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The primary functions of the crystalline lens are to transmit incident light and to focus it on the retina. This requires that the lens be transparent, a condition dependent on the highly regular organisation of the cells of the lens and the high degree of order of the proteins in the lens cytoplasm.

Protein concentration in lens fibre cells is extremely high, resulting in an index of refraction significantly greater than that of the surrounding fluids and enabling the lens to refract incident light. Cataract occurs when the lens loses its transparency by either scattering or absorbing light such that visual acuity is compromised.

Cataracts can result from genetic, metabolic, nutritional, or environmental insults or may be secondary to other ocular or systemic diseases, such as diabetes or retinal degenerative diseases (Table 1). By far the most important risk factor is age ageing-related cataract constitutes the great majority of all cataracts and is a major public health problem in the world. In developing countries, where the availability of surgical facilities is limited, ageing-related cataract is the leading cause of blindness. Because at present there is no efficacious non-surgical therapy for cataract, the problem is expected to increase in magnitude as the world population becomes progressively older in coming decades.

Lens growth

Although the lens grows throughout life, none of the cells are cast off. Component cells are added to the lens as time goes by, with those in the centre being as old as the individual themselves. The lens grows by regular addition of fibres to the lens mass. Growth rate is not uniform throughout the human life span, it appears to be maximal in foetal life. Foetal lens mass increases by about 180mg/year (90 mg at birth) and drops significantly after birth to 1.3 mg/year between 10-90 years. Estimates of average lens density suggest that protein content remains relatively fixed at around 33 per cent of the wet weight over the age span.

The dimensions of the lens changes in a complex manner as the lens grows. In early foetal life, the lens is almost perfectly spherical but by birth, the sagittal profile is ellipsoidal, as equatorial growth outstrips growth in the sagittal plane. At birth, equatorial lens diameter is about 6.5mm while sagittal width is about 3mm. By the age of 90 years, this changes to about 10mm in the equatorial plane and 6mm in the sagittal plane.

Presbyopia

Loss of accommodative power is a lens-related problem initiated in infancy. This steady loss of accommodation is completed by about the age of 50 years and is coupled with difficulty restoring focus from near to far. The factors responsible for presbyopia are multiple and the relative contribution of these changes to the presbyopic state is not established.

Some of the factors involved are:

● Forward shift of the apical part of the ciliary body with age which reduces the function of the muscle

● Increase in the connective tissue of the ciliary muscle

● Increase in the stiffness of the cortex and nucleus of the lens

● Increase curvature of the anterior lens surface which reduces the ability of ciliary muscle contraction to change the lens shape during accommodation.

Oxidation is a key factor in lens ageing

Oxidative damage to lens constituents, including nucleic acids, proteins, and lipids, is believed to be a primary factor in ageing-related cataract. This belief is based on a large body of evidence of various types. That oxidative stress can be cataractogenic is clear from abundant data demonstrating, both in animals and in humans, that exposure of the eye to x-rays or to high levels of other types of radiation, including ultraviolet (UV) and microwaves, can cause cataract with definitive oxidative effects in the lens. Likewise, exposure to hyperbaric oxygen, either experimentally in animals or therapeutically in patients, can cause cataracts. Further support for the oxidation hypothesis comes from epidemiologic studies that have found an association between increased exposure to sunlight, particularly its UV component, and ageing-related cataract.

Scatter and absorption changes in the ageing lens

The young human lens is colourless and transmits almost 100 per cent of the incident light. With age, both scatter and absorption of optical radiation increase, with the result that the lens becomes yellow.

The absorption of optical radiation by the lens increases exponentially with age. The rise in absorption is greatest for wavelengths at the blue end of the spectrum (460nm) and this is due to accumulation of yellow chromophores. Increasing yellowing of the lens and absorption of wavelengths at the blue end of the spectrum play a protective role against the harmful effects of optical radiation on the macula. This is one of the factors that led to the development of yellow-tinted intraocular lenses, for example Acrysof Natural (Alcon), for cataract surgery.

