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The World Health Organisation reports that by 2020 the number of the blind will escalate to 54 million due to all the eye diseases in people over 60 years old (Liutkeviciene et al, 2010). Age related macular degeneration (AMD) and retinitis pigmentosa (RP) are one of the leading causes of visual impairment and irreversible blindness in adults of the Western world (Wan et al, 2010). These two degenerative diseases severely disrupt the quality of life of adults with an abrupt reduction in visual acuity.
AMD is characterised by photoreceptor degeneration in the macula lutea associated with aging, family history, light coloured irides and cigarette smoking which are the greatest risk factors (Rogers et al, 2007). AMD is diagnosed as non exudative (dry) or neovascular (wet) and prevalent in adults over 50 years old who lose their central visual field which generally has the highest acuity (Kim et al, 2010). Central visual field loss affects reading ability and reduces performance in activities that require fine manipulation. On the contrary, RP is a hereditary retinal disease that is characterised by photoreceptor degeneration; the degeneration of rods preceding the cones (Eballe et al, 2010). The X-linked recessive RP causes severe visual acuity impairment and expressed only in males, therefore affecting men more than women (Rogers et al, 2007). The prevalence of RP is approximately 1: 4000 worldwide and characteristically involves colour vision defect, central or peripheral vision loss, light flashes and night blindness with difficulty adjusting to dim lighting conditions (Lee et al, 2010).
Understanding the basic structure of the eye is essential to appreciate the pathophysiology of AMD and RP which in turn allows the comparison between these two degenerative diseases. The eye is one of the special senses which provide vision. The transparent lens situated at the front of the eye functions to focus images on the retina by permitting light rays to pass through into the eye. The retina is the innermost layer of the eye consisting of photoreceptors called rods and cones that convert light energy into nerve impulses through the optic nerve allowing the brain to interpret the image. Rods have high sensitivity and are responsible for vision in dim light whilst less sensitive cones provide colour vision during the day (Sherwood, 2010). These photoreceptors are provided with oxygen, sugars and other essential nutrients through the retinal pigment epithelium (RPE) which also removes waste products below to the choroid layer comprising blood vessels that nourish the retina. Bruchâ€™s membrane lies between the RPE and choroidal system. The macula lutea is situated in the centre of the retina and has a high density of cones responsible for central colour vision and high visual acuity (Sherwood, 2010).
The early stages of dry AMD which represents 80% of all AMD cases is characterised by the appearance of large yellow coloured drusen which are colloid material excresceneces that accumulate in the Bruchâ€™s membrane underlying the RPE in the macula region of the retina (Coscas, 2009). Drusen interrupts oxygen metabolism which determines photoreceptor and RPE degeneration consequently causing progressive central vision loss (Liutkeviciene, 2010). Dry AMD results in 20% of all vision loss in eyes with AMD and can progress to late stage geographic atrophy of the RPE which can lead to severe vision loss but does not cause abrupt loss of vision. In contrast, neovascular AMD causes sudden decline in visual acuity and represents 20% of all AMD cases and causes 80% of all vision loss in all eyes with AMD (Coscas, 2009). RPE detachment results in the inability to prevent abnormal growth of blood vessels from the chorioid layer into the retina which rapidly grows and leaks fluid and blood into the retina, eventually producing scarring and central vision loss in neovascular AMD. Vascular endothelial growth factor (VEGF) and other factors trigger the growth of choroidal neovascularisation (CNV) which is the principle process causing neovascular AMD and abnormal blood vessel growth in the choroid (Rogers et al, 2007).
