Glaucoma Blinding Condition And Optic Neuropathy Biology Essay

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Glaucoma is a potentially a blinding condition, a disease which exhibits a characteristic optic neuropathy which may result in progressive visual field loss. (Kanski, 2006). Glaucoma was considered a neurological disease, as the association in raising intraocular pressure (IOP) occurred over several centuries but was boosted by apparatus improvements in tonometer between 1880 and 1910. (Paul N.S. et. al, 2010). Development of ocular measurement apparatus and findings can create a new concept that may change the terminology of glaucoma. Yet, IOP is known to be the major risk factor among others which is proven in so many cases around the world. (Quigley et. al, 2006).

Glaucoma has been highlighted to be the second largest cause of blindness in the world and it getting increase yearly. It has been expected to increase by 79.6 million by year 2020 with 74% of these will be Open Angle Glaucoma (OAG). (Quigley et. al, 2006). The status as the second largest cause of blinding glaucoma were also agreed by Leske M.C., 2007 but there are various gap of etiology that need further clarification to allow better comparability across studies. Identifying through the risk factors helps glaucoma condition to be clearly understood the way it effect the patients and other related complications. The goal of identifying the risk factors for glaucoma is to recognize the possibility of patient with greater risk to have a symptomatic visual loss that effect their quality of life. (Keith E. et. al, 2009).

General risk factor for glaucoma had been long discussed which include the age, race, family history, ocular risk factors (IOP, Optic nerve head features), myopia, systemic risk factors (diabetes mellitus, young age, inadequate IOP control, high rate of progression despite treatment). (Allingham R.R. et. al, 2005). Elevation in intraocular pressure (IOP) has long been considered to be the primary cause of glaucoma. High in IOP usually defined as greater than 21mm mercury (Hg). However, even people with normal pressures can develop glaucoma too. Many research shows that taking eye drops can help to lower the IOP elevation to diminish the risk of glaucoma. Although having 21mmHG of IOP, this does not mean that we have glaucoma but considered as a risk factor. Upward of six million people in United States have elevated IOP without demonstrable perimetric visual field damage. (Kreuger D.E. et. al, 1980; Armaly M.F. et. al, 1980; Quigley H.A. et. al, 1994).

In black people, open angle glaucoma is the leading cause of blindness and is 6~8 times more common than in Caucasians. It is also been notified that the risk is high for people who is over 40 years of age. As for Asian and Eskimos, more likely to develop closed-angle glaucoma than other races.

A recent large clinical trial discovered that patients with thinner corneas (the clear structure at the front of the eye) are at an increased risk of developing glaucoma. They also found that African-Americans have thinner corneas than Caucasians.

Patient with corneal thickness less than 555microns have three fold greater risk of developing glaucoma as compared with those who’s cornea are more than 588microns thick. Some specialist believe that there is a common collagen abnormality found in both cornea and lamina cribosa that predisposes patient to glaucoma. Although corneal thickness is part glaucoma risk factor, this has not yet translated to clinical applicability as many question about corneal thickness remain unanswered.

Age is another risk of getting glaucoma after age of 50. Some other finding suggest that at age of 40 the risk of getting glaucoma is true for black people. However, glaucoma can occur in anyone at any age which include congenital & juvenile glaucoma. Since majority glaucoma recorded comes from older citizen, priority of the risk glaucoma were classified to be 50 years of age.

Hereditary were also considered as a risk factor for glaucoma disease. In case of congenital glaucoma that appears in the first months of life, eventually at birth or in utero. Congenital glaucoma is characterized by minor malformations of the irido-corneal angle of the anterior chamber of the eye. Other observation finding include tearing, photophobia and enlargement of the globe appearing in the first month of life. Heredity of congenital glaucoma is autosomal recessive which involve CYP1B1, GLC3A and GLC3B. Juvenile glaucoma is considered as a primary open angle glaucoma which appearing during first two decades of life. Primary open-angle glaucoma has high IOP elevation (> 21mmHG), excavation of the optic nerve head & lost of visual field. POAG is the most prevalence type of glaucoma, affecting 1 in 100 population of 40 years of age. Treatment involve medical and often surgical. Heredity is autosomal dominant, and there are two genes that have been identified, MYOC (myocilin genes) on chromosome 1q21-q31 and optineurin gene in the GLC-1E interval on chromosome 10p. (Kanski, 2007, Wiggs J.L, 1994). Myocilin is still poorly understood in POAG and a study of group of unrelated POAG patients found myocilin mutations in at least 4% of the adult patients. The equivalent of hereditary process to have genetic myocilin mutation is up to 33% if any of family member at age 35 who develop glaucoma to be pass on to their heir.

