Dilated fundus examination: when and how
- Examination
Through an undilated pupil the ophthalmoscope shows little more than the disc. Dilating drops open the view to the macula and periphery, with few risks.
See it in 3D: Optic disc · Retina · Macula
What it is
The fundus is the back of the eye: optic disc, retinal vessels, macula and peripheral retina. A direct ophthalmoscope through a small pupil shows a narrow area around the disc. Dilating the pupil with short-acting drops widens the view; ophthalmologists add binocular indirect ophthalmoscopy or slit lamp lenses to see the periphery, where retinal tears hide. Diabetic retinopathy screening needs a dilated exam or retinal photographs.
Normal values and scales
- Tropicamide acts quickly and lasts a few hours; phenylephrine adds dilation through the sympathetic route.
- Dilation rarely triggers acute angle closure; a shallow anterior chamber is a reason for caution, not an absolute contraindication.
- In neurological observation (head injury, coma) pupils are not dilated without agreement, and if they are it must be recorded clearly.
- Normal cup-to-disc ratio is usually under about 0.5, and similar in both eyes.
How it is done
- Measure visual acuity and check the pupils before dilating, and look at the anterior chamber depth with a penlight.
- Instil a short-acting dilating drop (tropicamide, often with phenylephrine) in each eye.
- Wait about 15 to 30 minutes for full dilation.
- With the direct ophthalmoscope, find the red reflex, follow a vessel to the disc, then look at each quadrant and finally at the macula.
- Describe the disc (colour, margins, cup), the vessels, haemorrhages or exudates, and the macula.
- Warn the patient about blurred near vision and glare for several hours, and advise not to drive until it settles.
How to interpret it
- Peripheral retinal tears cannot be excluded with a direct ophthalmoscope even through a dilated pupil.
- The disc and vessels show raised intracranial pressure, hypertension, diabetes and glaucoma.
- If dilation causes a painful red eye with halos, it is acute angle closure: better found in the clinic than later.
Next step / Initial management
- Abnormal disc (swelling, pallor, large or asymmetric cup) or retina: refer according to the finding (urgent for swelling or haemorrhages with symptoms).
- Diabetes: dilated exam or retinal photography at diagnosis of type 2 and then periodically.
- New floaters, flashes or a shadow: dilated peripheral exam by ophthalmology.
- Painful red eye after dilation: emergency angle closure pathway.
Test yourself
Think of the answer, then tap the question to see it.
Why is the pupil dilated for a fundus exam?
Through a small pupil only a narrow area around the disc is seen; dilation shows the macula and more of the retina.
What should be done before dilating the pupils?
Measure visual acuity, check the pupil reactions and estimate the anterior chamber depth.
When should dilation be avoided or clearly documented?
In patients under neurological observation, where pupil changes are monitored.
Elsewhere in DurgiPedia
Connections
See it in 3D3
The structures of Durgical's 3D anatomy where it is found.
In Pharmacology3
Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.
Sources: Auxiliary Lenses for Slit-Lamp Examination of the Retina (StatPearls Publishing (NCBI Bookshelf), 2023); Acute Angle-Closure Glaucoma (StatPearls Publishing (NCBI Bookshelf), 2026); Diabetic Retinopathy (StatPearls Publishing (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.