The etiology of AMD is unknown but the two most consistent risk factors that have been established to be associated with the onset and development of AMD comprise genetic markers like the complement factor H gene which is a plasma glycoprotein, the Ago E gene which codes the plasma protein found in drusen and cigarette smoking which causes oxidative RPE damage and promotes CNV (Liutkeviciene et al 2010, Lavanya et al, 2010). Ageing, hypertension, ethnicity and diet have also been confirmed to influence AMD onset (Liutkeviciene, 2010). Smoking, blue coloured irides and female gender are associated with lower retinal antioxidant concentrations which increase the risk of developing AMD (Falsini et al, 2010). AMD is detected and diagnosed with optical coherence tomography which measures central macula thickness and fluorescein angiography which assesses CNV (Ghazi et al, 2010). It is incurable, however a combination of photodynamic therapy (PDT) and bevacizumab which is a humanised monoclonal antibody against VEGF has been shown to be effective in treatment of neovascular AMD by preserving visual acuity (Wan et al, 2010). Bevacizumab and PDT inhibit angiogenesis by targeting VEGF therefore stabilising neovascular AMD. Kim and others (2010) study also demonstrated that PDT improved the general health and vision related quality of life of AMD patients. It is evident that bevacizumab and PDT are effective in the treatment of AMD, however with any study there will be limitations and this study is no exception. Wan and others (2010) study did not perform uniform follow up visits for each subject; furthermore the study used a Snellen chart which lacks accuracy and consistency in the measurement of visual acuity.
RP is a hereditary retinal disease that progressively impairs vision through the degeneration of photoreceptors, ultimately leading to incurable blindness and implicating quality of life (Seo et al, 2009). Unlike AMD it is inherited and highly heterogenous, with at least 44 underlying mutated genes identified and demonstrated to show variance in hereditary and clinical features across genetic backgrounds (Lee et al, 2010). RP is genetically transmitted in an autosomal dominant, autosomal recessive or X-linked recessive mode (Eballe et al, 2010). The progressive degeneration of rods and less common genetic defects of the RPE and cones result in eventual loss of central or peripheral vision. RP can be detected with an electroretinogram (ERG) which detects electrical potentials from the photoreceptors determining rod and cone cell function (Lee et al, 2010). The rod ERG amplitude is affected greater than the cones in early RP but with progressive impairment both will eventually display comparable responses (Rogers et al, 2007). Alternatively, visual field testing examines the presence of a midperipheral ring scotoma caused by RP which enlarges at the centre and periphery with disease progression (Rogers et al, 2007).
RP has no effective treatment in the progressive vision loss, however Clemson and others (2010) study demonstrated that treatment with valproic acid (VPA) in patients with RP can provide therapeutic benefits. Improvements in visual field and acuity were observed by subjects; moreover recent research findings suggest VPA has the ability to reverse photoreceptor damage and may also stimulate glial cells into differentiating into photoreceptor-like cells (Clemson et al, 2010). Daily doses of vitamin A palmitate have also been established to slow the progression of RP but the effects are minimal (Rogers, 2007). Future therapeutic approaches for both AMD and RP are advancing as more evidence is being documented for stem cell transplantation which has the potential to restore vision loss (Wang et al, 2010). Wang and others (2010) study found differentiated embryonic stem (ES) cells could develop into RPE-like cells which potentially can be utilised as a therapeutic approach for RPE specific diseases such as AMD and RP.
AMD and RP are two chronic degenerative retinal diseases which are the leading causes for the escalation of irreversible blindness in adults of the Western world (Wan et al, 2010). AMD causes central visual field loss characterised by photoreceptor and RPE degeneration (Liutkeviciene, 2010). Conversely, RP is a hereditary retinal disease which results in photoreceptor degeneration and causes central vision loss and night blindness (Lee et al, 2010). RP and AMD are irreversible but treatment to slow progression of vision loss comprise valproic acid (VPA) and a combination of photodynamic therapy (PDT) and bevacizumab respectively for each disease (Wan et al 2010, Clemson et al 2010). Research in stem cell transplantation shows potential in the treatment of AMD and RP but requires development (Wand et al, 2010). Further progression in the investigations for new treatment in AMD and RP is required to improve prognosis for patients diagnosed with these two principal causes of visual impairment and irreversible blindness in adults (Wan et al, 2010).