What high blood pressure does to the back of the eye

Hypertension is common in Nigeria, frequently undiagnosed, and largely symptomless until it causes damage. The eye is unusual in that the damage can be seen — the retinal vessels are the only blood vessels in the body that can be examined directly, without instruments entering anything.

That makes a dilated eye examination a genuinely useful assessment of vascular health, and it is why optometrists not infrequently send someone to have their blood pressure measured.

What is visible

Sustained high pressure changes the arteries. The walls thicken, the vessels narrow, and where an artery crosses a vein it begins to compress it — a sign with the unlovely name of arteriovenous nicking. As the process advances, small haemorrhages appear, along with cotton-wool spots marking areas where the nerve fibre layer has been starved of blood, and lipid deposits leaking from damaged vessel walls.

In severe, rapidly rising hypertension the optic disc itself swells. That is a medical emergency, and the eye may be where it is first recognised.

What it can cost you

Hypertensive retinopathy itself often does not affect vision until it is advanced. The sight-threatening consequences tend to be the events it makes more likely:

Retinal vein occlusion. A vein blocks and the area it drains floods with blood. Typically a sudden, painless loss or blurring of vision in one eye. Hypertension is the leading risk factor.

Retinal artery occlusion. A blocked artery, producing sudden painless loss of vision — often described as a curtain coming down. This is a stroke of the eye, it is a medical emergency, and it demands urgent investigation of the heart and carotid arteries, because the same embolus could next go to the brain.

Macular problems from chronic leakage and swelling.

Hypertension also compounds diabetic retinopathy substantially. Someone with both conditions is at considerably higher risk than either alone, and blood pressure control has been shown in large trials to slow retinopathy independently of glucose control.

The wider signal

Changes in the retinal vessels reflect what is happening in small vessels throughout the body — including in the kidneys and the brain. Retinopathy found on examination is associated with raised risk of stroke and of cardiovascular events. It is a finding about the whole person, not only about the eye.

Which is why the referral goes both ways: an optometrist who sees these changes will send you to have your blood pressure and general cardiovascular risk assessed, and a physician managing hypertension should be sending patients for retinal examination.

What to do about it

Know your blood pressure. It cannot be felt, and a very large number of Nigerians with hypertension do not know they have it.

If you are being treated, keep taking the medication. The commonest reason for poor control is that treatment is stopped once the person feels well — which they did before starting, and will continue to until damage accumulates.

Have a dilated retinal examination if you are hypertensive, and annually if you are also diabetic.

See someone urgently for sudden painless loss of vision in one eye, a curtain or shadow across the field, or sudden severe headache with visual disturbance. These are not things to sleep on.