Introduction

Millions of individuals worldwide suffer from diabetes mellitus, a chronic illness, and its prevalence is steadily increasing. According to the International Diabetes Federation's (IDF) atlas (2021), 10.5% of the adult population between the ages of 20-79 years has diabetes, with nearly half unaware of their condition, and approximately 783 million adults will have diabetes by 2045, which is a 46% increase in prevalence.

The effects of diabetes go beyond blood sugar levels and affect many parts of health, such as eye health. Cataract is a lesser-known complication of diabetes, and the risk of cataract development is higher in people with diabetes.

It is important to understand the relationship between diabetes and cataract formation in order to develop effective management and prevention strategies. In this article, we will try to understand this interlinkage. Let's discuss diabetes first.

What is diabetes?

Diabetes is a metabolic disease characterised by hyperglycaemia (elevated blood glucose (blood sugar) levels in the body). Glucose is the primary energy source for the body. A hormone called insulin, which controls blood glucose levels, is produced by the pancreas. It facilitates the transport of glucose into cells for energy use.

Diabetes is classified into further types, based on the onset of the disease. These are Type 1 and Type 2 diabetes.

Type 1 diabetes

Previously, type 1 diabetes was referred to as juvenile diabetes or insulin-dependent diabetes. In this type, insulin production by the pancreas is reduced, which impacts blood glucose regulation.  The exact cause of type 1 diabetes is unknown, but it is thought to be the result of an autoimmune reaction. Insulin needs to be administered daily in this type of diabetes.

Type 2 diabetes

Type 2 diabetes was previously referred to as non-insulin-dependent or adult-onset diabetes. It was only observed in adults earlier, but it is now also seen in children. In this type, either the body stops making insulin or it becomes resistant to insulin. Because of this, the body cannot use insulin properly, which can cause blood sugar levels to rise.

Obesity, insufficient physical activity, and genetics are all risk factors for developing type 2 diabetes. More than 95% of diabetics have type 2 diabetes. It can cause major harm to the body over time, particularly to the blood vessels and nerves.

Gestational diabetes

During pregnancy, a condition known as gestational diabetes can develop. It is distinguished by blood glucose levels that are higher than usual but lower than those that indicate diabetes. Women with gestational diabetes are more likely to experience delivery complications. These women, along with their children, are more likely to develop type 2 diabetes in the future.

Once diabetes develops, it can have a long-term impact on overall health. It has the potential to seriously impair numerous bodily systems over time, especially the blood vessels and neurons. It can harm the blood vessels in the kidneys, nerves, eyes, and heart.

What are cataracts?

As we look at things, our eyes work like cameras. The cornea and crystalline lens in the front of the eye focus light on the retina in the back of the eye, which is like the film. Any irregularities or opacities in the cornea or crystalline lens get in the way of the brain's ability to see clearly, like how a smudge on a camera lens makes a picture look blurry. The crystalline lens is one of the major structures that focus light as it enters the eye. The eye lens is normally clear but gets cloudy with age, which makes it harder to see clearly. 

Cataract is an age-related condition. The protein inside the lens breaks down and clots, causing the lens to become cloudy. This causes the lens to become cloudy, affecting vision. Because of clouding, light rays cannot focus on the retina after passing through the lens.

Cataracts are progressive and can exist for many years before affecting a person's vision or causing symptoms.

What are the symptoms?

Cataracts grow slowly, causing vision to deteriorate gradually. Cataract symptoms may resemble those of other eye conditions. It could be different for each person. They might include:

• Cloudy and blurred vision
• Lighting that generates a halo or glare
• Poor night vision
• Double vision
• Colours seem to have faded
• Increased nearsightedness and the need to change eyeglass prescriptions

What are the different types of cataracts?

There are numerous forms of cataracts, including:

• Age-related cataracts: This is the most prevalent form of cataract
• Congenital cataracts: Some infants are born with cataracts, and some kids grow up with them, usually in both eyes
• Secondary cataracts: These result from a pre-existing disease in the body (such as diabetes)
• Traumatic cataracts: Traumatic cataracts can occur as a result of an injury to one or both eyes. This can occur immediately after the accident or years later.

How can diabetes lead to cataract formation?

Cataract is one of the main causes of visual impairment in diabetic people. According to reports, those with diabetes have a fivefold increased risk of cataract development, especially if they get it early. 

Mechanisms that contribute to the development of cataract in diabetics

Cataract develops via different mechanisms. The basic process that leads to cloudiness in the lens starts within the aqueous humour, which is fluid that fills the front portion of the eye.  It acts as a source of energy for the eye lens and provides oxygen and glucose as nutrients to the lens of the eye. When the glucose levels are raised in the blood, it affects the sugar levels in the aqueous humour. High glucose levels in the lens cause swelling, reducing vision clarity. In addition, the lens contains an enzyme that converts glucose into sorbitol. Sorbitol can impact the cells present and the naturally occurring proteins in the lens, making it less clear and more opaque. 

The various mechanisms proposed by different studies behind cataract development include:

Polyol pathway

The polyol pathway involves converting glucose to sorbitol using an enzyme called AR (aldose reductase). Diabetics produce sorbitol more quickly than non-diabetics. The accumulation of sorbitol leads to the infusion of fluid and the liquefaction of lens fibres, which in turn produces opacity.

Osmotic and oxidative stress

Sorbitol accumulates in the endoplasmic reticulum (ER), the primary site of protein synthesis, causing stress and the formation of free radicals. Changes in glucose levels can also cause stress in the ER. These changes cause an unfolded protein response that creates reactive oxygen species and damages lens fibres through oxidative stress (an imbalance of free radicals and antioxidants in the body that leads to cell damage).

Autoimmunity

The autoimmune hypothesis is a recently proposed mechanism. The authors found that insulin autoantibodies became positive within three months of starting insulin treatment, which coincided with cataract formation. This requires further investigation to be confirmed.

How can we prevent and manage cataracts in diabetics?

There is no way to prevent cataracts from forming, but people with diabetes can reduce their risk of developing cataracts by taking some preventive measures, such as:

Maintain stable blood sugar levels

To maintain healthy blood glucose levels, consult your doctor frequently. Maintaining proper glucose levels can help to keep the eyes' lenses clear and healthy.

Schedule regular eye examinations with an ophthalmologist

An ophthalmologist can detect and treat problematic changes in your eye before you notice any symptoms.

Lifestyle and dietary changes

• Quitting smoking
• Eating a balanced diet that includes fruits and vegetables that are high in antioxidants
• Regular physical exercise

Conclusion

Maintaining good eye health requires an understanding of the relationship between diabetes and cataract formation. High blood glucose levels can alter the eye's lens, which can result in blurred vision, and therefore, people with diabetes must properly control their blood sugar to reduce this risk. Regular eye examinations can help detect cataracts early on, allowing timely treatment. Individuals can protect their vision and improve their quality of life by managing their diabetes and prioritising eye health.