accommodation

• 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
  • 2 things that stimulate accommodation are blurring and looking at the near

  
• Blurring


  A certain amount of refractive error. There is a little astigmatism when the patient is refracted but the vision is still okay because that minimal eye grade can be compensated by the accommodative range. However, when the eye grade is high the accommodative range will not be enough to compensate
• Accommodative range


  The extent of what our eyes can accommodate
• Accommodative range


  • When the patient has -3.00 D, and the accommodative range is not high enough to compensate it will result in blurring. However, some insignificant amount of grades such as +0.50, - 0.50, and astigmatism of 0.50, normally the eye or lens can still compensate with this grade due to accommodation
• Accommodation


  1. Stimulation of blur can increase the accommodative power

  2. Looking at near task / object, accommodation will occur

  3. The lens will increase its curvature to the anterior area of the crystalline lens resulting in auto-focusing power
• Distance visual acuity


  1 meter to 20 feet is the standard
• Accommodation


  When we look near, there would be an increased curvature in the anterior segment of the lens, to have the object focus exactly at the retina
• Ciliary muscle


  • Smooth muscle that is shaped like a ring and it's located in the middle of the eyes. It holds the lens with the suspensory ligaments and also adjusts the shape of the lens during accommodation
  • Belong in the vascular or uveal coat
• Zonule of Zinn / suspensory ligaments

  • Tiny fibers suspense the lens at an exact location. When the zonule of zinn cannot hold the crystalline lens it will result in dislocation
• Ectopia lentis


  Dislocation of the lens causing double vision because some portion of the light passes through without passing the lens while some other passes the lenses
• Marfan's syndrome


  • They are mostly diagnosed with Ectopia lentis (dislocation of the lens) because of the bone structures and skeletal formation. There's no cure, only management
  • Long phalanges (unusually long arms and legs)
  • Thin people
  • Bone deformities
  • Arachnodactyly – curved chest because of bone deformation
• Crystalline lens


  • One of the transparent structures of the eye that is biconvex in shape (anterior and posterior are both convex). It forms with the posterior side of the iris and it is held by the suspensory ligaments
  • Made up of regularly arranged collagen fibers (fibers – flexible can reshape, posterior to anterior portion shape more curved), when we accommodate the lens are reshaping. Resulting in auto focus power, increasing the power of the lens, when there's an increase of curvature
• Presbyopic stage


  Natural generative disorder, 40 above - blurring at near, as we age the crystalline lens will be unable to shape because of the hardening of the lens. Until opacity occurs, cataract
• Cataract


  Opacity of the lens, 56 above until it matures, normal - extraction, surgery of cataract removal, replacing the lens with an artificial lens
• Risk factors for cataract


  • People who's always exposed at sunlight
  • High altitude places
• Systemic diseases that can affect the lenses

  • Hyperthyroidism
  • Diabetic mellitus
  • Hypertension
  • Systemic lupus erythematosus
• Lifestyle factors that can affect the lenses

  • Smoking
  • Drinking
  • Taking drugs
• Pupil


  • Located in the middle of the eyes, it is black and constricts to prevent light rays that have diverged from touching the retina and causing blurring
  • The iris constricts (regulates the light that enters the eye) constricts and dilates
• RAPD (Relative Afferent Pupillary Defect)


  Too much constriction or dilation, when the regulation of light has a problem
• Adie's pupil


  Always dilates, problem in parasympathetic CN # 3 (oculomotor), when they go out in well-lit places because of the dilation there would be sensitivity of light (photophobia). Cannot be operated because there's no innervation in the iris
• 3 Synkinetic action (3 C's)

  1. Convergence of the eyeball

  2. Contraction of ciliary muscle

  3. Constriction of pupil
• Accommodation


  1. Convergence of the eyeball to focus the eyes in a certain angle to have the right and left vision fuse, binocular vision

  2. Contraction of the ciliary muscle to activate accommodation, the zonular fibers will relax, increasing the lens curvature, resulting to accommodation

  3. Constriction of the pupil, the pupil constrict because of the nerve innervation or parasympathetic portion CN #3 (oculomotor nerve) is the stimulator of ciliary muscle and iris
• Mechanism of accommodation


  Contraction of the ciliary muscle, and relaxation of the zonules that attach the ciliary body to the lens; as a result, the lens thickens and becomes more steeply curved increasing the refractive power of the eye to focus the image exactly at the retina
• Accommodative insufficiency

  Occurs when the amplitude of accommodation (AA) is lower than the expected age of the patient. Not attributed to hardening of the crystalline lens (presbyopia). The patient normally experienced poor accommodative sustaining ability. Amount of accommodation is not enough to compensate the reading distance because the patient has insufficiency
• Donders table


  Estimate basis of the power of crystalline lens. Gross estimation of amplitude accommodation depending on the age. Young people has a stronger accommodation (flexible crystalline lens), however, as we age the accommodation weakens (70 -75, zero accommodative amplitude)
• Ill-sustained accommodation

  Condition in which the AA is normal (align with age), but fatigue occurs with repeated accommodative stimulation
• Accommodative infacility

  Also known as accommodative inertia. Occurs when the accommodative system is slow in making a change, or when there is a considerate lag between stimulus to accommodation and the accommodative response. The patient normally experienced blurred distance vision after doing near work
• Accommodative flippers test

  Flipping plus and minus lenses to see if the lens can compensate. Minus lenses will stimulate accommodation and plus lenses the accommodation will relax, timing the period of time of enough accommodation after the state of relaxation and stimulation
• Paralysis of accommodation

  Simple means the inability of ciliary muscled to change the shape of the lens to focus the image at far and near distance. The condition may be unilateral or bilateral associated with fixed dilated pupil
• Tonic accommodation


  Rare condition and is defined as a total or complete loss of accommodation, either in one or both eyes wherein the accommodative system fails to respond to any stimulus. Paralysis of accommodation rarely occurs on its own
• Causes of tonic accommodation

  • Trauma
  • Toxicity
  • Adie's pupil
  • Neuropathy
  • Drugs like cycloplegic (tropicamide and cyclopentolate are Diagnostic Pharmaceutical Agent or DPA are used to diagnose this type of disorder – paralysis disorder) such as atropine and homatropine
• Spasm of accommodation

  Also called as accommodative excess. Refers to prolonged contraction of ciliary muscle, cannot relax, even when looking at distance. Common in children
• Accommodative esotropia

  Manifested in children, both eyes are converging, and the accommodation is super stimulated even at far. Most commonly causing pseudomyopia to varying degrees in both eyes by keeping the lens in a state of short sightedness
• Fogging technique

  Relaxation of accommodation using plus lenses, to accurately measure the grade of the patient