ANAESTHESIA FOR STRABISMUS SURGERY.

One of the commonly performed ophthalmic surgery in the pediatric age group.The surgical procedure is simple and short but often associated with unexpected peri operative complications, but most of them can be successfully managed by immediate intervention by a vigilant anaesthetist.The anaesthetic concerns include controversial use of suxamethonium for induction, difficult airway, high incidence of post operative nausea and vomiting, systemic effect of topical medications,other associated congenital mal formations, oculocardiac reflex, need for post op analgesia and propensity for malignant hyperpyrexia.

Suxamethonium:  Suxamethonium if used for induction can cause sustained contracture of intraocular muscles and this may affect the forced duction test performed by surgeons during surgery to estimate the amount of restriction in movement of extraocular muscles. Hence to ensure patient immobility during surgery non depolarising agents like atracurium, vecuronium or rocuronium are considered. But because of its faster action,effective relaxation of laryngeal muscles and that raise in intra ocular pressure is not very significant in strabismus surgery,some anaesthetists still prefer[1] suxamethonium for induction.The use of suxamethonium also helps the surgeon, as the globe adducts exposing a large bare area of sclera allowing for easy re insertion of cut lateral rectus[2]
Incidence of Difficult Airway: Patients with squint may have associated congenital malformations like Down's syndrome,Marfans syndrome or Muscular dystrophies, with involvement of airway. Careful assessmemt of airway is mandatory before planning anaesthetic management.
Effect of medications placed on eye:  Eye drops are readily absorbed through hyperemic, incised conjunctiva causing systemic effects Phenylephrine is placed in the eye to produce mydriasis and haemostasis, however absorption of the phenylephrine can cause profound systemic vasoconstriction and hypertension.It can also cause arrhythmia and head ache. To prevent systemic hypertension only 1 to 2% phenylephrine should be used and only one drop should be put into each eye.Adrenaline(2%) cause hypertension & arrhythmias Timolol (B-blocker) causes bradycardia, hypotension & exacerbation of asthma  Phospoline iodide(echothiophate iodide) is a long acting anti-cholinesterase used in glaucoma prolongs suxamethonium induced muscle relaxation.A patient who has been treated with echothiophate iodide can retain low blood levels of pseudocholinesterase for weeks or even months after discontinuing the medicine.Thus the use of suxamethonium is contraindicated when phospholine iodide is used for fear of post op apnea.[3]systemic effects of cyclopentolate hydrochloride include disorientation dysarthria and seizures.
Oculocardiac reflex: Bernard Ashner and Guiseppe Dagnini first described this reflex in 1908.This reflex is a trigemino vagal reflex and is triggered by pressure on the globe or by traction on the extraocular muscles. The reflex is also triggered by ocular trauma, retrobulbar block, severe pain or by orbital compression due to hematoma or edema. the afferent impulse travels via the long and short ciliary nerves to the ciliary ganglion, then continues to the gasserian ganglion along the ophthalmological division of the trigeminal nerve and terminates at the main trigeminal sensory nucleus in the floor of the fourth ventricle. The efferent impulse travels by way of of the nucleus of the vagus nerve to the vagal cardiac depressor nerve, producing negative inotropic and conduction effects consisting of bradycardia, nodal rhythm, ectopic beats, ventricular fibrillation and rarely asystole.The surgeon should be informed immediately when this arrhythmia develops, to remove pressure or traction on the globe.

