cataract

• Lens


  Unique, can change its refractive error unlike the cornea, tear film and vitreous
• Ways the lens changes refractive error

  1. Ocular accommodation

  2. Age related changes
• Function of the lens

  Transmit as much light as possible with minimum degree of optical defocus
• Lens


  • Wants optical focus and light transmission
  • Tries to minimise light scatter (can lead to glare)
• Anterior part of the lens


  Single layer of epithelial cells
• Changes in lens epithelial cells near the equator

  1. Cell body flattens and elongates towards the anterior and posterior poles where they join to form the anterior and posterior sutures

  2. Cells lose their organelles leading to an organelle free zone (OFZ)
• The fibres that make up the lens cortex are like two halves of a Russian doll type structure, they don't go all the way around but stop at the sutures
• Features that help light transmission and reduce light scatter in the lens

  • Absence of vessels
  • Lack of cell organelles
  • Tightly packed and regular shaped fibres
• Cataract


  The lens is changing, leading to: loss in lens fibre elasticity (loss of accommodation), increased aberrations, and increased light scatter
• Cataract development happens sooner in some patients and later in others (non-linear)
• All cells in humans are subject to oxidative stress which can change cell structures over time
• Changes to lens fibre proteins are a form of protein denaturation


  Denatured proteins are opaque, which increases light scatter
• Age-related changes to protein structure cause proteins to change size and shape, which changes how closely they can be packed together
• Nuclear fibres are present at birth, while cortical fibres are added throughout life
• The maximum size of a nuclear cataract is limited to its fixed region, which reduces in size due to the compacting effect of cortical fibres being added
• There is an increase in lens thickness with age which is correlated to increased light scatter
• Cortical fibre production increases lens thickness in the sagittal and axial planes, but equatorial thickness remains the same
• Two types of light scatter

  • Forwards light scatter - towards the retina (causes cataract symptoms)
  • Backwards light scatter - away from the retina (seen on slit-lamp)
• The deeper the cortical fibre, the greater the degree of compaction, leading to a more rapid increase in light scatter with age
• Point spread function (PSF)

  Measures how much a 'point source' (narrowest beam) is spread by the ocular media
• Factors that influence the PSF

  • Aberrations
  • Light scatter
• Lower-order aberrations


  Defocus (myopia, hyperopia) and regular astigmatism - can be corrected by changing refractive power
• Higher-order aberrations


  Spherical aberration, coma, trefoil - cannot be corrected by changing refractive power
• Patients with more aberrations will have significant uncorrected refractive error and complain of blur
• Light scatter is not correlated with visual acuity, but is associated with glare symptoms
• Glare symptoms


  • Stars and/or halos around lights
  • Discomfort when fixation is close to a light source
  • Vision deteriorating with increasing proximity between fixation and light source
• Light scatter helps explain why patients may complain of poor vision but have good visual acuity in the testing room
• Nuclear cataract


  • Characterised by age-related sclerosis (hardening) of the lens nucleus
  • Opacity due to protein-related changes and accumulation of light-absorbing molecules called chromophores
• Symptoms of nuclear cataract

  • Blurred vision
  • Glare
  • Altered colour vision
• Risk factors for nuclear cataract

  • Age
  • Smoking
  • Diabetes
• Cortical cataract


  • Lens fibres become overhydrated, reducing transparency
  • Opacities begin in the peripheral cortex and spread towards the nucleus
  • Causes astigmatic shift
• Around 30% of patients over 45 have some form of cortical cataract, but they are usually asymptomatic due to their peripheral position
• Symptoms of cortical cataract

  • Blurred vision (astigmatic blur)
  • Monocular diplopia
  • Glare
• Risk factors for cortical cataract

  • Age
  • UV exposure
  • Diabetes
• Posterior subcapsular cataract (PSC)

  Some lens epithelial cells migrate towards the posterior pole and become vacuole-shaped, forming a thin opaque layer
• Symptoms of PSC


  • Blurred vision (due to aberrations and light scatter)
  • Glare
• Factors influencing PSC symptoms

  • Pupil size (light levels, degree of convergence, age)
  • Proximity of light source
• Congenital cataract


  • 10% of cataracts are present at birth
  • Most common cause is unknown, but can be due to metabolic disorders, systemic abnormalities, or chromosomal abnormalities
• Early diagnosis and treatment of congenital cataracts is important to avoid amblyopia
• Subluxated lens


  Lens is not centred, can cause unusual refractive error