Ophthalmic Surgery—The Cutting Edge Sandeep Saxena
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Anesthesia in Ophthalmology1

Shashi K Bhasker,
Sandeep Saxena,
Rajiv Nath
 
INTRODUCTION
In 1776, Priestly discovered anesthetic nitrous oxide and described its effects after inhalation. Horace Wells, a dentist, in Hartford, USA, in 1844, used it for the first time to extract a tooth without pain. Ether, as a general anesthetic, was used in the operating room of Massachusetts General Hospital, Boston USA, by Morton in 1846. Since then, the science of anesthesia has made advances with newer drugs and techniques.
Anesthesia literally means loss of sensations. This may be general anesthesia or local anesthesia. In general anesthesia, all modalities of sensation are lost, particularly pain along with reversible loss of consciousness. Local anesthetics abolish pain sensation in a particular localized area. There is no loss of consciousness in local anesthesia.
 
Features
In ophthalmology, anesthesia should have the following features:
  • Akinesia, anesthesia of globe, adnexa and lids
  • Intraocular pressure (IOP) stability
  • Systemic blood pressure maintenance
  • Patient's relaxation undergoing local anesthesia
  • Absence of any untoward events like oculocardiac reflex and malignant hyperthermia
  • Smooth emergence from anesthesia with no vomiting, blood pressure fluctuations, coughing or respiratory depression
  • Postoperative analgesia.
 
GENERAL ANESTHESIA
It has been said previously that in general anesthesia, all modalities of sensation are lost along with loss of consciousness. Nearly all ophthalmic cases may be managed with local anesthesia.
 
Indications
There are, however, a few indications for general anesthesia. Some of them are:
  • Inability of the patient to cooperate with local anesthesia (e.g. children, adults with mental or psychological deficits, tremor, etc.)
  • Surgery requiring complete ocular akinesia
  • Surgery requiring more than 3 to 4 hours
  • Surgical field not amenable to regional, local, or topical anesthesia
  • Regional block technically difficult or contraindicated (e.g. large myopic globe, coagulopathy, etc.)
  • Following inadvertent intrathecal or intravascular injection of local anesthetic
  • The surgeon's or the patient's preference to general anesthesia.
 
Stages
In 1920, Guedel outlined four stages of general anesthesia. In modern times, these stages are never discerned separately. The stages are:
  1. Stage I: Stage of analgesia.
  2. Stage II: Stage of delirium.
  3. Stage III: Stage of surgical anesthesia:
    • Plane i
    • Plane ii
    • Plane iii
    • Plane iv.
  4. Stage IV: Stage of respiratory paralysis.
 
Stage I
Stage of analgesia. The stage of analgesia starts from the beginning of inhalation till the loss of consciousness, and there is a gradual depression of cortical centers. With more administration of the anesthetic agent, the patient passes to the second stage.
 
Stage II
Stage of delirium. The extent of the stage of delirium is from the loss of consciousness till the beginning of surgical anesthesia. Its associations, like shouting, increased muscular activity, tachypnea, breath-holding, and hyperventilation, are due to loss of control of the higher centers over the lower centers. Struggling, muscle tone increase, retching, and vomiting, which are unwanted features, can be minimized by proper premedication.
 
Stage III
Stage of surgical anesthesia. In the stage of surgical anesthesia, reflex activity is lost, with relaxation of muscles. It is used for surgical procedures and is divided into four planes:4
Plane i In this, pupils are of normal size. The eyeballs are roving. Respiration is thoracoabdominal in character, and is full regular and deep. Corneal sensitivity is present. Skeletal muscles are partially relaxed.
Plane ii The eyeballs are fixed. Respiration is regular with diminished amplitude. Endotracheal intubation is possible due to abolished laryngeal reflex.
Plane iii Thoracic and abdominal respiratory asynchrony starts. Later, after paralysis of intercostal muscles, the respiration becomes abdominal. Corneal sensitivity and pupillary reactions are lost.
Plane iv Muscles are flaccid. Pupils are dilated and light reaction is lost. Blood pressure is low. Secretions progressively reduce from plane i.
 