The treatment of oculocardiac reflex should be immediate and with intravenous injection of atropine at doses of 0.005 to 0.4 mg/kg. Anticholinergic premedication helps to prevent occurence of this reflex.Recurrent episodes require local infiltration of lignocaine near the extrinsic muscles.IV Epinephrine 6-12 mg  is used for patients with cardiovascular collapse. IV Lidocaine 1.5-2 mg/kg is given for Ventricular arrhythmias along with cardiac massage. Retrobulbar block with 1 to 3 ml of 1% or 2% lidocaine (Xylocaine) may prevent oculo cardiac reflex by blocking the afferent limb of trigeminal vagal reflex.
Malignant hyperpyrexia: Patients with strabismus have a higher incidence of malignant hyperthermia.In cases where susceptibility to malignant hyperpyrexia or a family history is suspected pre-treatment with dantrolene is required[3]. Study of phenylketopyruvate serum levels can be useful in predicting susceptibility to malignant hyperthermia in a patient with a questionable family history[3].The triggering agents include suxamethonium and halothane and hence the preferred general anesthetic regimen for patients with  susceptibility to malignant hyperthermia is propofol,fentanyl, nitrous oxide, and a non depolarising muscle relaxant.Classicaly malignant hyperthermia occurs intraoperatively and results in rapid rise in temperature, muscle rigidity, dysrrythmias, rhabdomyolysis, acidosis and hyperkalemia. Approximately, one half of patients who develop muscle rigidity after succinylcholine are susceptible to malignant hyperthermia by the muscle biopsy and contracture test. In these patients, if creatinine phosphokinase level is more than 20,000 IU, malignant hyperthermia susceptiblity is strongly suggested. If massester muscle spasm occurs, a muscle biopsy and contacture test is indicated to confirm malignant hyperthermia.[4].The treatment of malignant hyperpyrexia include discontinuation of all anaesthetic agents, 100% oxygen,dantrolene 2.5mg/kg IV,rapid cooling to bring down body temperature correction of acidosis and hyperkalemia, and ventilatory support.
Post operative nausea and vomiting:is very common following strabismus correction. The exact mechanism is not known. It may be secondary to altered visual perception or an oculoemetic reflex, which is analogous to the oculocardiac reflex.It is more common in opioid premedicated patients.Oral midazolam 0.5 mg/kg seems to be a better premedicant for strbismus cases.Intraoperative use of metoclopramide 0.1-0.15mg/kg IV,droperidol 70 mic/kg,ondansetron 0.1mg/kg, and intravenous induction of anaesthesia by propofol etc, helps to reduce the incidence of PONV.[5]
Post operative pain management:is also equally important to reduce pain and discomfort in children.rectal paracetamol or diclofenac suppositories are commonly used for this purpose.Pre operative subtenon's instillation of levobupivacaine is also helpful.
Anaesthetic management:  Strabismus surgery in adult can be performed under local anaesthesia(retrobulbar or peribulbar block) with or without sedation. Adult Un-coperative patient can be managed with total intravenous anaesthetic technique with sedative and narcotic drugs. Children will always require general anaesthesia for corrective surgery. Premedication may be given with oral midazolam 0.5 mg/kg along with atropine 0.02 mg/kg. Inhalational induction with sevoflurane in oxygen and nitrous oxide,fentanyl 1mg/kg IV,rocuronium 1 mg/kg IV or atrcurium 0.5 mg/kg IV, proseal LMA/ETT, controlled ventilation.Intravenous induction is with fentanyl 1mg/kg, propofol 2.5mg/kg,vecuronium/atracurium with nitrous oxide in oxygen and isoflurane.The use of neuromuscular monitoring is strongly advised and ECG monitoring is mandatory. It is essential to maintain normocarbia throughout the procedure.Extubation or removal of LMA attempted in deep plane of anaesthesia.Intraop prophylaxis for PONV with ondansetron or metoclopramide in suggested doses should be administered.
References:
1)J.C Stanley, Hand Book of Clinical anaesthesia, Chrchill livingstone, 1996.
2)D Abrams, British Journal of Ophthalmology,1984,64:218
3)Eugene M. Helveston, M.D.Surgical Management of Strabismus: A practical and updated approach, 5th edition;http://telemedicine.orbis.org/bins/content_page.asp?cid=1-2161
4)Practical case notes;Dr. R.C. Agarwal,Dept. of Anaesthesiology & Critical Care, Bhopal Memorial Hospital & Research Centre, Raisen by-pass Road Karond, Bhopal.
5)Kenneth Davison, Clinical Anaesthesia Procedures of MGH, fifth edition.
Image cortesy: www.nysora.com(subspeciality/ophthal)