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The Eye

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The structure of the eye

Light reflected off an object enters the eye through the cornea, the clear layer at the front of the eye. The cornea bends the light, which then passes through the aqueous humour, a watery substance filling the area behind the cornea.

The light then travels through the pupil, the black opening at the centre of the iris. (The iris, the coloured section of your eye that surrounds the pupil, has muscles that can expand or contract the size of the pupil to control the intensity of the light travelling in.)

The light passes through the pupil to the lens. The lens adjusts its shape to focus the light a second time, to produce a clear image.

The light passes from the lens to the back of the eye, which is filled with a clear jelly called the vitreous humour. It then hits the retina, the light sensitive layer at the back of the eye. 

The retina contains photoreceptor nerves called rods and cones, which turn the light into electrical impulses. These impulses then travel through the optic nerve at the very back of the eye up to the occipital lobe in the back of the brain, where they are interpreted. 

 

Salivary gland cancer and the eye

Although it’s not common, some salivary gland and salivary gland-type cancers can begin or metastasise in the eye. Eyesight can also be affected by radiotherapy or surgical treatments for salivary gland cancers, even when they do not begin in the eye itself.

Effects of salivary gland cancer surgery on the eye

 Head and neck surgery for salivary gland cancers often doesn’t affect the eyes at all. However, there are some exceptions. 

  • After surgery to remove a tumour in the parotid gland, some people experience difficulty closing their eye on the affected side. This is often temporary, and usually clears up within a few weeks or months. It is caused by damage to, or irritation of, the facial nerve. This nerve runs through the parotid gland and is responsible for moving the eyelid.
  • In some cases, salivary gland tumours can affect the optic nerve – especially if the tumour occurs in the tear ducts or the skull base. The skull base is made up of the five bones behind the eyes and nose, including the hard palette, which contains many minor salivary glands. Cancers whose habit is to grow along nerves, such as Adenoid Cystic Carcinoma, are more likely to have optic nerve involvement. Surgery may then include removing parts of the optic nerve, or the nerve sheath in order to remove the tumour. At all times, the surgeon will prioritise retaining as much vision as possible in the affected eye.
  • In the rare cases where an eye has to be removed during surgery, the brain adapts over the following weeks and months, reinterpreting the incoming information from the remaining eye so that it ‘takes over’ much of the work of the missing eye. However, some parts of vision will be more permanently affected. Depth perception which allows you to judge how deep a step is, or the level of liquid in a glass, can be reduced. (Although, again, over time, your sense of this can improve). Peripheral vision - what you see to your side rather than in front of you - is usually reduced by around 30% on the side of the missing eye.

 

Effects of head and neck radiotherapy on the eye

Many people who have head and neck radiotherapy for a salivary gland cancer do not have problems with their eyes as a result of this treatment. Whether you experience difficulties depends on the dose and type of radiation you receive, and where it is directed in your head. Treatments directed at the eye itself or near to it carry greater risk.

There are several side effects that can occur either during radiotherapy or in the weeks, months or even years afterwards. That’s why it’s important to have regular eye tests and be aware of some of the problems that can come up over time. Remember to always mention to your doctor or Ophthalmologist that you have had radiotherapy in the head and neck when you are discussing any symptoms you may be experiencing in the eye, or you go for an eye test.

 

Short-term side effects

Temporary swelling
During radiotherapy, temporary swelling can cause the pressure inside your eye to rise. The use of eye drops or steroid tablets can help to treat this. Where these treatments do not work, laser therapy is occasionally necessary.

Loss of eyelashes
Radiotherapy, especially if delivered to the head or eye area, can damage hair follicles, leading to eyelash loss. Eyelashes usually grow back after treatment, but this can take a little time. (Occasionally, if the radiation dose is high, this loss can be permanent.) Some people like to use false eyelashes or use eyeliner to create the illusion of eyelashes. Eyelashes also help protect the eye from dust and other small particles, so using eye drops to soothe any irritation can also be useful.

Longer-term side effects

Dry eyes
Radiation can damage the lacrimal glands, which produce tears, leading to dry eyes. Symptoms can include a burning sensation, redness, itching and pain. Artificial tears in the form of eye drops, and medication to stimulate tear production can help treat the issue.

