Anesthesia for ophthalmic surgery can be said to have had its origin when Karl Koller, an Austrian Ophthalmologist used cocaine to deaden sensation in the cornea. The anesthetic drugs and techniques available in the early period of anesthesia were not very satisfactory from ophthalmologic view point. Straining associated with postoperative nausea and vomiting and airway compromise were rampant complications of general anesthesia which sometimes were severe enough to undo the benefits of the well executed surgical procedure. However, modern anesthetic drugs and techniques have introduced an element of safety and are considerably free from complications that hinder postsurgical well-being.
Ophthalmic surgery has witnessed an explosive deve-lopment in its scope with the introduction of laser and other modern electronic gadgets; the range of patients who are subjected to this type of surgery encompasses all age groups – from the very young to the very old. This has implications for the anesthetist who must be proficient in the knowledge of factors concerning ophthalmic surgery as well as the myriad medical conditions which affect the conduct of anesthesia as well as recovery and safety.
It is always difficult to decide the age at which local anesthesia can be preferred over general anesthesia. Atkinson suggested that all patients younger than 10 years undergo general anesthesia and patients older than 65 years receive local anesthesia because of their increased medical risk due to coexisting medical conditions. Von Noorden indicated that apprehensive or nervous patients and those undergoing re-do surgery, surgery on the inferior rectus muscle as a result of thyroid disease and surgery on the muscles of both eyes should have general anesthesia. However, no hard and fast rule can be laid down and the decision has to be made individually based on 5the condition of each case. As a general rule, long procedures, extensive or destructive procedures like evisceration or enucleation should be done on general anesthesia. The safety and comfort of the patient are paramount and the anesthetist's/surgeon's personal preferences should not stand in the way of choosing an appropriate technique for the patient.
PREOPERATIVE ANESTHETIC EVALUATION
Every surgery, however trivial, should be preceded by an anesthetic evaluation in order to identify medical problems that may compromise the outcome and also anesthetic problems which may interfere with the conduct of anesthesia. Parents of the pediatric patients are generally very anxious since the vision and future of their offspring hang in a balance. The opportunity to interact with them is well spent in explaining the risks associated with the procedure and anesthesia, if there are any. It is always prudent to give a moderate and realistic picture rather than a rosy and vivid scenario to avoid disappointment later if unexpected developments occur.
Presence of clear respiratory system is mandatory for smooth induction and maintenance of anesthesia. In the case of children with running nose, purulent discharge, productive cough and fever, it is better to defer anesthesia. However, the child with running nose due to incessant crying in the unfamiliar hospital environment can be taken for anesthesia with due caution. If the patient gives the history of infectious diseases like Chickenpox it is better to take up the patient for surgery after 3 weeks.
Medications for asthma, seizure, and other comorbid conditions should be continued perioperatively unless advice to the contrary is given by the anesthetist.6
Juvenile diabetes patients present special problems. Insulin therapy is the rule with blood sugar levels taken as control. It is important to avoid hypoglycemia as well as ketoacidosis. Prolonged fasting is to be avoided and generally these patients are taken early in the list.
Congenital anomalies could be multiple and very often strabismus and Down's syndrome may coexist. Down's syndrome is associated with increased incidence of C1-C2 instability. If detected, excessive extension of the neck should be avoided, fiberoptic laryngoscopes and laryngeal mask airway should be ready for intubation. Cerebral palsy children are prone to recurrent seizures and since they often have increased incidence of gastroesophageal reflux, longer preoperative starvation is recommended for these patients. Craniofacial anomalies and airway abnormalities may also be present and special gadgets may be necessary for airway management.
Preoperative Laboratory Test
Minimum investigations in normal healthy children include baseline hemoglobin and hematocrit values to rule out occult anemia. Urine is examined for sugar. All asthmatic patients should have a preoperative chest X-ray.
Further investigations may be required if coexisting disease is present. This may include a cardiological evaluation in children suffering from congenital heart disease. In juvenile diabetics, HbA1c level estimation gives a better idea of glycemic control than random blood sugar estimation. Preoperative blood sugar and urine for acetone are done on the day of surgery.
STARVATION AND PREMEDICATION
Preoperative starvation is mandatory for all patients undergoing surgery under general anesthesia. It is safe to give clear fluids up to 3 hours before induction, and this helps increase the child's comfort. It is mandatory to withhold solids for 6 hours before surgery. The importance of preoperative starvation and the sequence of anesthesia are explained to the parents and an informed consent is taken from them before surgery.
