TSM44: case launch – a lady with failing eyesight22/10/08

Case overview
  • 74-year-old woman who normally wears glasses presents with:
  • Blurring of vision when watching television and reading text
  • Increasing difficulties driving
  • Optician’s examination reveals:
  • Visual acuities without glasses6/24 R, 6/36 L (worse in left)
  • Visual acuities with glasses 6/12 R, 6/18 L(half normal acuity in right)
  • Intraocular pressure 17 in each eye (NR 10-21mmHg)
  • Streaky, hazy appearances in the lens of each eye
  • Referred to GP who diagnoses bilateral cataractsand is referred for surgery
  • Cataract surgery is the most commonsurgical procedure performed on the NHS
  • Cataracts are in the top four causes of blindness worldwide
Snellen visual acuity
  • Using a Snellen chart a patient’s visual acuity can be measured – their ability to see fine details
  • Result given as a fraction in which:
  • The numerator represents the distance from which the patient can clearly identify a letter
  • The denominator represents the distance from which a typical individual can do the same
  • Distances are usually given in feet with the standard (perfect acuity) being 20/20 vision
  • Focussing at 20 feet (or 6 metres) requires roughly the same optical power as at infinity
  • The smaller the resulting fraction (less than 1) the poorer the visual acuity
  • To drive in the UK you are required by law to have a visual acuity of at least 6/10

Learning Outcomes

Describe the normal anatomy of the eye

  • The eye is spherical with an anterior bulge made by the corneaand corneoscleral limbus[TG1]
  • Anterior to the lens are the posterior andanterior chambersfilled with aqueous humour
  • Posterior to the lens is the vitreous chamber filled with vitreous humour
  • The sclerais the outermost coating of the eye;white and tough, continuous anteriorly with the cornea
  • The choroid liesbeneath the sclera;vascularand continuous anteriorly with the ciliary bodies and lens
  • The retina is the innermost layer; sensitive to light and continuous posteriorly with the optic nerve

Describe basic physiological optics

  • Light passes through a 10μm layer of tearsfollowed by thecorneaand then thelens:
  • Abiconvex discattached circumferentially by zonule fibres[TG2]to the ciliary bodies
  • The shape of the lens in part dictates the way light is refracted through the vitreous chamber
  • The image is inverted and focussed in refraction,mostly by the cornea but also the lens
  • It then reaches the retina which is the only layer to contain the actual photoreceptors

Describe the basic structure of the retina

  • The retina is the neural component of the eye and is essentially a displaced element of the CNS
  • It consists of several distinct layers arranged in a tightly laminated structure:
  • The nerve fibresand associated ganglia make up the innermost (chamber-facing) layer
  • A complex connecting cell layercontains various cells bridging neural/receptor connections
  • The actual photoreceptivelayercontains functional photoreceptive units – rods and cones
  • The outermost layer is a pigmented epitheliumwhich minimises unwanted reflections
  • There are significantly more rods than there are cones, though there are many cone subtypes
  • Rods convey monochromeinformation and are primarily dark-adaptive(scotopic vision)
  • Cones are responsible for colour vision

Describe the formation, circulation and removal of aqueous humour

  • Aqueous humour is produced by the ciliary epitheliumin the posterior chamber
  • Its production is reliant on carbonic anhydrase (CA)
  • Is it a transparent liquid that is aprotein-free plasma ultrafiltrate
  • It then flows through the pupilto fill the anterior chamber
  • The aqueous humour pressurises the anterior chamber (20mmHg) to maintain eye shape
  • The trabecular meshwork encircles the inner anterior chamber posterior to the cornea
  • Through the trabecular meshwork are thecanals of Schlemm at the corneoscleral limbus
  • The aqueous humourdrains into the canals of Schlemm and into the venous system

Be aware of how clinical problems such as glaucoma are related to normal structure and function of the eye

  • Glaucoma is a condition in which elevated intraocular pressure causes visual loss and can be:
  • Open (chronic) – trabecular meshwork becomes blocked, gradual onset
  • Closed (acute) –sudden closure of angle between cornea and iris
  • Vitreous humour is not constantly produced unlike aqueous humour and cannot be replaced
  • Normally gelatinous containing collagen fibrils
  • With age may become more aqueous or shrink which may result in retinal detachment

[TG1]Transitional area between sclera and cornea

[TG2]Suspensory ligaments