Cataracts are among the most common causes of visual impairment globally, particularly affecting the ageing population. While cataracts are often classified into distinct types — such as nuclear sclerotic, cortical, and posterior subcapsular — many individuals develop mixed cataracts, where more than one type coexists within the same eye. This complex presentation can have unique implications for diagnosis, symptomatology, and treatment.
What Are Mixed Cataracts?
Mixed cataracts refer to the simultaneous presence of two or more morphological types of cataracts within a single lens. Instead of a uniform opacity confined to one region of the lens, the clouding appears in multiple zones, reflecting different patterns of degeneration. The most frequent combinations include nuclear sclerotic with cortical changes, or cortical with posterior subcapsular involvement. In some cases, all three types may be present, leading to a multifaceted impact on vision.
Pathophysiology
The human crystalline lens is a transparent, biconvex structure composed of highly organised cells and proteins arranged in concentric layers. At its centre lies the nucleus, which forms during embryonic development, surrounded by the softer, more metabolically active cortex. Encasing these is a thin yet resilient capsule that maintains the structural integrity of the lens. Over time, age and environmental influences disrupt the delicate balance of proteins and water within the lens, leading to aggregation and loss of transparency. When these degenerative processes occur in multiple zones of the lens simultaneously, the result is a mixed cataract — a complex form involving overlapping morphologies.
The pathogenesis of mixed cataracts is multifactorial. It arises from a confluence of ageing, systemic conditions, environmental exposures, and medication-related influences. Each contributing factor initiates or accelerates specific types of opacification within the lens. When several such mechanisms act in parallel or sequentially, more than one type of cataract develops within the same lens, resulting in mixed patterns that may progress at different rates and present with a range of symptoms. Understanding the underlying triggers offers insight into prevention, monitoring, and early intervention.
Age-related oxidative stress can cause both nuclear hardening and cortical fibre disruption.
With advancing age, the lens becomes increasingly vulnerable to oxidative damage due to the gradual decline in antioxidant defence mechanisms. Reactive oxygen species (ROS) accumulate within the lens fibres, leading to chemical modifications of crystallin proteins. These modified proteins clump together and form aggregates, reducing lens transparency. In the nucleus, this results in a gradual compaction and yellowing, known as nuclear sclerosis, which contributes to blurred distance vision and a shift towards myopia.
Oxidative stress doesn’t just affect the central nucleus; it also disrupts the cortical fibres located peripherally. These fibres lose their regular alignment and may develop clefts or vacuoles. As damage accumulates, wedge-shaped opacities or spoke-like appearances become visible in the lens cortex. Because oxidative stress acts systemically within the lens, it can simultaneously trigger both nuclear and cortical changes. This dual impact lays the groundwork for mixed cataract formation, particularly in individuals with reduced dietary antioxidant intake or long-term exposure to oxidative stressors.
Metabolic diseases, such as diabetes mellitus, often predispose individuals to cortical and posterior subcapsular changes concurrently.
In diabetes mellitus, persistently elevated blood glucose levels alter the internal chemistry of the lens. Glucose is converted into sorbitol via the polyol pathway, leading to osmotic stress. This causes the lens fibres, especially in the cortex, to swell and eventually rupture, initiating the formation of cortical cataracts. These changes are often bilateral and may appear earlier in diabetic individuals compared to the general population.
Simultaneously, the posterior subcapsular region of the lens — an area with high metabolic activity and minimal fibre turnover — is particularly sensitive to metabolic imbalances. In diabetes, this zone often accumulates opacities due to impaired nutrient transport and increased oxidative damage. The co-occurrence of cortical and posterior subcapsular changes is therefore not uncommon in diabetic patients. When both areas are involved, the resulting mixed cataract may develop more rapidly and cause pronounced visual symptoms, especially under bright lighting or during reading.
Chronic use of corticosteroids can lead to posterior subcapsular opacities, which may coexist with age-related nuclear changes.
Long-term corticosteroid therapy, whether systemic, inhaled, or topical, is a recognised risk factor for posterior subcapsular cataract formation. These medications disrupt lens epithelial cell metabolism and promote abnormal migration of these cells toward the posterior pole of the lens. Over time, this leads to plaque-like opacities beneath the posterior capsule, directly in the visual axis, resulting in symptoms such as glare and difficulty with near vision.
In patients who are also ageing, the likelihood of coexisting nuclear sclerosis increases. The age-related process of nuclear hardening may progress concurrently with the corticosteroid-induced posterior changes. This overlap creates a mixed cataract scenario in which the central nucleus becomes progressively denser while the posterior capsule develops focal opacities. Such a combination can be particularly visually disabling and may require earlier surgical intervention due to the central location of the visual obstruction.
