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Miosis

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Miosis, Pupil Constriction, Consensual Light Reaction, Near Reaction, Visual Accommodation

  • Physiology
  • Pupil Constriction as a normal response
  1. Parasympathetic stimulation
    1. Parasympathetic fibers course with CN 3
    2. Parasympathetic fibers originate in Edinger-Westphal Nucleus and transmit to ciliary Ganglion
  2. Bright light results in bilateral Pupil Constriction
    1. Direct reaction to light in exposed eye
      1. Light strikes Retina of affected eye
      2. Signal passed via Optic Nerve to Midbrain
      3. CN 3 associated parasympathetic fibers signal same iris to constrict
    2. Consensual Light Reaction in opposite eye
      1. Midbrain receives signal from exposed eye
      2. CN 3 associated parasympathetic fibers signal opposite iris to constrict
  3. Accommodation
    1. Visual focusing
      1. Near Reaction (e.g. Reading)
        1. Also results in eye convergence (under CN 3 control as well)
    2. Pathway
      1. Ciliary body smooth muscle contracts
      2. Lens changes shape (more convex)
      3. Pupil constricts during accommodation to aid focusing
      4. Both ciliary body contraction and Pupil Constriction are mediated by CN 3 related parasympathetic fibers
      5. Visual cortex controls accommodation as part of a reflex loop based on Optic Nerve signals
  • Causes
  • Abnormal Miosis
  1. Horner's Syndrome
  2. Mnemonic: COPS
    1. Cholinergic Toxicity
    2. Clonidine
    3. Opioids (e.g. Heroin, Morphine)
    4. Organophosphate Toxicity
    5. Phenothiazines
    6. Pilocarpine
    7. Pontine bleed or lesion
    8. Sedative-Hypnotics (e.g. Benzodiazepines)
  • Causes
  • Other Abnormalities
  1. Syphilitic Pupil (Argyll-Robertson Pupil, Prostitute's Pupil)
    1. Pupil constricts during accommodation, but does not constrict to light