Stage IV
Stage of respiratory paralysis. In this stage, there is severe depression of vital medullary centers. Respiration is irregular, finally leading to respiratory arrest which is also accompanied by vasomotor collapse and the heart ceases to beat.
 
Technique
The technique can be discussed under the following headings.
 
Premedication
Premedication is used to reduce the anxiety of the patient and to produce some sedation. Injectable or oral medications may be used for it. The most commonly used oral drugs are diazepam and nitrazepam. Synthetic opiate and fentanyl may also be given. Other drugs used are atropine or scopolamine, pentobarbital, mepridine, promethazine in various combinations (Table 1.1)
 
Induction
Induction is done either by inhalation or by intravenous route. Ketamine causes increase in the IOP. Therefore, it is not used for intraocular surgeries. Thiopental sodium has a smooth induction and a transient fall of the IOP. The other drug is etomidate, a derivative of imidazole used for induction. Halothane in nitrous oxide and oxygen is commonly used for inhalation induction.
Table 1.1   Premedication
Sedatives
Opioids
Anticholinergics
Barbiturates
  • Phenobarbital
  • Pentobarbital
  • Thiopental
Natural
  • Morphine sulfate
  • Codeine
  • Pantopon
  • Atropine
  • Scopolamine
  • Glycopyrrolate
Nonbarbiturates
  • Ketamine
  • Diazepam
  • Flurazepam
Tranquilizers
  • Meprobamate
  • Chloropromazine
  • Promethazine
Semisynthetic
  • Hydromorphone
Synthetic
  • Mepridine
  • Pentazocine
  • Sublimaze
 
Endotracheal Intubation
Endotracheal intubation is done to allow access to eye and to maintain clear airway. Controlled ventilation by cuffed endotracheal tube is possible, if desired. Trachea and larynx sprayed with 4 percent lidocaine before intubation reduces coughing or straining. This is helpful for smooth extubation.
 
Maintenance
Spontaneous or controlled ventilations are two techniques used for maintenance of anesthesia. Volatile agents like halothane, enflurane or isoflurane in nitrous oxide and oxygen are used. Halothane and enflurane lower the IOP by depression of midbrain and hypothalamus.1,2
 
Extubation
Pharyngeal suction is done to clear the airway before the endotracheal tube is taken out. Carbon dioxide is administered to resume spontaneous respiration. As the respiration is adequate, extubation is done.
 
Recovery
The patient is turned to the opposite side of the eye operated to avoid accidental compression. Airway is supported till the patient is awake and oxygen administered for a variable period.
 
Special Considerations
 
Diabetes
Diabetes mellitus requires close monitoring of blood sugar before, during and after the surgery. Patients should miss oral hypoglycemics on day of surgery. Patients can resume their diet in the evening or the next morning after anesthesia. So the oral hypoglycemics can be started the next morning. Patients on insulin are managed on 5 percent dextrose in normal saline by an intravenous drip (no regular dose of insulin is administered on day of operation). Regular insulin is given to cover the amount of dextrose administered. The blood samples are withdrawn at regular intervals for monitoring. With the start of oral diet, the regular dose of insulin is started after the surgery. In uncontrolled diabetic patients, the surgery should be done under local anesthesia.
 
Hypertension
Hypertensive patients should take their medications preoperatively. More than minimal dose of induction is given to prevent temporary elevation of blood pressure under light anesthesia, mainly during intubation.35
 
Intraocular Pressure Maintenance
Intraocular pressure maintenance is important as any elevation may be harmful to the vision, especially in glaucomatous patients. Carbondioxide retention and hypoxemia tend to increase the IOP.4 Most of the tranquilizers, narcotics, hypnotics, barbiturates and neuroleptics cause decrease in pressure. Ketamine elevates the pressure by increasing the extraocular muscle tone. Conversely, nondepolarizing agents (like curare, pancuronium) decrease the pressure by lowering the muscle tone. Succinylcholine and decamethonium cause transient rise of IOP. Table 1.2 shows effect of various anesthetic drugs on IOP.
 
