PIGMENTARY GLAUCOMAOphthalmologist Carolina Valdivia, MD explains pigmentary glaucoma, its causes, risk factors and treatment options. Pigmentary glaucoma is a special form of open-angle glaucoma. This means that the angle formed by the iris and the cornea in the front part of the eye called the iridocorneal angle is open. However, drainage of fluid called aqueous humor out of the eye is impeded by alterations in a structure known as the trabecular meshwork. When aqueous humor drainage is impeded, pressure increases inside the eye (intraocular pressure), and this pressure can cause damage to the optic nerve, a bundle of nerve fibers in the back of the eye that carries visual images to the brain. This can result in a progressive loss of vision. Pigmentary glaucoma is related to another eye disorder known as
pigment dispersion syndrome (PDS).
In this condition, pigment granules that normally are found in specialized cells known as melanocytes at the back of the colored part of the eye (iris) slough off into the aqueous humor. These granules then become dispersed throughout the front part of the eye (anterior chamber), including the trabecular meshwork. However, having PDS does not necessarily mean that you have, or will develop, pigmentary glaucoma.
PIGMENT SHOWERSExtensive fluctuations in intraocular pressure can occur in pigmentary glaucoma, often reaching very high levels. Spikes due to the sudden release of large amounts of pigment from the posterior iris can occur spontaneously, due to cycloplegia, or after vigorous physical activity. Cycloplegia is paralysis of the ciliary muscle of the eye. This muscle plays an essential role in the focusing ability of the eye through a process called accommodation. Cycloplegia also compromises ciliary muscle tone on the trabecular meshwork, which negatively affects its filtering function and can result in elevated intraocular pressure. Cycloplegia can be induced by drugs, trauma, or disease. For pigmentary glaucoma, the primary cause is disease, due to a condition called reverse pupillary block. Normally, pressure between the anterior, posterior, and vitreous chambers of the eye is equalized. However, during pigment showers, outflow of aqueous humor through the trabecular meshwork temporarily can be impeded, raising pressure in the anterior chamber. The increased pressure within the anterior chamber forces the iris against the lens, causing the lens to block the open space of the pupil and thus maintaining the pressure disequilibrium between the chambers. The pressure difference causes a backward bowing of the iris. This concave configuration can cause a slight retraction of iris, so that it improperly inserts into the ciliary body, causing cycloplegia. Although pigment showers caused by vigorous physical activity do not occur in every patient, release of pigment from the iris, regardless of the mechanism, is thought to obstruct aqueous humor outflow by overwhelming the drainage angle of the anterior chamber. A majority of the liberated pigment appears to pass eventually through the trabecular meshwork and into Schlemm’s canal, as elevations in intraocular pressure tend to be transient and last just a few hours. Once in Schlemm's canal, the pigment granules are absorbed into pores within and between cells. Damage to the optic nerve varies with the extent and duration of elevated intraocular pressure. Beyond this, pigmentary glaucoma displays typical
signs and symptoms
observed in primary open-angle glaucoma.
PIGMENTARY GLAUCOMA SYMPTOMS AND DIAGNOSISAs with most types of glaucoma, patients with pigmentary glaucoma usually do not recognize any symptoms. However, a few patients have reported blurred vision and colored halos around lights after vigorous exercise. These symptoms are correlated with elevated intraocular pressure. The key feature of pigmentary glaucoma is a shedding of melanin pigment from cells called melanocytes in the iris pigment epithelium, located at the back of the iris and its subsequent accumulation on eye structures in the anterior chamber of the eye. I have explained the mechanism behind this on the pigment dispersion syndrome page, so I will not repeat the details here. Although the iris pigment can be distributed extensively, it tends to collect in characteristic regions inside the eye. This results in a distinctive array of diagnostic symptoms associated with pigmentary glaucoma.
