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A. Definitions
[Figure]: "Schematic of the Eye" navigator

  1. Accomodation: increase in refractive power of natural lens for use in near visual tasks (that is, pupil constricts on moving object nearer)
  2. Afferent Pupillary Defect: evidence of optic nerve disease detected by swingling light test
  3. Amblyopia: lazy eye, due to abnormal visual experience when young; no structural anomaly
  4. Anisocoria: inequality of pupil size; may be due to nerve damage or occur congenitally
  5. Aphakia: absence of natural crystalline lens
  6. Bruch's Membrane: basement membrane of retinal pigmented epithelium (RPE) and choriocapillaris
  7. Cataract: any opacification of crystalline lens
  8. Chemosis: edema of bulbar conjunctiva (on surface of globe); swelling around the cornea
  9. Choriocapillaris: large capillary network located beneath the RPE
  10. Choroid: posterior aspect of uveal tract (anterior aspect is iris and ciliary body)
  11. Congruity: refers to symmetry of homonymous field defects
  12. Conjunctiva
    1. Squamous epithelial membrane
    2. Covers globe (bulbar conjunctiva) and eye lids (tarsal conjunctiva)
    3. Containes tear glands and lymphatics
  13. Cornea: clear avascular anterior refractive surface of eye, continuous with sclera
  14. Fovea: central portion of macula, avascular, with greatest light sensitivity
  15. Glaucoma: generally refers to optic nerve damage with visual field loss due to increased intraocular pressure
  16. Hemianopsia: deficit in one half of visual field, either nasal or temporal; same side of field is affected in each eye
  17. Homonymous: high similar (identical) field defects in both visual fields
  18. Hyperopia: far-sightedness; able to see distant objects well. Often due to short axial eye length or to a relatively flat cornea (see below)
  19. Hyphema: blood in anterior chamber of the eye
  20. Hypopium: layered inflammatory cells in atnerior chamber of eye (inflammation)
  21. Iris: anterior aspect of uveal tract, composed of pigmented epithelium which contain dilator and sphincter muscles which alter pupilary size in response to light
  22. Limbus: transition zone between cornea and sclera
  23. Macula: most light-sensitive area in retina, located lateral to the optic disk [2]
  24. Marcus-Gunn Pupil: paradoxical pupil dilation on swinging light test (this is a form of afferent pupillary defect)
  25. Myopia: near-sightedness, often due to long axial eye length or relatively steep cornea
  26. Neovascularization: generation of abnormal new vessels at level of choroid or retina
  27. Optic Disk
    1. Retina area where optic nerves enters; ophthalmic artery and vein pass
    2. Causes the classical "Blind spot"
  28. Presbyopia: age related loss of accomodation leading to loss of ability to view near objects
  29. Quadranopsia: deficit involving one quadrant of visual field
  30. Retina
    1. Transparent structure containing primary visual sensory and processing neurons of eye
    2. Outer Retina: layers closer to retinal pigment epithelium, supplied by choriocapillares
    3. Inner Retina: layers closer to vitreous, supplied by retinal arteries
    4. Macula is posterior aspect of retina with highest concentration of photoreceptors [2]
  31. Retinal Pigment Epithelium (RPE): single columnar cell lining, beneath retina, responsible for blood-retinal border and aspects of photoreceptor metabolism
  32. Schlemm's Canal: portion of drainage system including trabecular meshwork that allows aqueous to escape through episcleral venous plexus
  33. Sclera: whites of eye; external collagenous coat of eye
  34. Syneresis: breakdown of vitreous leading to posterior vitreous detachment
  35. Uveal Tract: vascular middle coat of eye consisting of iris, ciliary body, and choroid
  36. Vitreous Humor: transparent gel-like tissue within posterior segment of eye

B. Pupilnavigator

  1. Autonomic Nervous System (iris)
    1. Parasympathetic
    2. Sympathetic
  2. Parasympathetic
    1. Pupillary constriction iva iris sphincter
    2. Ciliary body contraction allowing accomodation
    3. Midbrain connections (Edinger-Westphal Nuclei)
    4. Efferents travel with CN III (inferior division, synapse in ciliary ganglioin)
    5. Post-synaptic fibers travel to iris via short ciliary nerves
  3. Sympathetic
    1. Pupillary dilation via iris dilator
    2. Mueller's muscle of lid - secondary lid elevation
    3. System begins in hypothalamus and descends to C8-T2
    4. Synapse in superior cervical ganglion
    5. Post-ganglionic fibers travel along internal carotid artery into cavernous sinus
    6. Reach iris via nasocililary and long ciliary nerves
  4. Examination
    [Figure]: "Schematic of the Eye"
    1. Size in light and dark
    2. Anisocoria - in light suggests parasympathetic problem; sympathetic problem in dark
    3. Shape
    4. Reaction to light, direct and consensual (that is, other pupil normally same reaction)
    5. Swinging flashlight test to check for afferent pupillary defect
    6. A cycloplegic (anticholinergic, such as cyclopentolate) is given to relax ciliary body for ophthalmological examination and measurement of refractive error of eye