Complications
 
Hyperthermia
Hyperthermia is due to alteration in muscle metabolism. It may occur during the anesthesia or within a few hours of administration of anesthesia. Body temperature rises rapidly and may go beyond 44°C (111°F) leading to death if not identified and managed timely. Patients with increased creatine phosphokinase levels and musculoskeletal abnormalities are at greater risk. There is release of calcium within the muscle due to change in metabolism. According to a hypothesis, there is uncoupling of oxidation-phosphorylation leading to self-perpetuating hypermetabolism and hyperthermia. Administration of halothane and succinylcholine may trigger the disturbance. In ophthalmology, strabismus patients are at greater risk for hyperthermia. It is managed by stopping the drug and terminating the surgery as quickly as possible. Oxygenation and hydration with chilled fluids are started. Cardiac arrhythmias, pulmonary edema, respiratory and metabolic acidosis are managed accordingly.
 
Oculocardiac Reflex
Oculocardiac reflex is slowing of heart rate, i.e. bradycardia due to traction on extraocular muscles or pressure on the globe. It may also manifest as bigeminy, ectopic beats, nodal rhythms, atrioventricular block, arrhythmias or periods of asystole. It most commonly occurs during eye muscle surgery.5,6 It also occurs during retinal detachment repair surgery and enucleation. Oculocardiac reflex has also been seen after retrobulbar block and retrobulbar hemorrhage. The pathway of this reflex starts from long and short ciliary nerves. Through the ophthalmic division of trigeminal nerve and trigeminal ganglion, the afferent limb terminates in the main sensory nucleus of trigeminal nerve in the fourth ventricle. The efferent limb is via vagus nerve.7 Management is aimed to target both the limbs and depends on the severity. Continuous monitoring of electrocardiogram (ECG) helps in the diagnosis of oculocardiac reflex. A retrobulbar block is said to block the reflex, but it itself has the risk to induce it.811 No treatment is required if the reflex manifests as bradycardia or infrequent ectopic beats, and the blood pressure remains stable. Cessation of the surgery is indicated if the dysrhythmias become significant. Surgery may be resumed after a brief pause as oculocardiac reflex fatigues easily; and usually, there is little or no activity after a brief pause in surgical stimuli. In severe cases, the oculocardiac reflex patient is managed with anticholinergics (atropine or glycopyrrolate). Caution must be exercised as more severe, prolonged tachydysrhythmias may result with large doses of atropine.12
Table 1.2   Effect of various anesthetic drugs on intraocular pressure
Agent
Dose
Effect on intraocular pressure
I. Unknown or No effect
Inhalation
Nitrous oxide
70%
No effect
Intramuscular
Merperidine
50–100 mg
May increase, normally no effect
Atropine
0.4–1.0 mg
No effect
Scopolamine
0.4 mg
No effect
Intravenous
Alfentanil
5 mcg/kg
No effect
Atracurium
0.4–0.5 mg/kg
No effect
Flumazenil
0.0025 mg/kg
No effect
Glycopyrrolate
0.2–0.4 mg
No effect
Remifentanil
0.5 mcg/kg
No effect
Vecuronium
0.08–0.1 mg/kg
No effect
II. Increase Intraocular Pressure
Intramuscular
Ketamine
5 mg/kg
Slight increase
Intravenous
Succinylcholine
1–2 mg/kg
18% increase
Ketamine
1–2 mg/kg
Increased
III. Decrease Intraocular Pressure
Inhalation
Halothane
1 MAC
14%-33% decrease
Desflurane
6–12%
30% decrease
Enflurane
1% with N2O
35–40% decrease
Isoflurane
1–3%
40% decrease
Sevoflurane
1–3% with N2O
40% decrease
Intramuscular
Fentanyl
50–100 mcg
20% decrease
Chlorpromazine
10–25 mg
20–30% decrease
Morphine
8–15 mg
Decrease
Intravenous
Droperidol
5–10 mg
12% decrease
Haloperidol
0.5 mg
15% decrease
Midazolam
0.15 mg/kg
25% decrease
Etomidate
0.3 mg/kg
30% decrease
Pentothal
3–5 mg/kg
30% decrease
Thiopentone
2.5 mg/kg
30% decrease
Dexmedetomidine
1 mcg/kg
40% decreased
Diazepam
10 mg
Decrease
Dilaudid
1–2 mg
Decrease
Lidocaine
1.5 mg/kg
Decrease
Propofol
1–2 mg/kg
Decrease
Methohexital
6 mg/kg
Decrease
Sufentanil
1–2 mcg/kg
Decrease
Thiamylal
4–5 mg/kg
Decrease
Curare
0.5–0.6 mg/kg
Slight decrease
Metocurine
0.3–0.4 mg/kg
Slight decrease
Metocurine + Pancuronium
0.4–0.5 mg/kg
Slight decrease
Pancuronium
0.05 mg/kg
Slight decrease
mcg = micrograms MAC = monitored anesthesia care
6
 