Anesthesia often begins with the administration of a sedative/hypnotic/narcotic drugs as premedication. Children presenting for surgery are found to be crying and struggling when separated from parents. Premedication paves the way for smooth transfer of the child to the operating room and also ensures smooth induction.
Nowadays, with the shift towards day-care-procedures, premedication for pediatric patients is often omitted.
A vagolytic component is generally added to the sedative which ensures reduction of salivary secretions and prevents intraoperative oculocardiac reflex.
Inj. atropine (0.01 mg/kg) or glycopyrrolate (0.005 mg/kg) with Inj. midazolam (0.05 mg/kg) is given 30 mt before surgery. Oral midazolam in the dose of 0.5 to 0.75 mg/kg in flavored and palatable liquid is a better alternative to injectable premedication in anxious patients. The intravenous anticholinergic given just before induction as an alternative to intramuscular injection, is equally effective in preventing oculocardiac reflex.
CHOICE OF ANESTHESIA
- General anesthesia
- Local anesthesia
General Anesthesia
Majority of the patients who undergo squint surgery are children, although occasionally adults also may present for cosmetic correction. The surgery is usually carried out with general anesthesia because of the age of the patient. Good anesthesia is achieved with soft globe devoid of vascular congestion. Normal intraocular pressure (IOP) ranges from 10 – 20 mm Hg. Most anesthetic agents will decrease this. Table 1.1 describes the effects of commonly used anesthetic agents on IOP.
The choice of induction technique is either inhalational or intravenous. Intravenous induction performed with fentanyl combined with propofol or thiopentone is common. The inhalational induction with sevoflurane is an alternative technique especially in younger children. The airway 9is best managed by intubation with paralysis and controlled ventilation. Access to the airway will be restricted during the surgery so it is important to secure the tracheal tube firmly. A preformed RAE tubes or a reinforced flexible tracheal tubes are preferable. Nowadays the reinforced LMA (Laryngeal Mask Airway) are used for most of the eye procedures. The advantages are reduced coughing at the end of the surgery and controlled ventilation with the use of muscle relaxants.
The nondepolarizing agents such as vecuronium are normally preferred over suxamethonium for two reasons. Firstly, patients who have been given suxamethonium have a prolonged increase in the extraocular muscle tone, which interferes with the FDT (Foreced Duction Test). This effect of suxamethonium lasts roughly 15–20 minutes Secondly, patients undergoing correction of strabismus may be at increased risk of developing malignant hyperthermia. Anesthesia is usually maintained with oxygen, N2O and Sevoflurane/Propofol. As with induction, the choice of maintenance technique rests largely on the preferences of the anesthetist. Where halothane is used there is an increased risk of dysrhythmias, particularly where eye preparations containing adrenaline are used, and in the presence of hypercapnia; isoflurane or sevoflurane may be preferable.
Propofol has advantages in reducing the risk of postoperative nausea and vomiting (PONV) since propofol has antiemetic effects. At the end of surgery, the nondepolarizing muscle relaxant effect is reversed with neostigmine and glycopyrrolate. The use of glycopyrrolate is associated with a more stable cardiovascular system, fewer arrhythmias and superior control of oropharyngeal secretions at the time of reversal.10
Anesthetic Challenges in Squint Surgery
Intraocular pressure: Anesthetic maneuvers like laryngo-scopy and intubation, straining and coughing during induction and bucking on the tube by inadequately paralyzed patient will have the effect of causing an increase in the intraocular pressure. Attenuation of this effect is essential to obtain a soft globe. This effect may be attenuated by a dose of lidocaine 1 mg/kg 3 minutes prior to intubation or extubation. Use of the LMA permits smoother induction and emergence from anesthesia and has much less effect on IOP.
Unobstructed ventilation and lowering the PaCO2 by moderate hyperventilation during anesthesia and a slight head up tilt ensure reduction in intraocular pressure and prevent venous congestion of the globe.
Hypoxia and hypercapnia both increase IOP and should be scrupulously avoided.
Oculocardiac reflex: Initially, described in 1908 by Bernard Aschner and Giuseppe Dagnini, the oculocardiac reflex is elicited by the pressure on the globe and by traction on the conjunctiva, orbital structures and extraocular muscles, especially the medial rectus. It is a trigeminovagal reflex. It is characterized by sinus bradycardia, nodal rhythm, ectopic beats or sinus arrest. Afferent pathway is via the long and short ciliary nerves to the ciliary ganglion terminating in trigeminal sensory nucleus in the floor of the 4th ventricle. Efferent pathway is from the motor nucleus of vagus nerve via cardiac depressor nerve of 10th cranial nerve, which ends in myocardium. Pressure, torsion or pulling of extraocular muscle may elicit this reflex. Successive provocations decrease the reflex sensitivity.
