Introduction

Cataracts can be broadly defined as clouding of the eye lens, a transparent disc in the eyes that focuses light onto the retina (a layer in the eye that detects light).¹ This loss of lens transparency results in reduced vision clarity and, with time, could result in blindness. Although there are many risk factors for cataract development, including existing health conditions such as diabetes and genetic composition, the leading cause of clinically-relevant cataracts is ageing.² Different types of cataracts can be classified based on the age at onset, in which age-related cataracts (ARC) are most commonly found in adults.³ Generally, age-related cataracts are diagnosed in patients over 50 or 60 years of age. 

In 2014, the WHO estimated that 95 million people worldwide were visually impaired due to cataract development.² Large-scale population studies have indicated that cataract prevalence rises from approximately 4% among individuals aged 55-64 to nearly 93% in those aged 80 and older. Although the prevalence has decreased due to the wider availability of cataract surgery, cataracts remain the primary cause of blindness in low- and middle-income countries.

Mechanism of cataract development

Approximately 90% of the lens comprises a type of protein called crystallins that is crucial for its function.⁴ Over time, these proteins undergo various alterations with age, many of which are accelerated by stress, such as the presence of unstable oxygen molecules called reactive oxygen species (ROS).⁵ With age and oxidative stress caused by ROS, crystallins become less soluble, causing them to form clusters through aggregation, hence increasing lens opacity.⁴ This results in light being scattered or blocked by the clouded lens, further reducing vision clarity. 

ARC can be divided into three main types based on where they develop in the lens of the eye:^1,2

• Nuclear cataracts form in the centre of the lens
• Cortical cataracts have a wedge shape and start at the outer layer (the cortex) of the lens, moving inward toward the centre
• Posterior subcapsular cataracts develop at the back of the lens, specifically in the layer that is closest to the centre

Patients often display more than one type of cataract.^1,2

Risk factors include smoking, exposure to UV radiation, and alcohol use, among others.^1,4 Furthermore, various studies have demonstrated a genetic influence on the development of ARC.^3,5 Genetic factors were discovered to account for about 48% of the risk for nuclear cataracts and contribute to up to 58% of the risk for overall ARC development.⁵ Each type of cataract is influenced by both genetic and environmental risk factors, however, complex inheritance patterns and the late age of onset make it challenging to draw definite conclusions regarding correlation and causation.³

Symptoms and impact on visual clarity

Common clinical manifestations of ARC include:^1,2 

• Blurred vision
• Glare from lights
• Difficulty with night vision
• Reduced contrast sensitivity
• Colour perception changes
• Monocular diplopia (double vision)

Symptoms typically develop gradually, are painless, and may affect one or both eyes. 

ARC are generally considered to be clinically significant if the decline in vision clarity interferes with daily activities, such as driving and reading. Different symptoms may also be observed in different types of ARC. For instance, nuclear cataracts often lead to a decline in distance vision rather than near vision, while posterior subcapsular cataracts are more commonly associated with reduced near vision.² 

Diagnosis of cataracts

ARC are only considered to be clinically relevant if the patient experiences a significant decline in vision clarity, as they may develop asymptomatically.¹ Patients suspected of being affected by ARC should undergo a thorough eye examination. This would include assessing the patient's prescription, measuring eye pressure, examining the eye with a special microscope (slit-lamp), and checking the back of the eye by dilating it using special eye drops (dilated fundus examination).¹ Vision clarity is typically tested using a vision chart that displays black letters on a white background called a visual acuity test. The patient will then be tested in a dark room for distance vision and with lights on for near vision. A full eye exam would also help to identify other eye conditions, such as dry eyes and glaucoma, which could affect vision or interfere with management options.