LOCAL ANESTHESIA
Local anesthesia can be given topically, intraocularly, or by orbital injection. The relative potencies and recommended maximum dosages of the various agents should be known.
 
Topical Anesthesia
Topical anesthesia can be achieved by instilling any of the available topical agents like tetracaine (0.5%) and proparacaine (0.5%). Injectable drugs such as bupivacaine (0.75%), carbocaine (4%), and lidocaine (1.0–4.0%) can also be used topically. Lidocaine with pH adjusted to 7.2 with sodium bicarbonate achieves a higher anterior chamber concentration.13 The effect of most agents is from 20 to 40 minutes. These drugs may cause superficial punctate keratopathy of varying degrees. Rosenthal in 1995 described a technique in which a small saturated piece of sponge is placed in the superior and inferior fornices to maintain corneal clarity.14 Bloomberg replaced the sponge in fornices by a ring at the paralimbal area/region saturated with local anesthetics.15
Topical medication achieves loss of sensations of the conjunctiva, cornea, and anterior sclera. The eyelids, posterior sclera, intraocular tissues, or extraocular muscles are not anesthetized.
 
Advantages
Advantages of topical anesthesia are:
  • Avoidance of orbital or intraocular injection and their complications.
  • No need for a patch on the eye after surgery.
  • No temporary vision loss from the eye undergoing surgery.
 
Intracameral Anesthesia
Patients with topical anesthesia are aware of the surgical procedure, and are conscious of the eyelid speculum and microscope light. They also experience discomfort or pain with intraocular manipulation or IOP fluctuation.
The advantage of intraocular anesthetic injection is decrease in visceral pain associated with pressure fluctuations in the anterior chamber and with surgical manipulations of the iris, ciliary body, and lens, especially in phacoemulsification and lens insertion.
Intraocular anesthesia is given through a 1-mm paracentesis or side-port incision. The drug should be preservative free. Benzalkonium chloride is toxic to the corneal endothelium, so anesthetic agents injected intraocularly must be free of this preservative.16 The drug is directed posterior to iris to achieve a maximal effect on the iris and ciliary body proprioceptors. The anesthetic agent is washed out by injection of viscoelastic material after 15 to 30 seconds. Preservative-free lidocaine (1%) and preservative-free tetracaine have been commonly studied.17 Anderson et al18 found no difference in intracameral anesthetic effect of bupivacaine 0.5 percent versus lidocaine (1%), but reported possibility of damage to corneal endothelium with bupivacaine.
 
Anesthesia by Orbital Injection
Orbital anesthesia may be parabulbar injection, peribulbar injection, and retrobulbar injection. It attains anesthesia of the conjunctiva, cornea, sclera, intraocular structures, and extraocular muscles. Extraocular movements are greatly reduced or eliminated.
Superior rectus suture or cautery can be used after injection anesthesia and there is no restriction to the extent of intraocular manipulation (anterior or posterior).
The disadvantage of orbital injection anesthesia is the need to patch the eye postoperatively. The other disadvantage is the complications arising from the orbital injections like retrobulbar hemorrhage, globe perforation, injury to optic nerve.
 
