Chronic open angle glaucoma

Glaucoma can be divided into open-angle or closed-angle glaucoma. Closed-angle glaucoma has been described in a separate article. Open-angle glaucoma accounts for the majority (74%) of glaucoma cases worldwide.

What is it?

Chronic open angle glaucoma is a progressive and irreversible optic neuropathy.

It is characterised by:

  • Progressive loss of peripheral vision
    Patient’s often complain of “tunnel vision”, difficulty driving, or running into objects around the home
  • In the late stages there can be central visual loss too
  • It normally affects both eyes but this is not strictly the case
  • There’s normally an associated increase in intraocular pressure but this is not a requirement (there are cases of normal-tension glaucoma)

To understand the causes, let’s look at the aqueous humour pathway:

Aqueous humour is produced by the ciliary body in the posterior chamber of the eye. It then drains into the anterior chamber of the eye via either the trabecular meshwork (majority) or the uveoscleral pathway (minority).

The exact cause of open angle glaucoma is not known, but is due to an increased resistance in trabecular meshwork drainage pathway.

Risk factors: remember to look out for these in your history and examination

  • Old age
  • Race (African-American)
  • Family history
  • Increased IOP
  • Myopia
  • Increased cup-to-disc ratio
  • Disc haemorrhage
  • Thin central corneal thickness
  • Low ocular perfusion pressure
  • Low blood pressure
  • High blood pressure
  • T2DM
  • Migraine / vasospasm
  • Low intracranial CSF pressure
  • Oral contraceptive pill
  • Smoking, obesity, alcohol, anxiety, stress, sleep apnoea

What to do when someone complains of the above features and you suspect open angle glaucoma:

  1. Perform direct ophthalmoscopy on both eyes looking at both optic discs. Look for signs of:
  2. Increased cup-disc ratio
  3. Cup-disc ratio asymmetry
  4. Disc haemorrhage
  5. Measure intraocular pressure (IOP)

Portable battery-powered automatic devices (e.g. Tono-pen) might be available to you. However, gold standard is to use Goldmann applanation tonometry.
Normal IOP = 12-21mmHg
Remember that IOP can have diurnal variation (higher in early morning), so do repeated measures at varied timepoints.
A difference of 3mmHg between the two eyes can be normal, however a difference greater than this should raise suspicion of glaucoma. Again, do repeated measurements.
Ensure patient is calm and comfortable when performing measurements as squeezing eyes can falsely elevate IOP.

  1. Should you have any suspicion of glaucoma, always refer for assessment by an ophthalmologist.

Further investigations by an ophthalmologist include:

  • Perimetry or visual field testing. Usually 40-50% ganglion cell loss has already occurred by the time perimetry gives abnormal results
  • Gonioscopy to exclude closed-angle glaucoma. Normal angle between iris and cornea is 20-45 degrees.
  • OCT looking for thinning of peripapillary retinal nerve fiber layer

Treatment is usually started when IOP is persistently >21mmHg.

Topical medication options include:

  • prostaglandin analogue
    • e.g. latanoprost, bimatoprost
    • Side effects: lengthening of eyelashes, pigmentation of lids/iris, exacerbation of uveitis/herpes, cystoid macular oedema
    • usual preferred therapy, reduces IOP by 25-33%
  • adrenergic agents
    • e.g. brimonidine, apraclonidine
    • Side effects: allergic blepharoconjunctivitis, apnoea, lethargy, bradycardia
    • reduces IOP by 20-25%
  • beta blockers
    • e.g. timolol (non-selective), betaxolol (selective)
    • reduce IOP by 20-25%
  • Carbonic anhydrase inhibitors:
    • e.g. dorzolamide, brinzolamide
  • Cholinergic (parasympathomimetic) agents:
    • e.g. pilocarpine

Systemic medications can be used in acute situations or when topical drops are not tolerated. These include:

  • Carbonic anhydrase inhibitor (acetazolamide)
  • Osmotic agents (e.g. mannitol, glycerol)

Longer term management options include:

  • Laser trabeculoplasty
  • Trabeculectomy
  • Glaucoma drainage device
  • Minimally invasive glaucoma surgery
  • Enucleation if pain remains intolerable
  • Retrobulbar injection of absolute alcohol if pain remains intolerable

Target IOP is usually set on an individual bases depending on severity of optic nerve damage and visual field abnormality.

Follow-up is based on the success of IOP reduction between visits and severity of optic nerve damage.

In resistant cases, always remember to check for compliance to topical medication! Non-compliance is commonly caused by forgetfulness, other priorities and lack of understanding. Therefore it is important for any medical staff to always ensure compliance and check patient understanding.

References:

  1. Mahabadi N, Foris LA, Tripathy K. Open Angle Glaucoma. 2022 Aug 22. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan–. PMID: 28722917.
  2. Philippin H. Management of chronic open-angle glaucoma. Community Eye Health. 2021;34(112):43-46. Epub 2022 Jan 31. PMID: 35210703; PMCID: PMC8862623.

Sheybani A, Jindal A, Salim S, Kozak A, Aref A, Akkara J, Khawaja A. Primary Open-Angle Glaucoma. American Academy of Ophthalmology EyeWiki.

Written by Dr. Sandra Halim IMT2. Reviewed by Dr. Shruti Chandra ST3 Ophthalmology

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