The eye is a complex organ, composed of many different structures with varying sensitivity to the effects of radiation. An easy way to look at the eye is to divide it into zones that can be examined with respect to their susceptibility to radiation effects. It is also noteworthy that the effects of radiation on the eye not only depend on the sensitivity of the tissue itself but also on the mode of radiation delivery.
External Eye
The tissues of the external eye include the conjunctival epithelium, glandular ducts, cornea and lacrimal drainage apparatus. In the first six months, which is the acute phase of radiation, effects include redness and swelling of the eyelid and conjunctiva with possible keratinization (process by which epithelium firms via deposits of keratin, a protein). Damage to the glandular ducts can result in a decrease in tears, which not only results in dry eyes but also an increase in infection rate. In the subacute phase (6 to 24 months), corneal damage can be prominent and scarring can result.
Anterior Intraocular Components
These include the iris, lens and anterior chamber angle. The cells of the iris are usually resistant to radiation up to 30-40 Gy (Gray) and if they are damaged, it is not apparent until 6 to 8 weeks later and can result in glaucoma. The lens has potential for radiation damage as it has regularly dividing cells around its equator, which are very susceptible to radiation damage. Cataracts can develop wihtin two to three years.
Posterior Intraocular Components
The components of the posterior intraocular zone include the retina, choroid and optic disc. They are relatively resistant to doses up to 50 Gy, at which point retinal swelling that is usually temporary can occur. Most of the damage is chronic in nature, with the mechanism of damage relating more to damage of the blood vessels in the eye. Radiation damage to the vessels can occlude or obliterate them, resulting in ischemia (decreased blood flow) to the posterior intraocular components. Retinal damage can result in vision loss, and visualization of neovascularization (formation of new blood vessels) and exudates can be seen. The optic nerve can also suffer from lack of blood flow which can lead to visual impairment, which can sometimes improve over several months. Usually six months to three years is the time frame it takes for these types of damage and changes to manifest.
Eyelid and Orbital Tissue
The main damage that occurs here is damage to the skin on the eyelid and loss of eyelashes. It begins with reddening and followed by desquamation (shedding of the outer layers of skin). Redness takes 1-2 days after radiation exposure to develop, with a slow delayed healing process. Scar tissue is not common unless a very high dosage of radiation was delivered to the tissue. Meibomian glands are also fairly sensitive and their production of sebum, an oily substance, is affected as well. The eyelash loss is usually temporary and occurs in roughly 3 weeks and returns after approximately 2 months.
Hypothalamus/Pituitary Gland
Radiation damage to these glands commonly results as deficiencies in growth hormone. An example of this is children irradiated for optic nerve glioma that suffer from stunted growth/.
Note on malignancies
Orbital radiation doses increase the risk of malignant tumors, especially in those patients with previous retinoblastomas or other childhood malignancy. This makes things complicated as radiation is typically the treatment of choice for those tumors; tragically it can lead to more cancer.
References
- "Abeloff's Clinical Oncology, 4th Edition;" Abeloff; 2008
- "Opthalmology, 3rd Edition;" Yanoff & Duker; 2008