Treatment options and their effectiveness

The standard treatment for ARC is cataract surgery, which involves removing the cloudy lens and replacing it with an artificial one called an intraocular lens (IOL).¹ Before surgery, patients typically undergo a series of detailed tests to help guide the treatment process. Following surgery, 84-94% of patients achieve a best-corrected visual acuity of 20/30 (6/9) within six months.² Studies on long-term outcomes have shown that most patients maintain good vision even 10 to 15 years after surgery. Cataract surgery significantly improves quality of life by allowing patients to perform daily activities independently, and it has been associated with lower mortality and an extended lifespan.

There are several surgical approaches to treat ARC:^2,6

• Extracapsular cataract extraction (ECCE) involves removing the entire central part of the lens, called the nucleus, through a large incision of about 10 mm
• Phacoemulsification uses an ultrasound-driven needle to break up and remove the lens through a smaller incision of about 3-4 mm
• Femtosecond laser-assisted cataract surgery employs a laser to perform many steps of the procedure, including making the main incision and fragmenting the lens

After surgery, patients are typically prescribed topical antibiotics, corticosteroids, or non-steroidal anti-inflammatory drugs (NSAIDs) for 1-4 weeks.² Corticosteroids and NSAIDs may be used more frequently and for longer periods in cases with complications or in patients at higher risk for post-surgical inflammation. The choice of drugs, frequency of use, and frequency of postoperative follow-up visits vary by surgeon and by country.

Technological advancements now allow patients to choose lenses tailored to their lifestyle needs, with options for monovision or lenses that offer enhanced performance across different visual ranges.² Some lenses are even pre-loaded with medications such as antibiotics and NSAIDs, effectively combining surgical intervention and post-surgery treatment into a single procedure. In many developing countries, however, the primary goal of cataract surgery remains to restore visual function. In cases where patients are intolerant of their vision with the chosen lens, a lens replacement procedure called an intraocular lens exchange (IOL) may be necessary.⁶

Studies have not shown that femtosecond laser-associated cataract surgery offers better outcomes than traditional manual phacoemulsification, with both approaches proving to be safe and effective.⁶ Another topic of debate in cataract treatment is whether both eyes should be operated on at the same time, known as immediately sequential bilateral cataract surgery.² The current standard is delayed sequential surgery, where each eye is operated on separately. This approach reduces the risk of infections and allows any complications from the first surgery to be addressed before performing surgery on the second eye. While immediate sequential surgery may offer more prompt visual recovery, less variation between eyes, and time and cost savings, more evidence is needed to confirm its safety and long-term benefits. 

Common intraoperative or postoperative complications of cataract surgery include:^1,2,6

• Postoperative capsule rupture
  • Involves a tear in the lens, which may lead to lens fragments being left in the eye
  • The most common intraoperative complication, with a prevalence of 0.5-5.2%
• Endophthalmitis
  • Severe inflammation of the eye
  • The most severe postoperative complication, with a prevalence of 0.006-0.04%
• Posterior capsule opacification
  • Involves the development of secondary cataracts following IOL implantation
  • The most common postoperative complication

Alternative treatment options for cataracts include eye drops, suspensions, or ointments.⁴ These options are convenient and widely used because they are easy to apply, can be self-administered, and do not require surgery. However, they are generally not very effective and must be used frequently, which can make it hard for patients to keep up with regular application. This frequent use can lead to inconsistent results and lower treatment adherence.

Summary

Age-related cataracts (ARC) are a common eye condition in which the lens of the eye gradually becomes blurry as part of the ageing process. This cloudiness occurs when proteins in the lens called crystallins break down and clump together, creating areas of opacity that block or scatter light, leading to reduced visual clarity. Symptoms include blurry vision, reduced colour perception and increased glare from lights. ARC are usually diagnosed through a comprehensive eye examination, including vision tests, eye pressure measurements, and a dilated eye assessment. When symptoms persist and significantly impact daily life, cataract surgery is recommended as the standard treatment to improve quality of life. This procedure involves replacing the cloudy lens with an artificial lens through various methods, hence restoring vision. While there are some risks of postoperative complications, cataract surgery has been shown to be a safe and effective treatment, with up to 94% of patients experiencing restored vision within six months.