Parabulbar Anesthesia
Bergman19 in 1993 described parabulbar anesthesia. This block is also known as sub-Tenon's block,20,21 pinpoint anesthesia22 and medial episcleral block.23
In this, a retrobulbar flush of bupivacaine (0.75%, 2.5 ml) and lidocaine (2.0%, 2.5 ml) is given by a blunt cannula in the sub-Tenon's space after dissecting it under topical anesthesia (Figs 1.1A to C).
Greenbaum20 and Aleman24 modified this topical and parabulbar anesthesia by using a flexible cannula. A postlimbal, sub-Tenon's incision (1 mm in length), in the inferotemporal or inferonasal quadrant, after topical anesthesia, is made. Greenbaum flexible polyethylene cannula is used for retrobulbar flush anesthesia by passing it posteriorly in the sub-Tenon's space. Bupivacaine (0.75%, 1.25 ml) and lidocaine (2.0 or 4.0%, 1.25 ml) are used for this anesthesia.20
Chuang et al25 found this segmental parabulbar anesthesia efficient and safe in segmental scleral buckling.
Parabulbar anesthesia was introduced as a safe technique, as there was increasing concern about the complications by sharp needle blocks. The short ciliary nerves are blocked as they pass from the sub-Tenon's space to the globe causing loss of sensation. All four quadrants can be used,20,22,23 but the inferonasal quadrant has been used frequently in the different studies published. This quadrant allows good fluid distribution superiorly while avoiding the area for surgery and damage to vortex vein.
 
Peribulbar Anesthesia
Davis and Mandel26 described the efficacy and complications of peribulbar anesthesia in 1994. In peribulbar anesthesia, two injections each 2–3 ml of mixed solution of 0.5 percent bupivacaine hydrochloride (4 ml), 2 percent lidocaine (4 ml), and 0.5 ml of 150 IU hyaluronidase are administered into the peribulbar space (upper nasal and lower temporal) of the eye, with the eye in primary position.7
zoom view
Figs 1.1A to C: Sub-Tenon's anesthesia
With a 24-mm, 23-G to 26-G needle, the superior injection is performed below the supraocular notch and the inferior just above the infraocular notch in the anterior third of the orbit. This is followed by ocular compression of 5–10 minutes. Prior to surgery, effectiveness of the block is assessed. Minimal ocular motility of orbicularis function is observed. Surgery is started after 10 to 25 minutes. This anesthesia is also known as extraconal anesthesia as the tip of the needle is outside the muscle cone (Fig. 1.2). In a double-masked randomized control trial it has been demonstrated that anesthetic outcomes are similar in peribulbar and retrobulbar block.27
zoom view
Fig. 1.2: Injection sites for retrobulbar and peribulbar anesthesia
 