Since most of you will not be familiar with these terms, I will explain them below. Pigment Accumulation in the Trabecular Meshwork. A gonioscopy exam usually reveals a solid, dense band of pigment in the trabecular meshwork. In severely affected individuals, this increased pigmentation may extend to Schwalbe’s line in a thin band called Sampaolesi’s Line. Iris Concavity. Defined as a bending inward of the iris toward the lens of the eye, iris concavity is associated with myopia (nearsightedness) and a phenomenon called reverse pupillary block. In reverse pupillary block, pressure in the anterior chamber of the eye exceeds that of the posterior chamber. The increased pressure causes a backward bowing of the iris, forcing it over the lens and accentuating contact between the iris and the Zonules of Zinn. The zonules are thin filaments that form a scaffolding that suspends the lens inside the eye. Mechanical abrasion between the zonules and the posterior iris then can cause dispersion of iris pigment.
Iris Transillumination Defects. People with pigmentary glaucoma often exhibit iris transillumination defects. These are areas of pigment loss on the iris that appear as bright spikes emanating outward from the pupil, much like the spokes on a bicycle wheel. They are believed to be caused by mechanical rubbing of packets of 65-80 Zonules of Zinn fibers against the back of the iris, which liberates pigment and causes it to inappropriately disperse throughout the eye.Ocular transillumination is a medical test that shines light from inside the eye to the outside. It is based on the simple principle that light is absorbed by various tissues within the body at different wavelengths. Children are fascinated by this phenomenon and often can be seen in a darkened room covering the lamp of a flashlight with their hand and observing how the light reveals structures inside their hand or closing their mouth around a small flashlight and turning it on. I obviously cannot insert a light inside your eye for direct illumination. However, I am able to accomplish the same thing indirectly using one of two procedures. For a general view, I place a concentrated light source on either the upper or lower eyelid toward the outside corner. All lights are turned off to maximize visibility of structures. Some of the light transmits to the interior of the eye, making the areas of pigment loss visible. This is seen as a negative image, in which areas of pigment loss glow and areas where pigment remains are dark. For a more extensive and specific view, I shine a bright beam of light through your pupil onto the retina using an apparatus called a slit-lamp microscope. Most of this light is absorbed, but a significant portion is reflected back onto the internal structures of the eye. I then examine the eye at a slight angle to avoid being blinded by reflected light coming back through the pupil. Other Areas of Dispersed Pigment. A characteristic sign of pigmentary glaucoma is Krukenberg's Spindle. This is a narrow, vertical, spindle-shaped brown band of pigment deposited on the cornea of the eye, created by flakes of pigment rubbed off the back of the iris. Dispersed pigment is also found scattered on the surface of the iris, on the Zonules of Zinn, and at the junction of the posterior lens capsule and zonules, referred to as Scheie's stripe.
CAUSES OF PIGMENTARY GLAUCOMAThe first explanation that comes to mind for most people is that accumulated pigment within the trabecular meshwork physically blocks aqueous humor outflow and causes elevated intraocular pressure in pigmentary glaucoma. This process would be analogous to plugging a kitchen-sink drain catcher with coffee grounds. Supporting this viewpoint, pigment showers in which large amounts of pigment are dislodged from the posterior iris are observed in some patients and cause spikes in intraocular pressure. The sudden and transient quality of these spikes can be explained by obstruction to aqueous humor outflow caused by accumulations of pigment within the trabecular meshwork. Studies also have examined pigment accumulation in what is called the juxtacanalicular tissue. This is defined as the tissue underlying Schlemm's canal. Remember that Schlemm's canal is located directly after the trabecular meshwork and functions to convey filtered aqueous humor back into the bloodstream. The amount of pigment consistently present in the juxtacanalicular tissue is insufficient to account for the inability of aqueous humor to pass through the trabecular meshwork. This means that despite evidence that pigment showers can temporarily influence intraocular pressure through a simple physical blockade of the trabecular meshwork, this does not appear to be the major mechanism causing pigmentary glaucoma. At present, the actual mechanism remains unknown. However, studies in animals suggest that pigmentary glaucoma is the outcome