Classification of cataracts

There appear to be three major types of ageing-related cataracts - cortical, nuclear, and posterior sub-capsular -which differ in both the location in which the opacity initially appears and in the pathology underlying the opacification. Many risk factors may be common to all three types of ageing-related cataracts, and although cataracts often begin as a pure type, as they mature they typically become mixed cataracts. Table 2 summarises the main types of cataracts seen in clinical practice.

Objective classification schemes (Table 3) use photographic standards to sub-divide each major type further into grades. These grades are based on density and colour (in the case of the nucleus), or according to the anatomic area of the cataract (in the case of the cortical and posterior sub-capsular areas). One may directly compare the patient's lens as seen on the slit lamp with a photographic copy of the various standard grades as set up in the various classification schemes (clinical grading), or one may take photographs of the lens being studied and later grade the photographs according to the classification scheme used (photographic grading).

Cortical cataracts

Cortical cataracts (Figure 1) are the most common of the three major pure cataract types. It is notable that although certain types of cataracts may initially occur as pure types, as the cataract progresses, it eventually becomes mixed as the other anatomic areas become affected. The cortical layer is less compact than the nucleus and is therefore more prone to becoming overhydrated as a result of electrolyte imbalance, which eventually leads to disruption of the lens cortical fibres, as demonstrated in diabetes and galactosaemia. It has, therefore, been proposed that this type of cataract may be partly caused by osmotic stress. Early changes may include signs such as vacuoles, water clefts, and lamellar separation. These changes may come and go over time, but eventually they may predispose the patient to damage and irreversible opacification of some fibres.

Most cortical cataracts remain in the periphery for many years, even decades, before the central axis of the lens becomes involved, causing loss of vision late in the development of the cataract. Patients with this type of cataract are usually reassured that their cataracts may not decrease their vision for a good number of years and that, until that happens, they do not need to worry about undergoing cataract surgery.

It has been observed that patients may have an advanced grade of cortical cataracts (for example, cortical opacities covering the entire anterior cortical and posterior cortical area) and yet have 6/12 or better Snellen visual acuity under standard testing conditions. This occurs especially when the cortical cataract is of a low density, allowing enough light to reach the macula to stimulate it adequately. However, these patients have severe glare disability as documented by the Brightness Acuity Tester (BAT) from Mentor O & O, Norwell, MA (Figure 2), such that under simulated bright lights, their visual acuity may decrease to 6/24 or worse. They also have decreased contrast sensitivity. These patients tend to do well indoors, but have difficulty driving during bright, sunny days and at night because of oncoming headlights. Treatment in these cases must be decided on an individual basis.

Nuclear cataracts

Nuclear cataracts (Figure 3) appear to be an exaggeration of the normal sclerosis or hardening and yellowing of the nucleus in older adult patients. Studies have documented a gradual increase in optical density of the nucleus with increasing age in normal adults with 6/6 vision.

Nuclear cataract with yellowing of the lens nucleus

Nuclear cataracts tend to progress slowly, with the visual acuity of patients remaining in the 6/9 range for prolonged periods. The refractive index of the lens changes as the nucleus progressively hardens, usually resulting in increasing myopia and astigmatism. In some patients, this is accompanied by optical distortion, especially of distant images, while near vision remains in the N6 range. This is especially true in patients with high axial myopia, in whom refraction sometimes fails to provide adequate restoration of distance vision.

This cataract is best seen with the narrow-beam direct illumination employed by the slit lamp, which reveals the colour and generalised haze or opalescence of the nucleus. In the early stages, the two halves ('cotyledons') of the embryonic nucleus remain visible. Later, the entire nucleus appears as a homogeneous mass in contrast to the cortex. Retro-illumination may show the 'oil droplet' effect. Sometimes one may notice crystals in the lens nucleus ('Christmas tree' cataract).

