PHYSIOLOGICAL OPTICS 2 MIDTERMS

• Accommodation


  Mechanism by which the eye changes refractive power by altering the shape of the lens in order to focus objects at variable distance
• Accommodation


  • Caused by the increased curvature of the anterior area of the crystalline lens
  • Stimulated by blurring and looking at near objects
• Accommodative range


  The extent of what our eyes can accommodate
• Refractive error


  If the accommodative range is not high enough to compensate, it will result in blurring
• Some insignificant amount of refractive errors can be compensated by accommodation
• Stimuli for accommodation


  • Blur can increase accommodative power
  • Looking at near task/object will cause accommodation
• Accommodation


  1. Lens increases curvature of anterior area

  2. Results in auto-focusing power
• Distance vision


  1 meter to 20 feet is the standard
• Near vision


  1. Increased curvature in the anterior segment of the lens

  2. Object focused exactly at the retina
• Ciliary muscle


  • Smooth muscle shaped like a ring located in the middle of the eye
  • Holds the lens with the suspensory ligaments
  • Adjusts the shape of the lens during accommodation
• Zonule of Zinn / suspensory ligaments

  Tiny fibers that suspend the lens at an exact location
• Ectopia lentis


  Dislocation of the lens causing double vision
• Marfan's syndrome


  • Diagnosed with ectopia lentis
  • Long phalanges
  • Thin people
  • Bone deformities
  • Arachnodactyly
• Crystalline lens


  • Transparent, biconvex in shape
  • Held by suspensory ligaments
  • Made up of regularly arranged collagen fibers
• Lens curvature


  More curved lenses result in increased lens power and accommodation
• Presbyopic stage


  Blurring at near due to hardening of the crystalline lens

  
• Cataract


  Opacity of the lens
• Risk factors for cataract include exposure to UV light, systemic diseases, and lifestyle factors
• Pupil


  • Located in the middle of the eye
  • Constricts to prevent light rays from causing blurring
• RAPD (Relative Afferent Pupillary Defect)


  Problem in the regulation of light entering the eye
• Adie's pupil


  Always dilated due to problem in parasympathetic CN #3 (oculomotor)
• 3 synkinetic actions (3 C's)

  1. Convergence of the eyeball

  2. Contraction of ciliary muscle

  3. Constriction of pupil
• Accommodation mechanism


  1. Contraction of ciliary muscle

  2. Relaxation of zonular fibers

  3. Increased lens curvature
• Accommodative insufficiency


  Occurs when the amplitude of accommodation is lower than expected for the patient's age
• Ill-sustained accommodation


  Condition where the amplitude of accommodation is normal but fatigue occurs with repeated accommodative stimulation
• Accommodative infacility


  Slow accommodative response or lag between stimulus and accommodative response
• Accommodative flippers test


  Flipping plus and minus lenses to test the lens' ability to compensate
• Paralysis of accommodation


  Inability of the ciliary muscle to change the shape of the lens

  
• Tonic accommodation


  Total or complete loss of accommodation in one or both eyes
• Spasm of accommodation


  Prolonged contraction of the ciliary muscle, cannot relax even when looking at distance
• Accommodative esotropia


  Both eyes converging and accommodation super-stimulated even at far distance
• Fogging technique


  Use of plus lenses to relax accommodation and accurately measure refractive error
• Convergence


  Ability to turn the two eyes inward toward each other to look at close objects
• Amplitude of convergence does not deteriorate with age, unlike accommodation</b>
• Tropia / Strabismus


  Permanent deviation of the eye

  
• Esotropia


  Eyes turning inward
• Exotropia


  Eyes turning outward
• Physiological convergence (normal convergence)


  Convergence only happens when looking at close objects
• Convergence


  1. Requires a coordinated stimulation of some extraocular muscles at the same time others are relaxed

  2. Occurs by stimulation of the medial rectus muscle of both eyes while simultaneously relaxing the lateral recti muscles
• Amplitude of convergence does not deteriorate with age, unlike accommodation