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Primary Open Angle Glaucoma
Primary open-angle glaucoma is the most common type of glaucoma and the most common cause of irreversible blindness worldwide. Unfortunately, the condition is asymptomatic and the diagnosis is made when a significant part of the visual field has been lost. It is a chronic, progressive condition of the optic nerve characterized by:
– Beginning in adulthood
– Intraocular pressure above normal ( > 21 mmHg)
– Open angle of the anterior chamber (space between the iris and the cornea where the aqueous humor drains)
– Characteristic changes of the optic nerve
– Characteristic loss of visual field corresponding to optic nerve damage
– Absence of an underlying cause
– Both eyes are usually affected, possibly not to the same degree
Pathophysiology
Glaucoma is a condition of the optic nerve. Its pathophysiology is not very well understood, but we know that some genetic factors are involved. In glaucomatous damage we observe a progressive loss of the ganglion cells and their axons. In the early stages, only peripheral vision is affected. In advanced stages, however, it can also affect central vision and lead to irreversible blindness. Usually in primary open-angle glaucoma, the aqueous humor (the fluid that circulates in the anterior chamber of the eye) encounters increased resistance to its drainage for reasons we do not know and which are a field of extensive research nowadays. Treatment consists of lowering the intraocular pressure initially with drops or laser. In more advanced stages of the disease, surgical treatment may be necessary.
Epidemiology
– It is the most common type of glaucoma
– About 1-2% of adults over 40 have glaucoma but more than 50% of them don’t know it
– Incidence increases with age, affecting approximately 8% of people over 80 years of age
Risk factors
The risk factors for developing glaucoma are the following:
- Age over 60 years.
- Increased intraocular pressure: The normal pressure in the eye ranges from 10 to 21 mm Hg. Most patients with glaucoma have eye pressure above 21 mm Hg, but not all.
- Race: People of African, Hispanic, or Asian descent are more likely to develop glaucoma at a younger age.
- Family History: People who have a first-degree relative (parents or siblings) with primary open-angle glaucoma are at greater risk of developing it.
- Long-term use of cortisone: Long-term use of cortisone in the form of eye drops, pills, inhaled sprays or skin ointments can cause an increase in intraocular pressure, increasing the risk of glaucoma.
- Conditions that affect or are related to blood flow: Migraine sufferers, diabetic patients, hypotensive patients as well as hypertensive patients who take a large dose of medication are more likely to suffer from glaucoma.
- Myopia and thin central corneal thickness also predispose to glaucoma.
Symptoms
Unfortunately, in most cases, patients suffering from open-angle glaucoma do not show any symptoms. In the initial stages of the disease, a loss of peripheral vision is observed and usually one eye can compensate for the other and thus this loss is not noticed. Patients usually experience vision loss when vision is now centrally affected and the damage is very extensive. In these cases, more than 90% of the optic nerve fibers have been damaged.
We can usually detect primary open-angle glaucoma early by examining relatives of patients who already have glaucoma. In addition, a high intraocular pressure measurement or an abnormal visual field in a random eye examination may raise the suspicion of glaucoma.
Clinical examination
The clinical examination is detailed and includes:
– Measurement of vision and refractive error (presence of myopia, hyperopia or astigmatism)
– Measurement of intraocular pressure using a Goldmann applanation tonometer
– Measurement of the central thickness of the cornea (if the cornea is very thin, then the real pressure of the eye is greater than the one we measure, while if the cornea is thicker, the opposite is true)
– Examination of the anterior chamber
– Gonioscopy (examination with a special lens that is adapted to the surface of the eye after the instillation of anesthetic): we examine the depth of the angle of the anterior chamber, i.e. the space that exists between the cornea and the iris
– Examination of the optic nerve, determination of the central concavity, identification of any focal loss of nerve fibers. We also recommend visualizing the optic nerve with color fundus photography
– Optical coherence tomography of the nerve fibers of the optic nerve as well as the ganglion cells of the macula
– Automated examination of the visual field
Treatment
Glaucoma is a chronic disease and the purpose of treatment is to inhibit the progression of glaucomatous damage to the optic nerve and unfortunately not to eliminate it. Treatment aims to lower the intraocular pressure. It involves the use of eye drops, laser therapy or surgery that improves drainage of the aqueous humor and reduces intraocular pressure.
Pharmaceutical treatment
As far as antiglaucoma eye drops are concerned, we have available 4 pharmaceutical categories of substances as well as their combinations. These categories are: prostaglandin analogues, β-adrenergic blockers, carbonic anhydrase inhibitors and α2-adrenergic agonists.
Prostaglandin analogues
Mode of action: Increase drainage of aqueous humor through the uveoscleral tract
Contraindications: active inflammation, pregnancy, lactation
Common side effects in the eye: change in color of the skin around the eye, change in the color of the iris (affects people with light-colored eyes, e.g., blue or green iris color), thickening and lengthening of the eyelashes. Rarer: inflammation, pruritus, photophobia, keratitis
Systemic adverse effects: very rarely cause hypotension and bradycardia
β-adrenergic blockers
Mode of action: They reduce the production of aqueous humor
Contraindications: bradycardia, AV block, heart failure, asthma, history of chronic obstructive pulmonary disease (COPD)
Common side effects in the eye: redness, erythema, dry eye, allergic conjunctivitis, eczema
Systemic side effects: bronchospasm, bradycardia, worsening heart failure, nightmares
Carbonic anhydrase inhibitors
Mode of action: They reduce the production of aqueous humor
Contraindications: reduced renal function, liver failure, hypersensitivity to sulfonamides, pregnancy, lactation
Common side effects in the eye: foreign body sensation, lacrimation, allergic conjunctivitis, superficial punctate keratopathy, inflammation, transient myopia
Systemic side effects: bitter taste, nausea/vomiting, headache, dizziness, fatigue, hallucinations
A2 adrenergic agonists
Mode of action: They reduce the production of aqueous humor and increase its drainage through the uveoscleral tract
Contraindications: oral intake of monoamine oxidase inhibitors, pediatric patients
Common side effects in the eye: eyelid retraction, allergic blepharoconjunctivitis, contact dermatitis
Systemic side effects: dry mouth, fatigue, sleepiness
Selective laser trabeculoplasty (SLT)
The procedure known as laser trabeculoplasty or SLT targets the drainage system of the eye and works by increasing the drainage of the aqueous humor. Done by instillation of local anesthetic drops, it is quick and safe and is now recommended for almost all patients diagnosed with primary open-angle glaucoma as it can regulate the pressure over a period of months or years without the use of eye drops on a daily basis. (SLT link)
Surgical treatment
Intraocular pressure can be adjusted surgically by opening the eye’s drainage canal or by draining the aqueous humor under the conjunctiva (the thin transparent membrane that covers the eye).
In primary open-angle glaucoma nowadays, tiny devices are used to open the drainage channel that belong to the category of minimally invasive glaucoma surgery. (MIGS link).
The best-known anti-glaucoma procedure to treat primary open-angle glaucoma is called a trabeculectomy, and it drains the aqueous humor through a small hole in the eye wall. (trabeculectomy link)