy, with higher doses increasing the rates of neuropathy rapidly. Monocular visual loss occurs from involvement of one optic nerve. Radiation induced optic neuropathy is due to the vascular changes described in the brain section - increased permeability of the endothelial cells, oedema and haemorrhage. These structures are typically dose limiting in head and neck treatments.

Other Topics

Back to Late Effects in Normal Tissues
Back to contents table of contents