Gonioscopy

Gonioscopy is the examination of the drainage angle of the eye, the junction between the iris and the cornea where aqueous humour leaves the eye. The angle cannot be seen with a slit lamp alone, because light from the angle is totally reflected at the corneal surface. A gonioscopy lens, with a mirror or prism, removes this reflection and shows the angle structures directly.

Gonioscopy is the reference method for classifying glaucoma, and it separates open angle from angle closure disease, which have different treatment.

Two types of lens are used. An indirect lens (a mirrored lens placed on the cornea with a coupling fluid) is used at the slit lamp, and shows the opposite side of the angle through a mirror. It is the most used form, and in the dynamic form a small lens that sits on the cornea without a coupling agent allows pressure on the cornea to open an angle that is closed by apposition.

A direct lens is used with the patient lying and with a hand held microscope (in infants under anaesthesia). The structures seen, from front to back, are Schwalbe line, the trabecular meshwork (with pigment in the posterior part), the scleral spur, and the ciliary body band, and the iris root.

The test takes 2 to 5 minutes for each eye. The angle is graded using a grading system based on the structures visible, and assessed for width, iris shape, pigment, new vessels, and adhesions (synechiae).

Anterior segment optical coherence tomography gives an objective, non-contact image, and agrees reasonably with gonioscopy, but gonioscopy still shows the colour, pigment, vessels and fine structures. Primary angle closure affects about 17 million people worldwide, mostly in Asia, and is more likely to lead to blindness than open angle disease.

Indications.

• All patients with glaucoma or suspected glaucoma, at diagnosis, and repeated at intervals, as the angle changes over time.
• Narrow or shallow anterior chamber, or suspected angle closure, with symptoms of intermittent pain, halos or headache.
• Acute angle closure, and the assessment of the fellow eye.
• Raised intraocular pressure with unclear cause: pigment dispersion, pseudoexfoliation, uveitic, steroid induced, and traumatic angle recession.
• Neovascularisation of the iris and angle in diabetes and vein occlusion.
• Before and after laser or surgery for glaucoma (iridotomy, trabeculoplasty, angle surgery).
• Eye trauma (angle recession, cyclodialysis, hyphaema after healing), tumours of the angle, and developmental glaucoma.
• Before dilation in patients at risk of angle closure, such as hyperopes and older patients with shallow chambers.

Not indicated in the acute setting when the cornea is too oedematous to view, and in patients with open globe injury or very recent surgery.

Contraindications and precautions.

• Open globe injury, or recent surgery with a fragile wound: no contact, no pressure.
• Active corneal infection, severe corneal epithelial defect, or recent corneal graft: contact is avoided or done with great care.
• Hyphaema: gonioscopy is delayed until the blood has cleared, as pressure on the eye risks rebleeding.
• Allergy to anaesthetic drops or coupling gel.
• Patients who cannot cooperate or keep still, or who have blepharospasm: examination under anaesthesia.
• Cleaning of lenses and disinfection between patients prevent spread of infection (adenovirus, herpes), and disposable lenses are used in some clinics.
• A corneal bandage lens, or a very dry cornea, reduces the view.

Preparation and technique.

The patient sits at the slit lamp with local anaesthetic in the eye. The room is dimmed, and the slit beam is narrowed to a short, thin beam, and the lowest light level that allows a view is used, since bright light narrows the pupil and widens the angle, giving a false impression. The lens is placed gently on the cornea, with coupling fluid when needed, and the patient looks straight ahead.

All four quadrants are viewed by rotating the lens, and the angle is graded in each. The patient is asked to look toward the mirror to view over the iris convexity. In dynamic (compression) gonioscopy, gentle pressure is applied with a four mirror lens, pushing aqueous toward the angle and opening an angle that is closed by apposition, and so distinguishing it from permanent adhesions.

Normal appearances.

In an open angle, the examiner sees all structures from Schwalbe line to the ciliary body band, in all four quadrants. The trabecular meshwork is greyish, with a speckled band of pigment at the back that increases with age, and the lower angle is usually the most pigmented. The iris approaches the angle with a gentle slope and no bulge. The angle is widest in myopes and in young people, and narrowest in hyperopes.

Interpretation.

