Primary Open Angle Glaucoma (POAG)
POAG is evenly distributed worldwide, and accounts for >70% of the overall incidence of glaucoma. The term “open angle” refers to the angle between the iris and sclera which is normal, unlike in ACG. POAG typically develops in adulthood and is usually bilateral.
POAG is characterized by degeneration of the RNFL in the area of the optic nerve head (ONH). The precise cause for this is unknown, but the mechanism of RNFL degeneration appears to be apoptosis of retinal ganglion cells (RGCs).
This leads to an increase in disc-to-cup ratio and a thinning of the RNFL as the RGCs are not replaced. This degeneration is progressive and may not result in symptomatic loss of visual field during a patient’s lifetime, but the rate of progression varies between patients.
Structural degeneration often precedes visual field loss in POAG.
Early detection of these changes is increasingly viewed as essential in preventing the progression of POAG to visual field loss. Indeed, early therapeutic intervention can prevent blindness in some patients in the long-term.
There is also a type of glaucoma called normal-tension glaucoma, which occurs without the elevated IOP associated with POAG. Incidence of NTG is particularly prevalent in Japan. NTG has been postulated to be the result of compromised optic nerve fibers or poor blood supply to the optic nerve. Current thinking is that NTG and POAG represent a continuum of OAG as there is considerable overlap between the two conditions.
Treatment for NTG is similar to that of POAG, with the aim of reducing IOP by at least 30%. Decreasing IOP (even though it is not elevated) is thought to slow progression of NTG. As with POAG, surgery is also an option, with trabeculoplasty as the primary option.
Angle Closure Glaucoma (ACG)
ACG is particularly prevalent in patients of Asian descent. The term “angle closure” refers to the angle between the iris and cornea (the junction of the cornea and the sclera). In angle closure glaucoma the anterior chamber tends to be smaller than average. Aqueous fluid is prevented from exiting the eye by a blockage of the drain. This leads to a rapid increase in intraocular pressure (IOP) giving rise to several symptoms including: eye pain; blurred vision; headache; and systemic effects such as nausea and vomiting.
Two types of ACG exist: acute and chronic. Acute angle-closure glaucoma is more severe and can result in long-term damage to the optic nerve, significant visual field loss or blindness within hours if not treated promptly. This highlights the importance of prompt diagnosis and treatment.
Chronic angle-closure glaucoma refers to an eye with a high IOP. The anterior chamber is narrow and closed in places by synechiae (adhesion). It is often not possible to identify what has caused the synechiae at the angle. But chronic angle-closure glaucoma is sometimes the result of slight intraocular inflammations that may have gone unnoticed.
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