The Aging Lens and Classification of Cataracts

Friday, May 20th, 2011 No Commented
Categorized Under: Vision

The primary functions of the crystalline lens are to transmit incident light and to focus it on the retina. This requires that the lens is transparent, a condition dependent on the highly regular organization of the cells of the lens and the high degree of order of the proteins in the lens cytoplasm. The protein concentration in lens fibre cells is extremely high, resulting in an index of refraction significantly greater than that of the surrounding fluids, and so 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 they may be secondary to other ocular or systemic diseases such as diabetes or retinal degenerative diseases (see box). By far the most important risk factor is age; aging-related cataract constitutes the great majority of all cataracts and is a major public health problem worldwide. In developing countries, where the availability of surgical facilities is limited, aging-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 in coming decades as the world population becomes progressively older.

Causes of cataracts

  • Aging
  • Inheritance
  • Metabolic disorders, e.g. Lowe’s syndrome, hypocalcaemia
  • Diabetes
  • Toxicity, e.g. drug-induced (steroids, amiodarone), chemical, metal ions
  • Nutrition
  • Physical dehydration
  • Trauma
  • Radiation
  • Eye disease, e.g. glaucoma, uveitis, post-vitrectomy
  • Systemic disease, e.g. atopy, renal failure

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. The lens grows by regular addition of fibres to the lens mass. Growth rate is not uniform throughout the human lifespan, and it appears to be maximal in foetal life. Foetal lens mass increases by about 180 mg/year (lens mass is 90 mg at birth), but the growth rate drops significantly after birth and is 1.3 mg/year between 10 and 90 years of age. Estimates of average lens density suggest that protein content remains relatively constant at around 33% of the wet weight over the age span. The dimensions of the lens change 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 6.5 mm while sagittal width is about 3 mm. By the age of 90 years, this changes to about 10 mm in the equatorial plane and 6 mm in the sagittal plane.

Classification of cataracts

There appears to be three major types of ageing-related cataracts:

  • cortical,
  • nuclear,
  • posterior subcapsular.

Which differ both in 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. The main types of cataracts seen in clinical practice are summarized in the box below. Objective classification schemes (see box) use photographic standards to subdivide each major type into grades. These grades are based on density and colour (in the case of the nucleus) or according to the anatomical area of the cataract (in the case of the cortical and posterior subcapsular areas). One may directly compare a 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).

Types of cataracts

  • Cortical
  • Nuclear
  • Posterior subcapsular
  • Mixed
  • Mature and hypermature
  • Capsular
  • Anterior subcapsular
  • Retrodots
  • Congenital and juvenile-total or partial
  • Traumatic

Objective classification schemes for cataracts

  • Lens Opacities Classification System II and III (LOCS II and LOCS III)
  • Oxford Cataract Classification System
  • Beaver Dam Eye Study