Types of glaucoma

There are four main types of glaucoma, which are primary-open angle glaucoma, primary angle closure glaucoma, secondary glaucoma and developmental glaucoma. All of these glaucoma shares one thing in common, visual field lost.

Primary open angle glaucoma (POAG), is the most common glaucoma. It is categorized as chronic (slowly-developing) condition which recorded to have high IOP elevation due to failure drainage of fluid going out from the eye. The elevation of IOP rises slowly without pain symptom even the optic disc have been damaged so badly until a visual field lost is notified. Generally it is bilateral but not always symmetrical disease, which it is characterized to be adult onset, IOP over 21mmHg, optic nerve head damages and visual field lost. POAG is the most prevalent type of glaucoma, affecting approximately 1 in 100 of general population over the age of 40 and above. It effects both sexes equally and is responsible for about 12% of all cases of blind registration in the UK and USA.

Primary angle closure of glaucoma (PACG) occur in anatomically predisposed eyes, without other pathology, in which vision is threatened by elevation of IOP due to obstruction of aqueous flow by means of occlusion of the trabecular meshwork by the peripheral iris. Again, the it may remain asymptomatic or manifest as ophthalmic emergency until visual field loss occur. Primary angle closure should only be used when there is optic disc damage and visual field loss. The predisposing anatomical structure involving PACG relatively anterior location of the iris-lens diaphragm secondary to short axial length, shallow anterior chamber, and narrow entrance to the chamber. The following three interrelated factors are responsible for these characteristics. Lens size is a structure that continues to grow through out our life, as it growth mature, the structure of anterior surface will get closer to the cornea. At the same time, the muscle of ligament is getting slacken where it allow the iris-lens diaphragm to move anteriorly. Other than that, the corneal diameter were found to be smaller on PACG patient. The axial length were also related to the diameter of cornea as a short eye has a small diameter and a relatively located lens. (Kanski, 2007).

Secondary glaucoma is a type of glaucoma that can be either open angle or close angle. This may due to traumatic condition secondary to blunt trauma or cataract surgery.

Developmental glaucoma is a very rare condition can be classified as a congenital glaucoma whereby it present in about 1 in 10,000 babies. This may be due to delivery during pregnancy upon pressure on infant head.

1.3 Diagnosis of glaucoma

Glaucoma diagnosis is no longer simply relies on the presence of pressure within the eye. A disrupted structure of optic nerve damage were able to be seen with the aid of dilating drugs. It was suggested that a dilated pupil eye examination should be done for at least 2 years. The goldmann applanation tonometry (GAT) has been the gold standard in tonometry for more than 50 years but there are possibilities that some error might lead to be under-estimate or over-estimate the real IOP measurement, especially when the central corneal thickness were accounted. (Boehm et. al, 2008; Sarkisian S.R., 2006; Stevens et. al, 2007).

The applanation tonometer touches the eye’s surface after the eye has been numbed, and measures the amount of pressure necessary to flatten the cornea. It is also known to be the most sensitive tonometer as it requires a clear regularly shaped cornea to obtain proper measurement. Drops are put in the eyes to numb the eye and then measurement is taken. This measurement measure the inner pressure of the eye by determining how much pressure is necessary to cause light indentation on the outer part of the eye.

Other than that, ophtalmoscopy is used to examine the inside of the eye, including optic disk and the peripheral fundus of the eye. Usually drop will be put on the eye to dilate the pupil so assessment can be evaluate easily by using ophthalmoscope that lights up and magnifies the inside of the eye. Recent technology like fundus camera is much more preferable to reduce doubtfulness among medical practitioner evaluation.

Visual field assessment by using Humphrey is the most acceptable test to determine visual field lost subjectively. Reduce visual field is a less sensitive but more specific indicator of glaucoma than IOP above 21mmHg. But, if a doctors rely on visual field to detect glaucoma, they would miss almost everybody with early glaucoma. Visual field lost may not only occur in glaucoma, but also occur with the patient who has retinal detachment, multiple sclerosis, or even optic neuropathy. Nevertheless, a reduced visual field is more likely to be a sign of glaucoma than an IOP pressure above 21mmHg.