Intraoperative management depends upon the severity of the reflex. The importance lies in its early recognition. 11On realizing this event the surgeon should relax the muscle hook immediately. The bradycardia resolves almost immediately after the stimulus has been removed. If it persists the patient should be given intravenous atropine (0.15 mg/kg). In cases where the response to this treatment is poor, a retrobulbar lignocaine is recommended to block the afferent loop. Sevoflurane is less likely to provoke the reflex than halothane; it is also less likely with deep anesthesia compared to light anesthesia. The incidence of significant bradycardia is doubled if the carbon dioxide level is high, so controlled ventilation should be preferred over spontaneous breathing.
Nausea and vomiting: PONV is most common after squint surgery. The incidence of PONV after strabismus surgery varies from 46 to 88%. Most children have some pain after eye surgery and should be given analgesics without an opioid as it induces emesis. Prophylactic antiemetic in the combination of Inj. ondansetron 0.15 mg/kg with Inj. dexamethasone 100 ug/kg may be given along with premedication.
Postoperative pain: Even though most of the eye procedures have mild to moderate pain, squint surgeries have moderate pain which require stronger intraoperative and postoperative analgesics. These include paracetamol, NSAID, intravenous fentanyl, and peribulbar or sub-Tenon's block before emergence from the anesthesia.
Others: Extubation should be considered in the deeper plane to avoid coughing and bucking in pediatric patients.
Malignant hyperthermia a rare complication is sometimes associated with strabismus patients; it is commonly triggered by inhalational agents such as halothane and succinylcholine.12
Local Anesthesia
Local anesthesia has become popular because of the in-creased use of adjustable sutures and the shift to day care procedures. Lignocaine 2 to 4% and Bupivacaine 0.25 to 0.75% are the commonly used anesthetic agents. It is often mixed with Epinephrine, 1:100,000 (or) Hyaluronidase. The choice depends on the anticipated duration of surgery. Adverse reaction can happen when the drug is accidentally injected into intravascular compartment, dural spread through the optic nerve sheath and drug sensitivity. Methods aimed at reducing these complications include proper techniques with careful positioning of the needle, aspiration before every injection and the use of a test dose. Direct injection into muscle can cause muscle necrosis. The inferior oblique, inferior rectus and medial rectus are most frequently involved. This can occur in sub-Tenon's injection where the local anesthetic pools around the muscle. The risk is reduced by the addition of hyaluronidase.
The choices of local anesthetic techniques are:
- Retrobulbar anesthesia
- Peribulbar anesthesia
- Sub-Tenon's anesthesia
- Topical anesthesia
Retrobulbar Anesthesia
The technique involves instillation of anesthetic solution into the intraconal space. A 1.5 inch, 25 gauge needle is used. Blunt tipped needles such as the Atkinson's needle are preferred as the risk of globe perforation is minimal. After skin preparation, the inferior orbital margin is palpated to identify the junction between the outer 1/3rd and inner 2/3rd. The patient is asked to gaze straight ahead. This keeps the optic nerve out of Harm's way. It should be 13noted that Bell's phenomenon will bring the optic nerve and the globe directly in the path of the needle and therefore, it is important to explain the procedure to the patient. The skin is entered with the needle parallel to the orbital floor for approximately 1 cm (Fig. 1.1). It is then directed medially towards the orbital apex as it advances posteriorly. A resistance is felt as the muscle cone is entered, more so, if a blunt needle is used. This is a subtle feeling and needs considerable experience to identify. The injection is now given slowly.
Usually 2–4 ml of solution is used. The needle is then withdrawn slowly and pressure is given on the globe over closed lids.
This technique achieves deep orbital anesthesia and akinesia and blocks the potential oculocardiac reflex with a minimal amount of anesthetic agent. The block effect is achieved in approximately 5 minutes. However, adjustment of sutures cannot be done at the earliest as the return of full extraocular motility takes long time, and this technique has increased the incidence of associated morbidity.
Peribulbar Anesthesia
Here the anesthetic solution is injected in the extraconal space. A 0.5 inch, 26 gauge disposable needle is preferred. More volume of anesthetic is needed and this technique is considered less effective than retrobulbar anesthesia. The injections are given at two sites. The first injection is given in the inferotemporal quadrant at the junction of the lateral 1/3rd and the medial 2/3rds. The needle is inserted parallel to the infraorbital margin along its entire length and may be canted upwards once the equator of the globe is crossed. No attempt is made to enter the muscle cone however. Gentle sideways movement may be done to identify engagement of the globe if any. Approximately 4–5 ml of solution is injected. The second injection is given in the superonasal quadrant at the junction of the lateral 2/3rd and medial 1/3rds. Again gentle sideways movement may be done to identify any accidental perforation. Once the needle is withdrawn, gentle pressure is given over the closed lids. The complications associated with this technique are less; but the risk of globe perforation exists, however it is less than with retrobulbar anesthesia. The block effect takes little longer than retrobulbar anesthesia, approximately 10 minutes. Pupillary dilatation indicates that adequate 15anesthesia has been achieved, as the drug has diffused to the ciliary ganglion.
