Glaucoma is an eye disorder which leads to progressive damage of the optic nerve in a characteristic pattern. The optic nerve is involved in carrying the visual impulse from the retina to the brain enabling us to see this impulse as vision. Glaucoma is generally but not always caused due to increase in the eyeball or intraocular pressure; in few glaucoma may be caused with normal eye pressure.
There is no set elevated value of intraocular pressure (IOP) which definitely leads to glaucoma; also there is no lower limit which eliminates the risk of glaucoma. Hence early diagnosis and treatment is absolutely essential.
According to WHO, glaucoma is the second leading cause of blindness worldwide after cataracts.
The eye is like a football, round and firm. The pressure inside the eyeball is known as the intraocular pressure which help maintains the shape and tone of the eye. This pressure is normally between 8-22 mm Hg. Increase in this pressure causes damage to the optic nerve located at the back of the eye as its fibres are most vulnerable to pressure changes.
This pressure is helped maintained by the fluid known as aqueous humor which provides nourishment to structures located in front of the eye. It is produced by the ciliary body which surrounds the lens; from there the fluid travels via the pupil to the angle formed by the iris and cornea to be drained via the trabecular meshwork out of the eye (Fig 1).
Fig 1 : Drainage pathway of aqueous humor of the eye
So if the trabecular meshwork which is like the sieve in the drainage pipe gets blocked, there is backup of aqueous humor and pressure rises as eye is closed system and there is no other way for the fluid to escape the eye. In angle closure variety the drainage angle is reduced so much that the outflow tract is altogether cut off leading to acute severe attacks.
As glaucoma causes optic nerve damage without producing and symptoms and the visual loss is peripheral which gets unnoticed, regular checkup in those at risk is essential to prevent visual loss.
• More than 45 years of age
• Family history of glaucoma
• History of raised intraocular pressure (IOP)
• High degree of Nearsightedness and Far sightedness
• History of injury to the eye
• Use of steroids (prednisone), either in the eye, orally or injected)
There are two major types of glaucoma, chronic - open angle glaucoma or acute- closed angle glaucoma. The angle refers to the drainage angle between the cornea and iris which helps regulates outflow of aqueous fluid via the trabecular meshwork in the eye.
Other variants include; normal tension glaucoma, congenital glaucoma, pigmentary glaucoma and secondary glaucoma.
Glaucoma is leading cause of blindness as it is painless and without any symptoms in the initial stage until permanent visual loss occurs due to damage to the optic nerve initially affecting the peripheral vision (Fig 2). This is commonly seen in the chronic open angle glaucoma. The eye appears normal externally. On examination, raised IOP, peripheral visual field disturbances and optic nerve damage may be found.
Fig 2 A: Normal Vision
Fig 2B: Loss of peripheral vision in glaucoma
The acute closed angle glaucoma normally presents with acute crisis characterised by sudden eye pain, blurry vision, nausea and vomiting, halos around light and raised IOP (>30 mmHg). The eye appears red and the pupil appears dilated and fixed. The cornea appears cloudy and hazy. On examination, the IOP is raised, visual acuity is decreased, the pupil is non reactive to light and a closed/acute drainage angle is noted (Fig 3). This is an emergency and immediate treatment should be sought to prevent permanent damage.
Acute angle closure glaucoma can be triggered by medications that dilate the pupil, eg eye drops like tropicamide, atropine, sea sickness and cold medications; in areas of dark room with focused bright light like movie halls.
Fig 3 : Acute red eye in angle closure glaucoma
Several tests are available to help the ophthalmologist detect the glaucoma. These tests include:
1. Tonometry: It measures the tone or the firmness of the eye. The eye is numbed and the tonometer is placed on the cornea of the eye. Greater the firmness, higher is the pressure.
2. Pachymeter: It is used to measure the thickness of the cornea, as too thick cornea can give false reading on tonometry.
3. Opthalmoscopy: The eye is dilated with eye drops and the ophthalmologist uses this scope with light at one end to look at the back of the eye and examine the optic nerve and the fundus for any changes like cupping or indentation of optic nerve head or paleness due to increased IOP.
4. Perimetry: This is used to examine and map the peripheral field of vision as glaucoma initially affects only peripheral field, which at most times is unnoticed. The patient looks straight ahead and asked to indicate when the light passes in his peripheral field of vision.
5. Gonioscopy – Used to visualise and measure the drainage angle between the cornea and iris using a lens based device. Helps identify angle closure glaucoma.
Other methods of monitoring glaucoma involve the use of sophisticated imaging technology — such as scanning laser polarimetry (SLP), optical coherence tomography (OCT) and confocal scanning laser ophthalmoscopy — to create baseline images and measurements of the eye's optic nerve and internal structures.
Glaucoma is treated by either medications or surgery.
Eye drops medications are first used to lower IOP by decreasing the production or increasing the outflow of aqueous humor via trabecular meshwork. These include beta blockers, prostaglandin analogues, carbonic anhydrase inhibitors etc.
Laser treatment includes laser iridotomy which creates a hole in the iris to allow for drainage by bypassing the pupil and relieving the pressure, commonly used in angle closure glaucoma. Laser trabeculoplasty is used in open angle variety to remove the blockage at the trabecular meshwork.
Surgical correction like the trabeculectomy is used to remove the clogged trabecular meshwork and create a new drainage pathway. If successful it is the most effective way of lowering IOP.