Ultraviolet light exposure and smoking are non-specific risk factors that can damage multiple parts of the lens.
Chronic exposure to ultraviolet (UV) radiation, particularly UV-B rays, is known to induce photochemical changes in the lens. These rays penetrate the eye and can cause oxidative damage to lens proteins and DNA within epithelial cells. UV exposure is linked to both nuclear sclerosis and cortical cataracts, depending on the intensity and duration of exposure. The oxidative burden from UV light does not discriminate between different lens regions, which is why mixed cataracts are more common in individuals with high cumulative sun exposure, such as outdoor workers.
Smoking, on the other hand, introduces a host of toxic substances into the bloodstream, many of which reach the ocular structures. Cigarette smoke contains free radicals and heavy metals that deplete the body’s natural antioxidants. This systemic oxidative stress accelerates protein denaturation within the lens. Studies have linked smoking to all major cataract types, and it is particularly implicated in the development of nuclear and posterior subcapsular changes. The non-specific nature of these risk factors means that individuals exposed to them are at increased risk of developing multiple cataract subtypes concurrently.
Clinical Presentation
The symptoms of mixed cataracts are typically more varied and may evolve more rapidly than those of isolated cataract types. Depending on the predominant component, patients may report:
- Gradual blurring of vision (common with nuclear changes)
- Glare and halos around lights, especially at night (often linked with cortical or posterior subcapsular cataracts)
- Difficulty reading in bright environments
- Reduced contrast sensitivity
- Rapid deterioration in near or distance vision, depending on the area most affected
Importantly, symptoms may not correlate precisely with the degree of visual acuity loss measured in clinic, especially if the posterior subcapsular component is dominant, as even small opacities in this location can significantly impair central vision.
Diagnosis
Diagnosing mixed cataracts involves a comprehensive eye examination:
- Slit-lamp biomicroscopy allows direct visualisation of the lens opacities and helps to distinguish their location and density.
- Dilated fundus examination ensures that no retinal pathology is contributing to the patient’s symptoms.
- Visual acuity testing and contrast sensitivity assessment provide objective measures of visual function.
- Potential acuity metre (PAM) testing may be used to estimate post-operative vision, especially when other ocular conditions exist.
Careful documentation of the type and extent of each cataract component is essential for guiding treatment planning.
Treatment Options
Surgical removal remains the definitive treatment for mixed cataracts, particularly when they begin to interfere with daily activities or reduce overall visual function. The surgery is typically considered once the visual impairment affects reading, driving, recognising faces, or performing routine tasks. Unlike early cataract types that may progress slowly, mixed cataracts often cause more unpredictable and variable symptoms due to the involvement of multiple lens zones. This variability can make it difficult to manage conservatively, and surgery becomes the most viable option.
The most widely used surgical technique is phacoemulsification, which involves the use of ultrasonic energy to break up the cloudy lens, followed by the implantation of an intraocular lens (IOL). In most cases, this procedure is performed under local anaesthesia and takes less than an hour. While outcomes are generally excellent, mixed cataracts may pose additional technical challenges due to the varied nature and density of the lens opacities. Customising the surgical approach to account for these complexities is essential to minimise risks and achieve optimal results.
Increased lens density in nuclear sclerotic cataracts may require more ultrasonic energy, increasing the risk of endothelial cell loss.
In patients with mixed cataracts that include a dense nuclear sclerotic component, the hardening of the central lens can demand greater phacoemulsification energy for effective fragmentation and removal. This increase in energy raises intraocular temperatures and mechanical stress, potentially leading to damage of the delicate corneal endothelium. As endothelial cells do not regenerate, excessive loss can result in postoperative corneal oedema and delayed visual recovery.
To mitigate this risk, surgeons often employ advanced techniques such as dividing the lens into smaller quadrants before emulsification, allowing for a more controlled and efficient removal. The use of high-viscosity ophthalmic viscoelastic devices (OVDs) can also help protect the corneal endothelium during energy delivery. Additionally, reducing phacoemulsification time by employing newer technologies or opting for femtosecond laser pre-fragmentation can further preserve corneal clarity and support better outcomes.
Cortical spokes can obscure the red reflex, complicating capsulorhexis.
Cortical cataracts often present as radial or spoke-like opacities that scatter light and interfere with the surgeon’s ability to visualise the red reflex — the reddish glow from the retina that aids in identifying anatomical landmarks during surgery. In mixed cataracts where cortical changes are present alongside nuclear sclerosis, this visual interference becomes more problematic, particularly during the creation of the capsulorhexis, a critical step where a circular opening is made in the anterior lens capsule.