Nuclear cataracts are associated with physiochemical changes in the lens structural proteins (α-, β-, and γ-crystallins). These proteins undergo oxidation, nonenzymatic glycosylation, proteolysis, deamidation, phosphorylation, and carbamylation leading to aggregation and formation of high-molecular-weight proteins. Chemical modification of the nuclear lens protein leads to yellowing, followed by browning and, in advanced stages, blackening.

Posterior subcapsular cataracts

Posterior subcapsular cataracts (PSC) (Figure 4) are less frequently encountered and often occur in combination with nuclear or cortical cataracts in the later stages. One easily notices them on retro-illumination, since they are usually located centrally, and they may interfere with funduscopy. In early stages, patients usually complain of subjective symptoms such as glare disability and difficulty focusing on objects, especially for near distance. This is due to the fact that when the pupil constricts during accommodation, the light entering the eye becomes concentrated centrally where the PSC is also located, causing light scattering and interfering with the ability of the eye to focus an image on the macula. In addition, these opacities lie at or near the nodal point of the eye, further interfering with focusing the image on the macula.

This cataract may be examined with direct illumination, using the narrow and broad beams of the slit lamp to show the characteristic granular (gravel-like) inner surface immediately in front of the posterior capsule.

The problem with this technique, however, is that patients may not tolerate any prolonged direct illumination due to intolerable glare. Retro-illumination is, therefore, more useful for revealing the outline of the opacity, since it is usually seen as an 'island' in the centre of the posterior capsule, which is further highlighted by the shadow cast by the opacities.

In the early stages of this cataract, however, the dust-like particles that might be noticeable in the central posterior subcapsular area with direct illumination disappear or are difficult to see with retro-illumination. Eventually, this 'dusting' becomes dense enough to cast a shadow and thus appear on retro-illumination. The smooth orange background of the fundus helps to highlight the rough, irregular pseudopodia-like edges of the central opacity. In advanced stages, the PSC may become a thick, calcified plaque.

PSCs may also result from irradiation or steroid ingestion, or it may be associated with diabetes, high myopia, retinal degeneration (for example, retinitis pigmentosa), and gyrate atrophy.

Mixed cataract

These are cataracts that have more than one variety of cataract occurring together. A cataract normally starts as one type, but may eventually become mixed as the other lens regions become involved in the degenerative process. A mixed cataract indicates that the cataract has already advanced to some degree and its progression should be watched more closely. Patients with these cataracts tend to have more visual symptoms.

Mature cataracts

The lens may swell and increase in volume rapidly because of rapid hydration of the lens cortex. Complete opacification of the lens is called a mature cataract (Figure 5). If the liquefied cortical material is not re-absorbed, the solid nucleus may 'sink' to the bottom of the lens bag (Morgagnian cataract), this is rarely seen in the UK. Re-absorption of the milky cortex causes reduction in the lens volume causing the capsule to form folds.

Capsular cataracts

The lens capsule may develop localised opacities in age-related cataracts. However, they may also occur in uveitis in association with posterior synechiae or secondary to injury caused by drugs, radiation, or trauma. Localised central capsular cataracts (polar cataracts) can occur in the anterior and posterior capsule and are usually congenital, although they may also be secondary to trauma.

Polar cataracts (Figure 6) are usually dense, localised, and non-progressive. Because they are stable, many patients may be able to tolerate them and may retain good, adequate vision with conservative treatment (for example, dilation of the pupil, wearing sunglasses on bright days, optical correction). For the surgeon, extra care has to be taken when removing this type of cataract as there is a high risk of posterior capsule rupture during surgery.

Anterior subcapsular cataracts

In contrast to PSCs, anterior sub-capsular cataracts consist of multi-layering of the anterior lens epithelium and deposition of abnormal lens capsule. They may occur together with PSC cataracts. They may also result from local injury or irritation, as in uveitis or injury due to chemicals or radiation.

