Lateral Canthotomy, AC Paracentesis and Pupil Examination: How-To for Residents

Last updated: October 2026

Three bedside skills every ophthalmology resident must be able to do confidently, often at 2 a.m.: a sight-saving lateral canthotomy, a safe anterior chamber paracentesis, and a pupil examination that actually answers the clinical question.

1. Lateral canthotomy and inferior cantholysis

When to do it

Orbital compartment syndrome, usually from a retrobulbar haemorrhage after trauma or surgery. Look for tense proptosis, a hard orbit on palpation, raised IOP, reduced eye movements, falling vision and a RAPD. This is a clinical diagnosis: do not wait for imaging.

Equipment

Povidone-iodine, 2% lidocaine with adrenaline, a 25G needle, a straight haemostat, blunt-tipped scissors (e.g., Stevens or Westcott), and toothed forceps.

Steps

1. Explain the procedure and take consent if the patient can give it. Clean the lateral canthal skin.
2. Inject 1–2 mL of local anaesthetic into the lateral canthus, with the needle pointing away from the globe.
3. Crush the lateral canthus with the straight haemostat from the canthal angle to the orbital rim for about a minute. This reduces bleeding and marks the line of the cut.
4. Canthotomy: cut horizontally along the crushed line, about 1 cm, down to the orbital rim.
5. Inferior cantholysis (the step that actually decompresses the orbit): grasp the lower lid and pull it forwards and away from the globe. With the scissor tips pointing down towards the chin, "strum" to feel the taut inferior crus of the lateral canthal tendon, then cut it. The lower lid should now move freely away from the globe.
6. Reassess IOP, vision and the pupil within 15 minutes. If the orbit is still tight, perform a superior cantholysis, taking care to avoid the lacrimal gland and levator.
7. Leave the wound open. It usually heals well, and formal repair can be done later if needed.

Common mistake: stopping after the canthotomy. The skin cut alone does not decompress the orbit; the cantholysis does.

2. Anterior chamber paracentesis (AC tap)

When to do it

• Diagnostic: aqueous sample for PCR (HSV, VZV, CMV, toxoplasma) in uveitis, or microbiology in suspected endophthalmitis
• Therapeutic: temporary IOP lowering, for example in a very high-pressure eye while medical treatment takes effect

Steps

1. Position the patient at the slit lamp, or lying flat under the microscope. Give topical anaesthesia.
2. Instil povidone-iodine 5% into the conjunctival sac and on the lashes. Insert a lid speculum.
3. Use a 27G or 30G needle on a 1 mL (tuberculin) syringe.
4. Brace your hand on the patient's cheek or forehead so that your hand moves with the patient's head.
5. Enter through clear cornea just inside the limbus (temporal is easiest), bevel up, with the needle parallel to the iris plane and the tip over the peripheral iris, never over the lens.
6. Withdraw 0.1–0.2 mL slowly while watching the anterior chamber shallow. Then withdraw the needle along the same path.
7. Check the IOP and the wound, and give a topical antibiotic. Send the sample immediately and label it clearly.

Complications to avoid: lens touch (cataract), iris trauma, hyphaema, and endophthalmitis. Most of these come from a needle angled towards the lens or a patient who moves; bracing prevents both.

3. The pupil examination, done properly

Set-up

Use a dim room, ask the patient to look at a distant target (to avoid near miosis), and use a bright torch. Measure pupil size in both light and dark.

Swinging flashlight test for a RAPD

• Shine the light on each eye for about 2–3 seconds, swinging briskly from one to the other. Watch the eye being illuminated.
• A RAPD is present if the pupil dilates when the light swings onto it.
• Quantify it with neutral density filters (0.3 log unit steps) over the good eye until the response balances, or grade it clinically from 1+ to 4+.
• Reverse RAPD: if one pupil is fixed (for example after trauma or drops), watch only the reactive pupil while swinging the light. The afferent defect can still be detected.
• A dense cataract or vitreous haemorrhage rarely causes a significant RAPD. If one is present, look for optic nerve or extensive retinal disease.

Anisocoria: which pupil is abnormal?

Pharmacological tests

• Apraclonidine 0.5%: in Horner syndrome, the small pupil dilates (denervation supersensitivity), so the anisocoria reverses. It may be falsely negative in very recent Horner.
• Cocaine 10%: fails to dilate a Horner pupil. Useful but hard to obtain.
• Hydroxyamphetamine 1%: dilates a pre-ganglionic (first or second order) Horner pupil, but not a post-ganglionic (third order) one. Use it to localise.
• Dilute pilocarpine 0.1%: constricts an Adie tonic pupil (cholinergic supersensitivity), but has little effect on a normal pupil.
• Pilocarpine 1%: constricts a third-nerve pupil, but not a pharmacologically blocked pupil (atropine, scopolamine patches, plant exposure).

Light-near dissociation

Test near response only if the light reaction is poor. Causes include Argyll Robertson pupils, Adie tonic pupil, dorsal midbrain (Parinaud) syndrome, diabetic autonomic neuropathy, and severe bilateral afferent visual loss. See our anisocoria page.