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Chapter 73. Ophthalmology

Manuel Corripio, MD

  • Regulation of intraocular pressure (IOP). Normal intraocular pressure: 12–20 mm Hg
  • Clinical pearl: Pressure from a tightly fitted mask, improper prone position or retrobulbar hemorrhage can lead to ↑ IOP
  • Mechanisms for the decrease of IOP: a drop in BP reduces choroidal volume, relaxation of the extraocular muscles (EOM) lowers wall tension, and papillary constriction facilitates aqueous outflow
  • Clinical pearl: Succinylcholine ↑ IOP by 5–10 mm Hg for 5–10 minutes, through prolonged contracture of the EOM. This may cause:
    • Extrusion of ocular contents through an open surgical or traumatic wound
    • Abnormal forced duction test for 20 minutes (test used in strabismus surgery)
  • The oculocardiac reflex:
    • Pathway: Afferent trigeminal nerve (V1)/Efferent vagus nerve (X)
    • Most common scenarios: pediatric patients, strabismus surgery
    • Symptoms/Signs: Cardiac dysrhythmias ranging from bradycardia to ventricular ectopy to sinus arrest. In awake patients, somnolence and nausea
    • Prevention:
      • Anticholinergic medication. IV is more effective than IM
      • Retrobulbar block and/or deep inhalation anesthesia may also help
      • Local infiltration by surgeon
    • Management:
      • Immediately notification of surgeon and temporary cessation of surgical stimulus until hemodynamic stability
      • Confirm adequate ventilation, oxygenation, and depth of anesthesia
      • IV atropine (10 mcg/kg) if hemodynamic instability
      • If more than one episode, infiltration of the rectus muscle with local anesthetic
  • Intraocular gas expansion:
    • The surgeon may inject air or gas into the posterior chamber to flatten a detached retina and allow better healing
    • The air bubble is usually absorbed in 5 days by diffusion
    • If the patient is breathing nitrous oxide (N2O) the bubble will increase in size → ↑IOP
    • N2O is 35 times more soluble than nitrogen (major component in air) in blood
    • Sulfur hexafluoride (SF6) is an inert gas that is less soluble in blood than nitrogen, and less soluble than N2O. It has a duration of action of 10 days
    • Bubble size doubles within 24 hours after injection because nitrogen from air enters the bubble more rapidly than the SF6 diffuses into the bloodstream
    • Unless high volumes of SF6 are injected, the slow bubble expansion will not increase IOP
    • However, if a patient is breathing N2O, the bubble will rapidly increase in size and ↑ IOP
    • N2O should be stopped at least 15 minutes before the injection of air and should be avoided until at least 5 days after the injection of air and 10 days after the injection of SF6
  • Systemic side effects of ophthalmic drugs:

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DrugMechanism of actionEffect
AcetylcholineCholinergic agonist (miosis)Bronchospasm, bradycardia, hypotension
AcetazolamideCarbonic anhydrase inhibitor (decreases IOP)Diuresis, hypokalemic metabolic acidosis
AtropineAnticholinergic (mydriasis)Central anticholinergic syndrome
CyclopentolateAnticholinergic (mydriasis)Disorientation, psychosis, convulsions
EchothiophateCholinesterase inhibitor (miosis, decreases IOP)Prolongation of succinylcholine and mivacurium paralysis, bronchospasm
EpinephrineSympathetic agonist (mydriasis, decreases IOP)Hypertension, bradycardia, tachycardia, headache
Phenylephrineα-Adrenergic agonist (mydriasis, vasoconstriction)Hypertension, tachycardia, dysrhythmias
ScopolamineAnticholinergic (mydriasis, vasoconstriction)Central anticholinergic syndrome
Timololβ-Adrenergic blocking agent (decreases IOP)Bradycardia, ...

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