C. Retina navigator

  1. Layers (from exterior to interior)
    1. (Choriocapillaris)
    2. (Bruch's Membrane)
    3. Pigmented Epithelium (RPE)
    4. External Limiting Membrane - attachments of photoreceptors and Muller cells
    5. Photoreceptors - rods (detect light level) and cones (color detection)
    6. Outer Nuclear Layer - photoreceptor nuclei
    7. Outer Plexiform Layer - bipolar horizontal cells synapse with photoreceptors
    8. Inner Nuclear Layer
    9. Inner Plexiform Layer - axons of bipolar / amacrine cells synapse with ganglion cells
    10. Ganglion Cell Layer
    11. Nerve Fiber Layer - ganglion cell axons
    12. Internal Limiting Membrane - footplate of Muller cells
  2. Blood Supply
    1. Retinal vessels: support inner retinal structures
    2. Choriocapillaris (choroidal vessels): support RPE and Photoreceptors
  3. Retinal Neurons
    1. Photoreceptors
    2. Inner Nuclear Layer
    3. Ganglion Cell layer
  4. Photoreceptors
    1. Rods: scotopic vision (120 million in retina)
    2. Cones: color vision (6 million in retina)
  5. Inner Nuclear Layer
    1. Bipolar Cells - vertical orientation, connect photoreceptors to ganglion cells
    2. Horizontal Cells - connect photoreceptors, processes extend in outer plexiform layer
    3. Muller Cells - glial cells providing structural support
    4. Amacrine Cells - interconnect ganglion cells, processes extend inner plexiform layer
  6. Ganglion Cell layer
    1. Axons form nerve fiber layer and optic nerve
    2. 1.2 million axons run to cerebral cortex

D. Cornea [3]navigator

  1. Clear avascular anterior refractive surface of eye
  2. Continuous with sclera
  3. Provides tension on lens and major structure for refraction
  4. Anatomy
    1. 5 distinct layers, total 0.5mm thick
    2. Epithelium - most exterior layer
    3. Bowman's membrane
    4. Stroma - 90% of corneal thickness, collagen fibrils
    5. Endothelium - maintain relative corneal dehydration necessary for corneal clarity
    6. Desemet's membrane
  5. Corneal shape abnormalities lead to inabilities to focus
    1. Emmetropia: eye with normal vision (cornea and lens work in concert); light rays perfectly focused on the fovea
    2. Myopia: near-sighted; rays converge in front of the retina; cornea too curved
    3. Hyperopia: far-sighted; rays converge in "back" of the retina; cornea too flat
    4. Astigmatism: cornea is not spherical (one meridian steeper or flatter than others)
    5. Presbyopia: far-sightedness caused by loss of elasticity, usually in middle or older age
    6. Keratoconus: progressive thinning of central cornea, usually with myopia, astigmatism
  6. Diseases Associated with Abnormal Cornea
    1. Severe myopia associated with detached retina
    2. Severe hyperopia associated with angle closure glaucoma
  7. Correction of Corneal Abnormalities [1]
    1. Goal is to reshape the cornea so that light rays are redirected to focus on retina
    2. Several surgical procedures are available with good results on myopia and hyperopia
    3. Results in astigmatism improving and are now nearly as good as non-astigmatism
    4. Glasses or contact lenses may still be required after corneal surgery
    5. Repeat laser surgery may be required years after initial surgery
  8. Refractive Surgery [1,3,5]
    1. LASIK - Lamellar Laser In Situ Keratomileusis, microkeratome produces thin flap in cornea, laser to etch away at flap, minimal pain, usually bilateral, ~90% success
    2. Three methods of surface ablation: PRK, LASEK, Epi-LASIK
    3. PRK - photorefractive keratectomy, laser reshapes stromal surface of cornea, but the epithelium is stripped and protocol is fairly painful, outcomes similar to LASIK
    4. Outcomes with LASIK superior to PRK for myopia >6 diopters [6]
    5. LASEK - Laser Epithelial Keratomileusis; patient's original epithelial shset is repositioned onto stromal bed after laser ablation
    6. Epi-LASIK - similar to LASEK, but a purely mechanical means of epithelial dissection; this is repositioned after excimer laser ablation
    7. RK - radial keratotomy, deep radial incisions weaken peripheral part of cornea causing peripheral cornea to bulge out and central corenal to become flatter (rarely used today)
    8. CK - conductive keratotomy, for correction of hyperopia +0.75-3.25 diopters in age >40 years; CK delivers radiowaves to spots along circumference of cornea
    9. CK also now approved by FDA for treatment of presbyopia in the nondominant eye by inducing myopia in this eye; effect may fade over time [5]
    10. Intracorneal Rings (Intaks®) - PMMA rings placed into peripheral corena for correction of mild myopia (up to 3 diopters); rings may be removed or replaced
    11. Phakic intraocular lenses - implanted plastic intraocular lenses in addition to normal lens for correction of high degrees of myopia and hyperopia

E. Lens
[Figure]: "Schematic of the Lens" navigator

  1. Second most important refractive surface of eye after cornea
  2. Surrounded by fibrous outer capsule
  3. Contains water soluble crystallins and water insoluble proteins
  4. Insoluble fraction increases with cataract-type opacification
  5. Accomodation
    1. Process by which refractive power (focus) of natural lens is increased
    2. This needs to occur for focusing on near vision tasks
    3. Parasympathetic innervation leads to ciliary muscle contraction
    4. This relaxes lens zonule tension allowing lens to thicken and increase power
    5. As the lens ages, it is less flexible to changing dimensions, leading to presbyopia

References navigator

  1. Sakimoto T, Rosenblatt MI, Azar DT. 2006. Lancet. 367(9520):1432 abstract
  2. Fine SL, Berger JW, Maguire MG, Ho AC. 2000. Lancet. 342(7):483 abstract
  3. Bower KS, Weichel ED, Kim TJ. 2002. Am Fam Phys. 64(7):1183
  4. LASIK and Its Alternatives. 2004. Med Let. 46(1174):5 abstract
  5. Conductive Keratoplasty for Presbyopia. 2004. Med Let. 46(1185):49 abstract
  6. Wilson SE. 2004. NEJM. 351(5):470 abstract