Measurement through gonioscopy lens is another assessment to evaluate the iridocorneal angle or the anatomical angle formed between the eyes cornea and iris. This is very important to relate the association of glaucomatous condition. Angle between the posterior corneal surface and the anterior surface of the iris constitutes the angle of anterior chamber. The gonioscopy technique allow 2 main groups of glaucoma to be evaluate, closed-angle glaucoma and open-angle glaucoma. By doing it this way, proper therapy will be conducted specifically to be effective.

Treatment of glaucoma

1.5.1 Laser

Laser iridotomy and trabeculoplasty are the types of laser therapy for glaucoma condition. Laser iridotomy approach involve the hole making on iris in primary angle closure glaucoma while laser trabeculoplasty perform on open angle glaucoma and claimed to be painless with the aids of anesthetic drugs. These therapy were carefully perform to prevent damaging the lens of the eye.

Selective laser trabeculoplasty (SLT) were applied to trabecular meshwork with Nd:YAG laser usually used to treat hypertension. Argon laser trabeculoplasty (ALT) also help to reduce the IOP elevation for the first year. Somehow, SLT reported to be more comfortable with the patient and may be repeated for couple times compared to ALT. Treatment with laser somehow wear off over years. SLT alone were found to be a considerable way for cataract surgery too.

Cyclophotocoagulation or laser cilioablation claimed to be reserved for patient with vision of 20/400 and lower in visual acuity. Application of laser burns is to destroy the cell that produce fluid helps to reduce the IOP elevation. This approach relatively suites for patient who are non-neovascular eyes and who had failed with previously surgery base. (Paul N.S. and John R.S., 2010).

1.5.2 Glaucoma surgery

Trabeculectomy were found to reduce IOP up to 30% for 43% of people with NTG. (Schulzer M., 1992). Partial removal of the eye’s drainage system were involve in trabeculectomy whereby a small bubble formed in between cornea and the sclera forming a tunnel to allow fluid outflow. This kind of surgery will be conducted if laser surgery and eye medication is not sufficient in controlling glaucomatous condition.

Viscocanalostomy was first described by Stegmann, a nonpenetrating procedure wherein two cut ends of the canal were inflated with viscoelastic. (Stegmann R. Et. al, 1999). In other words, it involves removing a portion of sclera to allow drainage of excessive fluid. Canaloplasty origin from catheterization of Schlemm’s canal via an abexterno approach to restore the outflow of the aqueous through conventional pathway. A non-invasive can be a successful way to reduce the chances of getting infection or inflammation but it may not works effectively like invasive ways. (Paul N.S. and John R.S., 2010).

1.5.3 Clinical therapeutic

Beta-blockers generally reduce the production of aqueous humour so that the continuation IOP elevation is reduced. There are few famous Beta blockers available which are, timolol, levobunolol, metipranolol and carteolol.

Prostaglandin such bimatoprost, latanoprost and travoprost were classified to reduce the IOP by improving the outflow of aqueous humour. Study by Parrish et. al, 2003, shows that there is no difference between bimatoprost, latanoprost, and travoprost in terms of lowering IOP elevation. This group of drug were associated with discoloration of iris and darkening of the eyelid skin.

Adrenergic agonist works by decreasing the aqueous humour and increasing the uveoscleral outflow. The side effect involve mouth dryness, fatigue, hyperaemia and headache. Both brimonidine and apraclonidine are example of adrenergic agonist that works reducing the elevation of IOP. There were risk reported with this classification of drug and because of that, the usage have been rarely applied (Virginia P.A. and Andrew M.P., 2006).

Carbonic anhydrase inhibitor (CAI) acts as a inhibitor to bicarbonate formation whereby finally reduce the IOP level. CAI were found to be less side effects compared to beta-blockers or prostaglandin. Few example like dorzolamide and brinzolamide falls under CAI classification which may be found in form of topical application or drops. CAI were used two or three times daily is considered to be the best prescription given. The side effect may involve gastrointestinal disturbances, central nervous disturbances, kidney stones, numbness or tingling sensations in the arms and legs, fatigue, and nausea. (Virginia P.A. and Andrew M.P., 2006).

Combination of drugs were currently being applied to treat glaucomatous condition. For example, the combination of dorzolamide 2% and timolol 0.5% show a positive effect better than a single drugs usage. (Harris A. et. al, 2001). The result shows an improvement to reduce the elevation of IOP. By using this approach, medical therapeutically have move ahead in finding the best way to eliminate IOP elevation constructively.

1.6 General Objective

To study the relationship between blood flow at the neuroretinal rim changes in glaucomatous eyes.

1.7 Hypothesis

Reduction in blood flow in defective neuroretinal rim in glaucomatous eyes.