of a multi-step process that involves a complex set of reactions to the dispersed pigment. For example, if pigment is injected into the anterior chamber in experimental animals, it results in trabecular meshwork pigmentation, but does not cause lasting changes in aqueous humor outflow. This inducible model mimicks pigment dispersion syndrome but not pigmentary glaucoma. Gareth Howell and colleagues provided another line of evidence using genetics. DBA/2J is an established mouse strain for inherited glaucoma that shows iris pigment dispersion, elevated intraocular pressure and optic nerve degeneration. Howell and his team demonstrated that changes in two genes of these mice can convert them from a strain exhibiting glaucoma symptoms to one that only shows signs of pigment dispersion syndrome. This research strongly suggests that pigmentary glaucoma has a genetic component, involving at least two genes.Other researchers have elaborated theories that involve alterations in the structural integrity of the trabecular meshwork. In this model, trabecular endothelial cells ingest large amounts of pigment through a process called phagocytosis, which causes them to be overloaded, producing cell injury and death. Trabecular beams containing dead or damaged cells then can adhere to each other, obliterating the aqueous spaces between them. Loss of these aqueous spaces in enough regions around the trabecular meshwork could lead to increased resistance to aqueous humor outflow. Glaucoma would then follow through the typical open-angle mechanism. The various theoretical mechanisms for pigmentary glaucoma should not be considered mutually exclusive, such that demonstrating one will automatically invalidate the others. On the contrary, they more than likely are complementary, accounting for portions of the explanation but not the entire thing. For example, the two genes that were modified in the DBA/2J mice code for proteins related to cell structure. While these have been studied relative to the iris pigment cells, they potentially also could have a role in maintaining trabecular meshwork integrity.
CHARACTERISTICS OF PIGMENTARY GLAUCOMAWith the exception of the accumulation of iris pigment in atypical areas of the eye, pigmentary glaucoma exhibits the same characteristics as primary open-angle glaucoma.
RISK FACTORS FOR PIGMENTARY GLAUCOMAThe following factors have been associated with an increased risk for developing this ocular disease.
Young males with myopia more frequently develop pigmentary glaucoma and males typically require more aggressive medical and surgical therapy.
TREATMENT OF PIGMENTARY GLAUCOMATreatment of this disease follows a similar course to that of other types of open-angle glaucoma. Medications are usually the first option, often with multiple medications required to control pressure fluctuations. First-line choices include Betagan (Levobunolol), Timoptic or Betimol (Timolol), Optipranolol (Metipranolol) and Xalatan (Latanoprost). These eyedrops have a relatively low incidence of side effects and generally are well-tolerated in younger patients. Sometimes another class of medications called miotics are used. Examples include Pilocar and Ocusert. Miotics cause pupillay constriction (make the pupil smaller), which can inhibit the iris from rubbing against the zonules (supporting fibers), helping to prevent further release of pigment. However, these medications have side effects that limit their use, such as a disabling visual blurring in younger patients. Because there really are two factors occurring that serve to raise intraocular pressure in patients with pigmentary glaucoma – alterations in trabecular meshwork integrity and reverse pupillary block, medications alone often are not sufficient to lower eye pressure to safe levels. In these cases surgery is warranted. Three procedures typically are used.
Because of the complex nature of pigmentary glaucoma, more than one type of surgery may be required in combination with the use of eye drops to achieve a sustainable drop in intraocular pressure. Although most types of glaucoma do not exhibit acute symptoms, at times the rapid elevations in intraocular pressure characteristic of pigmentary glaucoma can result in pain and blurred vision. Some patients report seeing halos around lights. You should report any symptoms like this to your ophthalmologist immediately.
REFERENCESHowell GR, Libby RT, Marchant JK, Wilson LA, Cosma IM, Smith RS, Anderson MG, John SW. Absence of glaucoma in DBA/2J mice homozygous for wild-type versions of Gpnmb and Tyrp1. BMC Genetics. 2007;8:45. Niyadurupola N and Broadway DC. Pigment dispersion syndrome and pigmentary glaucoma - a major review. Clinical and Experimental Ophthalmology. 2008;36(9):868-882. Yang JW, Sakiyalak D, Krupin T. Pigmentary glaucoma. Journal of Glaucoma. 2001;5 Suppl 1):S30-S32.
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