Retrodots

Retrodots are round, translucent opacities that usually occur in the deep cortex or perinuclear region. In general, they do not seem to affect vision until a mixed cataract appears (nuclear or cortical), and patients may have these retrodots for years and still retain good vision

Congenital and juvenile cataracts

Congenital cataracts are detected at birth, whereas juvenile cataracts develop during the first 12 years of life. Both range from mild and benign to advanced and sight-threatening. Congenital cataracts are the third most common cause of blindness in children. The following is a morphologic classification of congenital cataracts:

Total or complete

These cataracts are completely opaque or hazy at birth. Most of these are associated with systemic disorders or abnormalities such as galactosaemia, rubella, and Lowe's syndrome. They may also be hereditary (autosomal dominant or autosomal recessive).

Partial or incomplete

Anterior and posterior polar cataract

These cataracts (Figure 6) involve the lens capsule in the anterior or posterior pole of the lens. They are sometimes associated with a localised anatomic abnormality in the region. For example, posterior polar cataracts commonly occur in cases of posterior lenticonus. They may cause more visual symptoms because they are closer to the nodal point of the eye. However, they are usually stable, and patients may do well with conservative measures. The familial type is bilateral and inherited as an autosomal dominant trait.

Zonular cataract

In this type of cataract, only a region or zone of the lens is opaque. They may be stationary but may also progress.

There are various subtypes:

● Lamellar: This is the most common type of congenital cataract. Such cases are usually bilateral and symmetric, and the density of opacification may vary considerably. Less opaque lamellar cataracts may be compatible with good vision and minimal medical intervention (for example, optical correction, therapeutic mydriasis). These cataracts may be inherited as an autosomal-dominant trait, but in some cases they may be attributed to a transient intrauterine toxic agent, affecting only the layer of cells developing at the time of foetal exposure

● Stellate: These cataracts affect the region of the sutures. They may be Y-shaped if the cataract occurs in the intrauterine stage of development, since the sutures have this configuration during this period. Anterior sutural cataracts are Y-shaped posterior sutural cataracts are shaped like an inverted Y. Sutural cataracts that develop later on have a more stellate shape in keeping with the shape of the sutures after birth

● Nuclear: These cataracts are usually bilateral and involve the foetal or embryonal nucleus. They may be inherited as an autosomal-dominant, autosomal-recessive, or X-linked trait

● Coronary: These cataracts are radial, club-shaped discrete opacities located in the cortex and are called 'coronary' because they appear like the top of a crown. Because of their peripheral location, they do not decrease visual acuity. Coronary cataracts are dominantly inherited and have been described in cases of Down's syndrome and myotonic dystrophy

● Cerulean: These cataracts consist of small, discrete opacities that have a distinct bluish hue. These opacities are located in the cortex, are non-progressive, and do not cause visual symptoms. They may be present together with other congenital cataracts.

● C Membranous cataracts: These cataracts are thin but dense and contain fibrous tissue. They may occur when lens proteins are re-absorbed (for example, traumatised lens see following section), such that the anterior and posterior lens capsules fuse, producing a dense membrane.

Traumatic cataract

Cataracts can occur secondary to trauma to the lens. The morphologic characteristics differ between cataracts due to blunt trauma and cataracts secondary to penetrating trauma. Cataracts secondary to blunt trauma often have a rosette-shaped appearance, or are of the PSC variety. In cataracts secondary to penetrating trauma, the size of the opening in the lens capsule determines the morphology of the cataract. When the opening is large, the whole lens is cataractous when the opening is small, it may sometimes seal by itself and leave behind an opacity that is localised to the site of penetration.

Summary

There are many subjective ways of classifying cataract but the most common way employed is by the anatomic locations of lens opacities, namely, cortical, nuclear and posterior subcapsular. Several types of clinical grading and photographic standards have been developed to grade each type of cataract

Kenneth Fong is a consultant ophthalmologist based in London.

Raman Malhotra is an ophthalmic and oculoplastic surgeon based in East Grinstead, West Sussex

This article is based on a chapter in Cataract by Ramon Malhotra, a forthcoming book in the Eye Essentials series

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