Sub-Tenon's Anesthesia
Sub-Tenon's anesthetic technique is safe, when compared to the other anesthetic techniques for strabismus surgery performed under local anesthesia (Fig. 1.2). The optic nerve function is not altered, which helps rapid visual recovery for the patient and earlier adjustment of adjustable sutures. The operating eye is prepared and draped, topical Proparacaine 0.5% is applied on the conjunctiva. The conjunctival and Tenon's incisions are made; these incisions can be the same for the intended strabismus surgery.
Local anesthetic is injected into the sub-Tenon's space through an irrigation syringe. The onset of block is as fast as retrobulbar anesthesia, and the block is usually performed in the course of surgery.
Topical Anesthesia
Strabismus surgery may be performed in select adult patients under topical anesthesia alone. Success depends on the experience of the surgeon and careful patient counseling and selection. This technique offers considerable advantage to the surgeon as single stage adjustment can be done right after the conclusion of the surgery. In addition, this technique obviates all the complications described associated with local anesthesia.
Various drugs including 4% lignocaine, amethocaine, and proparacaine have been used. Lignocaine Jelly 2% has been reported to give superior analgesia. The lignocaine jelly is applied to fill the fornices at least 20 minutes prior to the procedure. The application is repeated prior to the commencement. The patient is asked to gaze away from the muscle being operated to gain maximum view. It is important to avoid excessive traction on the muscle throughout the procedure as this can cause severe pain. The services of an experienced assistant will be invaluable at this juncture. It is better to avoid operating on the obliques and the superior rectus with topical anesthesia. It is also better to use complete regional/general anesthesia for resurgeries and complicated strabismus procedures. The surgery otherwise proceeds as it normally would. Should the patient report pain at any point? the surgeon should not hesitate to supplement with topical proparacaine/sub-Tenon's lignocaine. It is important to remember that excessive supplementation can cause 17corneal toxicity and alter the contractility of the muscle which may require the adjustment procedure to be delayed. The jelly is washed with saline at the end of the procedure. Supplementation of oral analgesics to the patients at the conclusion of the surgery is often helpful.
It is possible to augment topical anesthesia with the aid of sub-Tenon's instillation of anesthetic solution or with intravenous sedation. In monitored sedation, patient is given Inj. Fentanyl 1microgram/kg with the continuous infusion of Inj. Propofol at the dose of 6 to 8 mg/kg/hr through the infusion pump. The patient is monitored with pulse oxymeter and blood pressure.
Monitored sedation is not a substitute for good patient counseling/selection. It is also not intended to replace general/topical anesthesia and only serves to make the surgeon's job a little easier. The speedy recovery afforded by the newer sedatives, facilitate early adjustment. The patient should be made to understand that he/she will be given mild sedation and that his/her cooperation is crucial to the success of the procedure.
CONCLUSION
Eye is a delicate and sensitive organ. Consequently, any ophthalmic intervention including anesthesia has to be very refined and skilfully administered. Faulty maneuvers during anesthesia may negate the benefits expected from surgery.
Careful attention to IOP and vascularity is essential; smooth extubation without coughing or straining is of paramount importance. Complete elimination/considerable attenuation of PONV is strongly indicated and the anesthetic technique and drugs administered should facilitate the achievement of this ideal. Adequate pain relief with drugs which do not induce PONV is indicated.18
In short, ophthalmic anesthesia should not only enable eventless surgery but also should aid and facilitate quick functional recovery postoperatively.
BIBLIOGRAPHY
- Anesthesia for correction of strabismus – issue 17(2003) Article 9;P 1.
- Anesthesia for pediatric eye surgery-Anesthesia tutorial of the week 144–2009- Dr. Grant Stuart, Great Ormond Street Hospital, London, UK.
- Greenbaum Ocular Anesthesia—Anesthesia for Eye Muscle Surgery; pp 125–150.
- Ophthalmology Clinics of North America 2006;19(2):269–78.
- Wylie and Churchill-Davidson's “A Practice of Anesthesia”, 6th edn, pp 1265–81.