Cataracts
Cataracts are a condition in which the lens of the eye, which is normally clear, becomes cloudy.  This reduces the amount of light coming into the eye and the ability of the lens to focus light correctly on the retina. Cataracts can cause blurry or double vision, light sensitivity, poor night vision, halos around lights, and faded colours.

Radiotherapy uses ionizing radiation to kill cancer cells, but it can also damage healthy tissue. The lens of the eye is particularly sensitive to radiation because it contains actively dividing cells. Cataracts caused by radiation often occur at the back of the lens and are known as posterior subcapsular cataracts.

In the months and years after head and neck radiotherapy, it’s important to be aware of the possibility of cataracts developing, to look out for symptoms, and have regular eye tests.

Because cataracts are a common age-related condition, cataract surgery is fairly routine and is usually effective in restoring vision. The cloudy lens can be removed, and replaced with a clear, artificial lens.

Radiation retinopathy
Radiation can damage the capillaries that carry blood through the retina, causing them to grow rapidly, bulge, widen, and leak into the eye. Over time, this can cause a reduction in the sharpness of vision, blurring, a blind spot in the centre of your vision and floaters (small black spots that appear to float across the field of vision). Laser treatment, steroid injections and surgery may all help to improve eyesight.

In some cases, it’s also possible to reduce ongoing radiation damage of the retina with injections of drugs like bevacizumab (Avastin). Such drugs stop the growth of new blood vessels in the eye which in turn can stop further leaking and damage occurring. These drugs are known as anti-VEGF drugs. They block VEGF (Vascular Epithelial Growth Factor), a protein in the body that plays a vital role in the formation and growth of new blood vessels.

Glaucoma
Glaucoma is a chronic, progressive eye disease that damages the optic nerve. The damage is often caused by increased pressure in the eye squashing the nerve. The most common type of Glaucoma caused by radiotherapy is Neovascular Glaucoma, or NVG. It causes the growth of abnormal blood vessels in the iris and the angle of the eye, which can lead to a rapid increase in eye pressure. NVG is a late-onset side effect of treatment and can occur within a few years of high dose radiotherapy. Treatments can include topical antiglaucoma medications, retinal laser therapy, and in some cases, injections of drugs such as bevacizumab, which block the growth of new blood vessels.

Corneal ulcers
Radiotherapy - particularly in higher doses, and when it is directed in or around the eye - can cause ulcers to develop on the cornea. Radiotherapy can disrupt the tear film that protects the cornea’s surface, as well as directly damaging the cornea itself, leading to erosion and ulceration. Radiation damage can also occur in the lacrimal glands which produce tears. This can result in dry eyes, further increasing the risk of ulcers developing. Dry eyes can be treated with lubricants, artificial tear solutions, and ointments, and this can play an important role in helping to protect the cornea and prevent ulcers from developing.

Symptoms of corneal ulcers include pain, redness, discharge, tearing, sensitivity to light, and blurred vision. Medicated eye drops are a common first line treatment, and corticosteroid eye drops may also be used to reduce swelling and inflammation. In severe cases, a healthy cornea from a donor can be transplanted to restore vision. It’s important to get corneal ulcers seen to quickly, as they can cause vision loss, particularly if they become infected.

Radiation induced optic neuropathy (RON)
Top of FormBottom of FormRadiation can damage the optic nerve, leading to vision loss. Symptoms include unexplained, painless visual loss, visual field defects, abnormal shaped pupils, and defective colour vision in one or both eyes. Early detection and management are crucial, as RON can develop rapidly, so regular eye exams are important. It’s also important to make sure your clinician knows you have had radiotherapy in the head and neck area when you explain your symptoms, so they can consider RON as a cause of your eye problems.

 

References:

  1. Kay MD. Radiation Optic Neuropathy. LWW Health Library.
  2. Farooq O, Lincoff NS, Saikali N, Prasad D, Miletich RS, Mechtler L. Novel Treatment for Radiation Optic Neuropathy With Intravenous Bevacizumab. J Neuro-Ophthalmology. 2012;32(4):321-324. doi:10.1097/WNO.0b013e3182607381
  3. Indaram M, Ali FS, Levin MH. In search of a treatment for radiation-induced optic neuropathy. Curr Treat Options Neurol. 2015;17(1):325. doi:10.1007/s11940-014-0325-2

 

Last Updated July 2025