Retrobulbar Anesthesia
In retrobulbar block, a 25-gauge, 31-mm long needle is used. It is inserted through the conjunctiva or skin in the inferotemporal quadrant at the junction of lateral one-thirds and medial two-thirds of the inferior orbital margin. The needle is directed posteriorly, upward and medially toward the lower edge of the superior orbital fissure at the apex of the orbit or towards midway between occiput and opposite mastoid process. It is allowed to go upwards and inwards to enter the central space just behind the globe (Fig. 1.3) The globe is continuously observed during the needle placement. A volume of 4 to 5 ml of local anesthetic agent is injected. This is intraconal anesthesia (Fig. 1.3). The regimen given by Straatsma for retrobulbar anesthesia is bupivacaine (0.75%, 1.5 ml) and lidocaine (2.0%, 1.5 ml) with hyaluronidase (5 units/ml). Oculopression after retrobulbar anesthetic injection enhances the effect dramatically.28
Multiple communications between extraconal and intraconal compartments have been demonstrated by computed tomography (CT) studies after injections of radio contrast material; and thus, the injected material diffuses from one to the other.29 This division of extraconal and intraconal is artificial because the globe, the extraocular muscles, and the septal compartments function as a unit and there are no anatomical discrete divisions.30 A combined intraconal and extraconal blocks is known as combined retroperibulbar block.318
zoom view
Fig. 1.3: Retrobulbar anesthesia
Complications The complications of retrobulbar anesthesia may be grouped into two:
  1. Due to technique:
    • Retrobulbar hemorrhage: This is an unfortunate complication of retrobulbar anesthesia. In this, there is increasing proptosis, sub-conjunctival hemorrhage due to seepage of blood anteriorly. Bleeding may stop as it is in a closed space and this raises the pressure to limit further bleeding; but this may raise the IOP. Surgery must be postponed in most cases. The retrobulbar anesthesia and hemorrhage can also trigger oculocardiac reflex.
    • Globe perforation: In this complication, the globe becomes too soft. Here also the surgery should be postponed and the perforation managed accordingly.3234
    • Optic nerve damage: As it is a blind procedure, the tip of needle may pierce the optic nerve, thus damaging it, leading to optic atrophy.34
    • Vascular occlusion: Due to hemorrhage in the orbit, vessels may be occluded as the pressure of hemorrhage collapses the vessels. Cawley et al35 in 1988 described retinal vasculature occlusion after retrobulbar injection without retrobulbar hemorrhage or hemorrhage in optic nerve sheath.
  2. Due to drugs:
    • Allergic reactions: Patients may be sensitive to any of the anesthetizing drug. This may lead to allergic drug reaction.
    • Others: Cardiopulmonary arrest, convulsions or loss of consciousness. These complications arise when the optic nerve is penetrated by the needle tip with subsequent injection of anesthetic into the subarachnoid space. The drug may go to the opposite optic nerve through subarachnoid space. It may also travel to other parts of the brain including the brainstem leading to convulsion, anesthesia of vital centers. This may cause coma or even death.3639
zoom view
Fig. 1.4: Van Lint block
 
Regional Blocks
 
Facial Nerve Block
Van Lint block This infiltrative block was first described by Van Lint40 along with local anesthesia for cataract surgery. Here, a small wheal is made at the lateral orbital margin. A needle is inserted through this wheal along the lower orbital margin and the anesthetic is infiltrated along the path. The needle is withdrawn and then inserted similarly along the superior orbital margin. Lidocaine and bupivacaine in 1 : 1 ratio are taken for infiltration. Generally, 5 ml is injected; but in obese patients, a larger volume (8 ml) may be required. The disadvantage is that it causes lid and periorbital edema (Fig. 1.4).
O'Brien block This method was developed by O'Brien. In this, the facial nerve is blocked as it crosses the condyle of the mandible. The patient is asked to open and close his mouth. The condyle can be felt moving in front of tragus of the ear. Drug is injected about 2 mm below the tragus. Pressure is applied so that the infiltrated drug spreads out (Fig. 1.5).
zoom view
Fig. 1.5: O'Brien block
9
zoom view
Fig. 1.6: Atkinson block
Atkinson block In this block, the facial nerve is blocked midway between stylomastoid foramen (emergence of facial nerve) and the orbicularis oculi muscle. The needle is directed at the inferior border of the zygomatic bone aiming at the top of the ear (Fig. 1.6).
Spaeth block This block avoids the inconsistencies of O'Brien technique. Here, the facial nerve is blocked before it divides after emerging from stylomastoid foramen. In this, the fingers are placed at posterior border of mandible. The needle is inserted just anterior to the most superior finger (Fig. 1.7).
 
Adjuvants of Anesthetic Agents
 
Vasoconstrictor
Commonly used vasoconstrictors with local anesthetics are epinephrine and felypressin. These cause vasoconstriction resulting in decreased absorption of the drug. So the duration of the anesthetic drug is prolonged and also prevents surge in plasma levels. These should be avoided in patients with cerebrovascular and cardiovascular disorders, especially the use of epinephrine should be restricted in such cases.41
zoom view
Fig. 1.7: Spaeth block
 