• Open angle: the trabecular meshwork is visible for 360 degrees. Glaucoma with an open angle is called primary open angle glaucoma, and other open angle causes are looked for.
• Narrow angle, or occludable angle: the trabecular meshwork is visible for less than 90 degrees (some definitions use 180 degrees) without compression. It carries a risk of closure.
• Primary angle closure: the angle is closed (iris touching the trabecular meshwork) for 180 degrees or more, with raised pressure or peripheral anterior synechiae. Primary angle closure suspect: appositional contact without raised pressure or damage. Acute angle closure: a closed angle with very high pressure and symptoms.
• Plateau iris: an anteriorly placed ciliary body, with a flat iris plane and a narrow angle, and a "double hump" sign on indentation.
• Secondary closure: peripheral anterior synechiae from uveitis, neovascularisation (new vessels crossing the angle in diabetes and vein occlusion), pulling by lens swelling or tumour, and pushing from lens dislocation or tumours behind the iris.
• Pigment dispersion syndrome: a dense, dark band of pigment on the trabecular meshwork, with an iris that bows backwards. Pseudoexfoliation: flaky material and patchy pigment, with a line of pigment anterior to Schwalbe line.
• Angle recession after blunt trauma: a wide ciliary body band with torn iris processes, and a risk of late glaucoma.
• Inflammation: nodules, keratic precipitates in the angle, and synechiae.
• Foreign body, tumour, or cyst in the angle, and abnormal vessels.
• Congenital abnormalities: iris processes, and an anterior insertion of the iris in developmental glaucoma.

Critical values and findings needing immediate action.

• Acute angle closure (closed angle, pressure high, pain, halos, vomiting): emergency treatment to lower the pressure, followed by laser iridotomy, and treatment of the other eye.
• Neovascularisation of the angle with raised pressure: urgent referral for treatment of the underlying ischaemia and the pressure.
• Closed angle in a patient about to be dilated or given drugs that dilate the pupil (anticholinergic, some antidepressants): caution, and assessment first.
• Angle tumour or bleeding in the angle: urgent specialist assessment.
• Signs of developing synechial closure in an eye with symptoms or rising pressure: urgent review.

Limitations and artefacts.

• Light intensity, beam size and lens pressure can change the angle width. Bright light causes pupil constriction and opens the angle falsely. Pressing on the cornea with an indirect lens distorts the angle and opens it, or causes folds in the cornea that make the view poor.
• Tilting the lens and the patient's gaze alters the view, and a tilted lens gives a false view of a narrow angle.
• Corneal haze or oedema, scarring, and poor cooperation limit the view.
• Experience is needed. Identification of the structures and recognition of pigment and synechiae takes practice, and grades differ between examiners.
• Examination is subjective, and a single result may not be repeatable. Anterior segment optical coherence tomography gives objective, documented images in the dark, and shows the angle in a reproducible way.
• An angle that is open in a lit room may close in the dark or on dilation.
• Gonioscopy cannot show structures behind the iris, such as plateau configuration and ciliary body disease, for which ultrasound biomicroscopy and optical coherence tomography are used.

Special groups.

• Patients of East Asian ancestry and hyperopes: a high risk of primary angle closure, and gonioscopy is done at diagnosis.
• Patients of African ancestry: open angle glaucoma is common, but angle closure and other secondary causes are still looked for.
• Infants and children with suspected developmental glaucoma: examination under anaesthesia with a direct lens.
• Older patients with cataract: the thick lens narrows the angle, and cataract surgery deepens it.
• Diabetic patients and those with vein occlusion: look for new vessels in the angle before dilating, and at each review.
• Patients after trauma: angle recession is looked for with the fellow eye as comparison, and followed for life for glaucoma.
• Regions: angle closure glaucoma is a leading cause of blindness, and is under diagnosed. A simple torchlight test (the oblique flashlight test, in which the shadow on the iris suggests a shallow chamber) and the slit lamp peripheral chamber depth give an approximate assessment where gonioscopy is not available. Laser iridotomy and cataract extraction can prevent blindness, but need early detection. Gonioscopy lenses are inexpensive and give a high yield, and the skill can be taught.

Follow up tests.

• Intraocular pressure measurement (tonometry).
• Anterior segment optical coherence tomography or ultrasound biomicroscopy.
• Dark room provocative tests, where used, and repeat gonioscopy after dilation or laser.
• Optic disc examination, nerve fibre layer optical coherence tomography, and visual field testing.
• Laser peripheral iridotomy, and review of the angle after laser.
• Assessment of the fellow eye.
• Review at set intervals, since the angle changes with age, lens growth and treatment.