National Board Ex- 6 Book .pmd - National Board Of Examination

Anesthesia for Ophthalmic SurgeryRenu SinhaDr Rajendra Prasad Centre for Ophthalmic Sciences, New Delhi5ReviewArticleSkillful anaesthetic managementis integral to optimaloutcomes after ophthalmicsurgery. Ophthalmic patients areoften at the extremes of age(from sick premature newborn tofrail elderly) and not uncommonlyhave extensive associatedsystemic or metabolic diseases.The knowledge of eye anatomy,mechanism of oculocardiacreflex, physiology of intraocularpressure (IOP) and effects ofanaesthetics on IOP, systemiceffects of the drugs used topicallyfor eye disease and effect ofsurgical manipulation isimportant for proper anaestheticmanagement. Patient selection,preoperative evaluation,preparation, monitoring, sedationand local anaesthesia techniquesare also important for ophthalmicsurgery especially in elderlypatient.Intraocular pressure dynamicsThe pressure within the eyeball isintraocular pressure (IOP) and isnormally in the range of 10-21mm Hg. The diurnal variation fornormal eyes is between 3 and 6mmHg. The aqueous humor isproduced by ciliary body. Then itflows through the posteriorchamber between lens and the iris,through pupil, and out intoanterior chamber. In the posteriorpart of the anterior chamber, iteventually comes to the angleformed by the juncture of the irisand the cornea. The aqueous isthen filtered through trabecularmeshwork and enters in the canalof Schlemm. Then it passesthrough collector channels intothe episcleral veins, where it mixeswith the blood.Figure1. Anatomy of the anterioreye.Blinking increases IOP by5mmHg and squeezing increasesIOP by 26mmHg. In the openglobe i.e. during surgery ortraumatic perforation, IOP isequal to atmospheric pressure. Arise in the IOP will tend todecrease intraocular volume bycausing drainage of aqueous orextrusion of vitreous through thewound, which can lead topermanent visual loss. IOPshould be controlled before,during and after the ophthalmicsurgeries.The most important influenceson IOP are movement ofaqueous humor, changes inchoroidal blood volume, centralvenous pressure and extraocularmuscle tone.Factors increasing IOPl Obstruction to aqueoushumor outflow by the use ofmydriatic drugs in shallowanterior chamberl External pressure on the eyefrom a tightly fitted face maskl Raised venous pressure i.e. bycoughing, straining, vomiting,valsalva maneuversl Increase in choroidal bloodvolume i.e. Raised arterialpressure, respiratory acidosis,hypoxia and hypercarbia(vasodilation of intraocularblood vessels)l Rise in the content of thesphere i.e. retrobulbarhemorrhage, injection of alarge volume of localanaesthetic.l Decrease in the size of theglobe without a proportionalchange in the volume of itscontents.l Prone position.l Suxamethonium - the precisemechanism is unknown butmay be due to contraction ofextraocular muscles duringfasiculation or dilation ofblood vessels. The effect ismaximal at 2-4 minutesreturning to normal within 7minutes.l Ketaminel Laryngoscopy and endotrachealintubation increaseIOP significantly i.e. at least10- 20mmHg, this increasemay be due to rise in thearterial pressure.Factors lowering IOPl Reduced venous pressure i.e.Head up tilt.l Lowered arterial pressure - atsystolic pressures