Aetiology

The most common cause of cataract is the normal ageing process. Other causes include ocular trauma and previous ocular surgery or laser treatment causing traumatic capsule damage. Certain medical conditions are associated with cataracts, particularly diabetes, but also hypertension, metabolic syndrome, renal impairment, systemic lupus erythematosus, and atopic dermatitis.[9][10]​ Other causes include genetic diseases (e.g., myotonic dystrophy, Down's syndrome, Marfan's syndrome, neurofibromatosis type 2, and Alport's syndrome), metabolic conditions (e.g., Wilson's disease, galactosaemia, and Fabry's disease), or congenital problems (e.g., rubella infection). Some types of medication can also induce cataracts, particularly corticosteroids.[1][11][12]​ Smoking and exposure to ultraviolet (UV) radiation may also cause cataract progression, especially nuclear sclerotic cataracts.[1][2]​ 

Pathophysiology

Changes in the lens proteins (crystallins) affect how the lens refracts light and reduce its clarity, therefore decreasing visual acuity. Chemical modification of these lens proteins leads to the change in lens colour. Oxidative stress is thought to be a major contributor to the pathogenesis of nuclear cataracts. The lens normally has a low oxygen environment maintained by an oxygen gradient created by the posterior vitreous gel. Disturbing the vitreous gel destroys this oxygen gradient and exposes the lens to higher oxygen levels. This is supported by studies showing high rates of cataract formation within 2 years of vitrectomy surgery.[13]

In nuclear sclerotic cataracts, new cortical fibres are produced concentrically and lead to thickening and hardening of the lens, which often appears yellow, and can increase its focusing power. Increasing myopia can therefore be evidence of a progressing nuclear sclerotic cataract, referred to as myopic shift.

Cortical cataracts are most often seen as whitish spokes peripherally in the lens, separated by fluid. Vacuoles and water clefts can also be seen.

Posterior sub-capsular cataracts are due to the migration and enlargement of lens epithelial cells (Wedl cells) posteriorly. Diabetes mellitus is a major factor in the formation of this type of cataract. Osmotic stress due to sorbitol accumulation has been linked with sudden worsening in patients with uncontrolled hyperglycaemia. However, research has also found that when sorbitol dehydrogenase was blocked, preventing sorbitol accumulation, oxidative stress was connected with slow-developing cataracts.[14]

Classification

By cataract features

  • Nuclear sclerotic: generally age-related opacification, primarily involving the nucleus of the lens

  • Cortical: primarily opacification of the cortex of the lens

  • Posterior sub-capsular: opacification of the posterior sub-capsular cortex, seen frequently in drug-related (e.g., with topical corticosteroids) or metabolic cataracts

  • Anterior sub-capsular: opacification of the anterior sub-capsular cortex, frequently seen in blunt traumatic injuries

  • Cerulean: small bluish opacity seen in the lens cortex (often also referred to as blue-dot cataracts)

  • Snowflake: grey-white sub-capsular opacities that can be seen in cataracts in uncontrolled diabetes mellitus

  • Sunflower: yellow or brown pigmentation of the lens capsule in a petal-shaped distribution, seen in patients with an intra-ocular copper-containing foreign body or Wilson's disease

  • Christmas tree: polychromatic iridescent crystals, seen in the lens cortex of patients with myotonic dystrophy

  • Oil droplet: on retro-illumination, seen in patients with galactosaemia. An accumulation of galactose and galactitol in lens cells causes an increase in osmotic pressure due to fluid influx

  • Traumatic: blunt or penetrating trauma, infrared energy, electric shock, or ionising radiation can all lead to classic rosette or lamellar stellate cataracts

  • Posterior polar: opacity with dense white spots at the edge and may be associated with thin or absent posterior capsule.

By age of onset

  • Acquired

    • Most common cause of cataract is the normal ageing process.

    • Other contributing conditions include trauma, metabolic disorders, and medications.

    • While there is still some debate, smoking, alcohol, and exposure to UV radiation may also cause cataract progression, especially nuclear sclerotic cataracts.[1][2]

    • If associated with another ocular disorder, the term cataracta complicata may be used.

  • Congenital

    • Lamellar or zonular cataracts are the most commonly seen form of congenital cataracts.

    • Usually bi-lateral and symmetrical.

    • May be due to transient toxic influences during lens development, or may be inherited in an autosomal-dominant pattern.

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