Without a clear red reflex, maintaining the ideal shape and centration of the capsulorhexis can be more challenging, increasing the risk of capsule tears or poor IOL positioning. To address this, surgeons may employ capsule-staining dyes such as trypan blue to improve visual contrast and ensure greater accuracy. Enhanced microscope illumination settings and intraoperative guidance systems may also be used to compensate for the reduced visibility, helping to maintain surgical safety and precision.
Posterior subcapsular opacities may reduce visibility of the posterior capsule during surgery.
Posterior subcapsular cataracts are located directly in front of the posterior capsule, often in the central visual axis. During surgery, especially after lens material has been emulsified and aspirated, these dense opacities can obscure the surgeon’s view of the posterior capsule. This reduced visibility poses a risk of unintentional capsule rupture, which can compromise IOL placement and potentially lead to complications such as vitreous prolapse or dropped lens fragments.
To navigate this challenge, careful hydrodissection and controlled lens rotation are employed to minimise stress on the capsule. Surgeons may also use low-flow settings and high-viscosity OVDs to maintain a stable anterior chamber and safeguard the posterior capsule. When posterior visualisation remains limited, intraoperative devices like endoilluminators or optical coherence tomography-guided systems may be helpful in providing additional depth perception and ensuring precise IOL placement without compromising posterior structures.
Intraocular Lens (IOL) Selection
Selecting the appropriate intraocular lens (IOL) is a vital component of cataract surgery, especially in patients with mixed cataracts, where visual impairment stems from multiple areas within the lens. These individuals may experience a combination of distance, near, and contrast sensitivity issues, making their visual demands more complex. The goal of IOL selection is not only to restore clarity but also to optimise visual function based on individual lifestyle, ocular health, and patient expectations. A thorough preoperative assessment, including optical biometry, corneal topography, and ocular surface evaluation, is essential for guiding lens choice.
Monofocal IOLs remain the most commonly used option and offer excellent quality of vision at a fixed focal point, typically for distance. They are generally well-tolerated and less likely to cause issues such as glare or halos. For many patients with mixed cataracts, especially those with co-existing ocular conditions like early macular degeneration, glaucoma, or irregular corneal astigmatism, monofocal lenses provide the most predictable outcomes. When necessary, reading glasses can supplement vision for near tasks. Their reliability and simplicity make them a practical choice in cases where optical clarity and low risk of visual disturbances are the priority.
For selected individuals with healthy eyes and no significant retinal pathology or corneal irregularities, advanced lens technologies such as multifocal or extended depth-of-focus (EDOF) lenses may be considered. These lenses aim to provide a broader range of vision, potentially reducing the need for spectacles after surgery. However, they require careful patient selection and realistic counselling, as they can introduce side effects like halos or reduced contrast in low light. Patients with mixed cataracts must also be informed that due to the variable nature of their lens opacities, their visual outcomes may not mirror those of individuals with uniform cataracts. Therefore, shared decision-making, clear communication, and an understanding of visual priorities are key when considering premium IOLs in this patient group.
Prognosis
With modern surgical techniques, the prognosis for mixed cataracts is generally excellent. Most patients experience significant improvements in visual acuity and quality of life. However, the presence of multiple types of lens opacity can sometimes delay surgical referral, especially if symptoms are mistakenly attributed to only mild nuclear changes.
Postoperative outcomes are influenced by:
- The degree of pre-existing lens opacity
- The presence of co-morbid ocular diseases (e.g. macular degeneration, diabetic retinopathy)
- The accuracy of biometry and IOL power calculations
Prevention and Monitoring
While some risk factors such as age are unavoidable, certain measures may help delay the onset or progression of cataracts, including:
- Regular eye examinations
- Good glycaemic control in diabetic patients
- Use of UV-protective eyewear
- Smoking cessation
- Judicious use of systemic steroids
Patients diagnosed with early-stage mixed cataracts should be monitored periodically, with emphasis on changes in visual function rather than lens appearance alone.
Conclusion
Mixed cataracts represent a complex but common presentation of lens opacity, often requiring a tailored approach to diagnosis, monitoring, and surgical management. Understanding the interplay between different types of cataracts within a single eye allows clinicians to provide more nuanced care and achieve optimal outcomes. As the global population continues to age, awareness and management of mixed cataracts will remain a crucial component of ophthalmic practice. If you are concerned about cataracts or would like to explore your treatment options, you are welcome to contact us at the London Cataract Centre to arrange a consultation with one of our specialists.