Introduction
Visual disturbances can occur in individuals in the last stages of kidney disease. Diabetes and high blood pressure can also aggravate one's risk of developing eye disease. A hypertensive crisis arises from increased blood pressure and is usually life-threatening. It is a common medical emergency. Unchecked high blood pressure can cause complications in the cardiovascular, kidney, central nervous, and ocular systems. Vision impairment occurs in around 35 to 60 percent of patients.
How Are the Kidneys and Eyes Related?
The kidneys and eyes share similar structural, physiological, developmental, and pathogenic pathways. Both the choroid and the glomerulus have a similar vascular network, the glomerular filtration barrier and inner part of the retina have the same developmental pathway, and the renin-angiotensin-aldosterone hormonal cascade is present in both the kidneys and eyes.
Chronic kidney disease and certain diseases of the eyes, like diabetic retinopathy, glaucoma, age-related macular degeneration, and cataracts, are related to age and risk factors like diabetes, hypertension, and smoking. Chronic kidney disease may also cause developmental abnormalities of the oculorenal syndromes. Many studies have suggested a close link between kidney and eye problems. Individuals with chronic kidney disease are at increased risk for age-related macular degeneration, diabetes retinopathy, glaucoma, and cataracts.
What Are the Various Visual Disturbances Caused by Renal Dysfunction?
1. Retinopathy:
Retinopathy is an eye disease wherein the eyes' blood vessels deteriorate due to diabetes or hypertension. If the damage to the blood vessels is caused by diabetes, then it is known as diabetic retinopathy. Diabetes increases blood sugar levels. This increased blood sugar level can damage the small blood vessels (capillaries) in the kidneys, eyes, and feet.
The increase in blood pressure tends to apply pressure on the walls of the blood vessels, thus making them weak, which could lead to a break or tear in them. Retinopathy gradually progresses, and the symptoms are not visible till late. These damaged blood vessels can become scar tissue that detaches the retina, leading to visual impairment and, eventually, blindness.
2. Chronic Kidney Disease and Glaucoma:
Glaucoma is a type of optic neuropathy that is caused due damage to the optic nerve due to high eye pressure. It is one of the leading causes of peripheral and central vision loss. Around 60 million people have glaucomatous optic neuropathy. Glaucoma occurs due to problems related to intraocular pressure and ischemic mechanisms.
Intraocular pressure is affected by the aqueous humor's production rate, the resistance of the outflow at the trabecular meshwork, and episcleral venous pressure. The increased urea concentration in the aqueous humor tends to exert osmotic pressure and repress excessive fluid in the anterior chamber of the eyes. The toxic substances that accumulate in the trabecular meshwork may block the outflow of the aqueous humor.
The intraocular pressure has been found to increase during hemodialysis. According to the reverse urea hypothesis, hemodialysis quickly removes urea from the blood. The higher level of urea in the cerebrospinal fluid draws water into the central nervous system, causing cerebral edema. A urea gradient is developed between the aqueous humor and blood, causing the water to move into the anterior chamber and thus increasing the intra-ocular pressure.
This increased intraocular pressure further damages the optic nerve, causing glaucoma. Several studies have been conducted to deduce the relationship between chronic kidney disease and the development of glaucoma, which suggests that factors like ischemia and neuroprotective mechanisms may affect the optic nerve in response to pressure-related damage.
3. Chronic Kidney Disease and Cataract:
A cataract is the appearance of a cloudy area in the eye's lens. It is an age-related condition. There are three types of cataracts - cortical, nuclear sclerotic, and posterior subscapular cataracts. Cortical cataracts are a result of changes in the hydration of the lens. Nuclear sclerotic cataracts occur when the cortical fibers' continuous deposition causes the nucleus's compression and hardening.
Posterior subscapular cataracts have opacities in the posterior cortical layers. It is not clear whether the formation and progression of cataracts are related to chronic kidney diseases. Some studies examined the relationship between serum cystatin C, a marker of kidney disease, and the occurrence of age-related cataracts.
Blood urea nitrogen and creatinine have also been found to be related to the incidence of cataracts. The cataract risk increased with an increase in the levels of these parameters. In mild cases of chronic kidney disease, serum cystatin C is more sensitive than serum creatinine. This leads to the formation of cataracts due to metabolic disturbances. Cystatin C is also an inflammation marker; however, its role in cataractogenesis is still lacking.
4. Chronic Kidney Disease and Age-Related Maculopathy:
Age-related macular degeneration is one of the most common causes of blindness. It is characterized by the deposition of lipofuscin and pigment changes in the macula, atrophy of the retinal pigment epithelium, and choroidal neovascularization. The association between chronic kidney disease and age-related macular degeneration came to light after some studies in this field. It was found that early age-related macular degeneration in the absence of atrophy and choroidal neovascularization was common in individuals with moderate forms of chronic kidney diseases compared to those with mild or no kidney disease.
5. Chronic Kidney Disease and Ocular Surface Disease:
In individuals with advanced chronic kidney disease, metastatic calcification can occur in the eyelids, conjunctiva, and cornea. As chronic kidney disease progresses, serum calcium and phosphorus concentrations can lead to precipitation in the interpalpebral conjunctiva and the cornea's Bowman's layer, known as band keratopathy.
Calcific plaques have also been reported on the eyelid margins. Chronic kidney disease can also lead to increased central corneal thickness due to changes in the corneal epithelium. Increased urea concentrations in the aqueous humor are toxic to the corneal endothelium, leading to a loss of normal morphology and function of the endothelial cells of the cornea.
How To Keep the Eyes Healthy in Renal Dysfunction?
One must pay close attention to eye health if one has chronic kidney disease.
The following precautions should be taken to maintain healthy eyes:
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Routine eye screening.
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Pay attention to the warning signs like seeing flashing lights, changes in vision, and the sudden appearance of floaters.
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Controlling high blood pressure and glucose levels.
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Maintaining a healthy lifestyle.
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Abstaining from smoking.
Conclusion:
Chronic kidney disease and eye diseases are closely associated. The changes in the retinal vasculature, like retinopathy, can predict chronic kidney disease. Kidney diseases are associated with age-related macular degradation, cataracts, and glaucoma. Proper know-how of the pathophysiology of kidney and eye diseases will help develop new treatment plans. Routine eye screening of kidney patients should be done, and research should be carried out to understand the relationship further.