Hyaluronidase
Hyaluronidase is a highly purified bovine testicular enzyme which hydrolyzes hyaluronic acid present in tissues. This is done by liquefying the interstitial cell barrier between cells by depolymerization of hyaluronic acid to a tetrasaccharide.42 Hyaluronidase is available as a readily soluble powder. The dose varies from 5 to 150 IU per ml. Hyaluronidase has been shown to improve the effectiveness and the quality of needle as well as sub-Tenon's block,43,44 but controversy still remains.45 In many studies, it was seen that hyaluronidase produces better akinesia when compared with a placebo. The results were inconsistent with some studies showing no benefit. Complications like diplopia and greater orbital swelling have been reported. A rare allergic orbital swelling has been reported by Ahluwalia et al46 in 2003 after a cataract surgery. Hyaluronidase-induced orbital pseudotumor has been seen by Kempeneers et al47 in 1992. However, complications are noted even when hyaluronidase is not used. Complications like diplopia, ptosis have been observed.4852 The protective effect of hyaluronidase cannot be established for the above complications when it was not used. It has been seen that inadvertent injection of the local anesthetic into an extraocular muscle can cause paresis of the muscle by the myotoxic effect of the anesthetic drugs.53 This causes diplopia which diminishes within a few days to weeks. In some patients, fibrosis may set in after paresis. This can be corrected by muscle surgery.
 
pH alteration
Lidocaine and bupivacaine are available as acidic solutions commercially. At low pH, they are present in the charged ionic form; and therefore, they cannot cross the lipid membrane of neuron to produce a block. It is the nonionized form that crosses the lipid membrane to produce a block. At higher pH, these drugs are present in nonionized form. Thus, alkalinization has been found to decrease the onset and prolong the duration of effect of anesthesia.54,55
 
SPECIAL CONDITIONS
 
Anesthesia in Children
Anesthesia in children is different from the adult anesthesia. Many procedures, which can be done in local anesthesia in adults, require general anesthesia in children. Anesthetic agents, laryngoscopy, and intubation affect IOP. Therefore, this should be measured before deep level of anesthesia is reached and intubation done. Moreover, drugs that do not increase IOP may be used (Table 1.2). For infants less than 3 months, the general anesthesia is complicated by immature muscle development. Therefore, risk-benefit ratio should be considered before operating them under general anesthesia. The 10conditions for such situations are congenital glaucoma, bilateral cataract or tumor. After 3 months or gaining weight of 5.5 kg, the use of general anesthesia becomes safer. Infants and children get easily dehydrated. So appropriate preoperative instructions should be given. Clear fluids may be given up to 6 hours before surgery. In premature infants, the concentration of oxygen should not be more than that in the room unless it is essential for life due to possible side effect of retinopathy of prematurity.
 
Anesthesia for Vitreoretinal Surgery
The patients undergoing vitreoretinal surgery are usually diabetic and/or hypertensive. This offers a challenge to anesthetists.56 Now-a-days, instead of general anesthesia, ophthalmologists are preferring local anesthesia such as a retrobulbar or sub-Tenon's block.25 Under general anesthesia, a larger sub-Tenon's block may be given to prevent scleral perforation.
In retinal detachment surgery, intravitreal gas (SF6 or C3F8) is frequently used. When nitrous oxide is being used as an anesthetic agent, it will diffuse into the intraocular gas bubble causing rise in the IOP. By stopping nitrous oxide, the IOP will normalize; but it will also lead to shrinkage of the gas bubble. This will result in inadequate tamponade. Therefore, it is recommended to stop nitrous oxide 20 minutes before injecting the intravitreal gas. Nitrous oxide should be avoided 3 to 4 weeks postoperatively to prevent rise in IOP.
 
Anesthesia for Lacrimal Surgery
Lacrimal surgery requires anesthesia around lacrimal drainage area and the nasal mucosa. In this, local infiltration is done in the skin above the caruncle at the upper inner angle of the orbit. The needle is then directed backward and medially to block infratrochlear nerve. The needle is withdrawn and then directed towards nose up to medial canthus. By this, the surgical area is anesthetized. To anesthetize nasal mucosa, a gauze ribbon soaked in 4 percent lidocaine with epinephrine (1:100, 000) is packed in nasal cavity.
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