Anesthesia for ophthalmic surgery

  Anesthesia for ophthalmic surgery

Intraocular pressure dynamics

The eye can be considered a hollow sphere with a rigid wall with intraocular pressureof 10-20 mmhg

Factors affecting IOP:

• Volume of the content of the globe: obstruction to aqueous humor outflow increase IOP as in case of glucouma
• External pressure on the eye due to decreasing the size of the globe without proportional change in the volume of its content e.g

• pressure on theeye from tightly fitted mask
• Improper prone position
• Retrobulbar hge

• Cardiac and respiratory variables:

1-Central venous pressure:

A rise in venous pressure will increase IOP by decreasing aqueous drainage and increasing choroidal blood volume

2-Arterial blood pressure:

Extreme changes in arterial pressure can affect IOP. Any anesthetic maneuver that affect arterial blood pressure can affect IOP such as laryngoscopy and intubation, coughing trendlenberg position.

Increase in BP--------------increase IOP

Decrease in IOP -----------decrease IOP

3-Paco2

Increase in paco2 level due to hypoventilation can increase IOP while decrease in paco2 level due to hyperventilation decrease IOP

4-Pao2

Incease pao2 has no effect on IOP while decrease in pao2 will decrease IOP.

N.B when the globe is open during certain surgical procedures or after traumatic perforation, IOP approaches atmospheric pressure. Any factor that normally increase IOP will tend to decrease intraocular volume by causing drainage of aqueous or extrusion of vitreous through the wound.

Effect of anesthetic drugs on IOP

Most anesthetic drugs lower or have no effect on IOP.

Inhalational anesthetics decrease IOP in proportion to the depth of anesthesia due to multiple causes:

• Drop in blood pressure reduce colloidal volume
• Relaxation of extraocular muscles
• Papillary constriction facilitates aqeous outflow

Intravenous anesthetics also decrease IOP with the exception of ketamine which usually raise arterial blood pressure and dose not relax extraocular muscles.

Topically administered antichlonirgic drugs result in papillary dilataion whichmay precipitate angle closure glaucoma. Premedication with systemically administered atropine is not associated with intraocular hypertension. Glucopyrrolate may provide a greater margin of safety by preventing its passage into the central nervous system.

Succinyl chlorine increase IOP by 5- 10 mmgh for 5- 10 minafter administration due to prolonged contracture of extraocular muscles. This will cause spurious measurements of IOP during examination anesthesia in glaucoma patients potentially leading to unneccssary surgery, may cause extrusion of ocular contents through an open surgical or tuamatic wound and finally abnormal forced duction test for 20 min( this maneuver evaluate the cause of extraocular muscles imbalance and may influence the type of strabismus surgery.

Systemic effects of ophthalmic drugs

Topically applied eye drops are absorbed by vessels in the conunctival sac and nasolcrimal duct mucosa.

Topically applied drugs are absorbed at a rate intermediate between absorption following iv and sc injection.

One drop of 10% of phenylephrine contains 5 mg of drug compared to 0.05 – 0.1 mg used to treat hypotension.

Echothiophate is an irreversible antichloinestrase used in the treatment of glaucoma. Topical administration lead to systemic absroptopn and reduction in plasa cholinesterase activity which will prolong duration of action of succinyl choline.

The inhibition of cholinesterase activity lasts for 3-7 weeks after discontinuation of eye drops.

Epinephrine eye drops can cause hypertension, tachycardia and vent dysrrhythmia which are potentiated by halothane.

Timolol, a non selective B blocker reduce IOP by decreasing aqeous production. It has been rarely associated with atropine resistant bradycardia, hypotension, and bronchospasm during general anesthesia.

The occulocardic reflex

Traction on extraocular muscles or pressure on the eye ball can elicit a wide variety ofmcardiac dysrhythmia ranging from bradycardia and vent ectopy to sinus arrest and vent fibrillation. It consists of afferent trigeminal and a vagal efferent pathway.

Anticholinergic medication is often helpful in preventing the occulocardic reflex. Intravenous atropine or glycopyrrolate immediately prior to surgery is more effective than intramuscular premedication.

Management of the oculcardic reflex when it occursnconsists of the following procedures:

• Immediate notification of the surgeon
• Confirmation of adequate ventilation, oxygenation and depth of anesthesia.
• Administration of iv atropine 10 ug/kg.
• Infiltration of the rectus muscle with local anesthetic.

General anesthesia a for ophthalmic surgery

Open-Eye Surgical Procedures.

Cataract extraction

Corneal laceration repair

Corneal transplant (penetrating keratoplasty)

Peripheral iridectomy

Removal of foreign body

Ruptured globe repair

Secondary intraocular lens implantation

Trabeculectomy (and other filtering procedures)

Vitrectomy (anterior and posterior)

Wound leak repair

The choice between general and regional anesthesia should be made jointly by the patient, anesthesiologist, and surgeon.

There is no conclusive evidence that one form of anesthesia is more safe than the other. However general anesthesia is indicated in uncooperative patients to prevent head movements which can prove disaterous during microsurgery or in other situations when local anesthesia is contraindicated such as high myopia, open eye injury, aphakic patient with single functioning eye.

The gools of general anesthesia include a smooth induction, stable IOP, avoidance of oculcardic reflex, a motionless field, and smooth emergence.

Premedication

Patients undergoing eye surgery may be apprehensive, particularly if they have undergone multiple procedures and there is a possibility of permanent blindness.

Pediatric patients often have associated congenital disorders (eg, rubella syndrome, Goldenhar's syndrome, Down syndrome).

Adult patients are usually elderly, with myriad systemic illnesses (eg, hypertension, diabetes mellitus, coronary artery disease). These factors must all be considered when selecting premedication.

Premedication may include:

Sedative: with dose reduction in elderly

Anticholinergic to prevent oculocardic reflex specially in children.

Antiemetics to avoid postoperative nausea and vomiting.

Induction

The choice of induction technique for eye surgery usually depends more on the patient's other medical problems than on the patient's eye disease or the type of surgery contemplated.

One exception is the patient with a ruptured globe. The key to inducing anesthesia in a patient with an open eye injury is controlling intraocular pressure with a smooth induction. Specifically, coughing during intubation must be avoided by achieving a deep level of anesthesia and profound paralysis.

The intraocular pressure response to laryngoscopy and endotracheal intubation can be somewhat blunted by prior administration of intravenous lidocaine (1.5 mg/kg) or an opioid (eg, remifentanil 0.5–1 g/kg or alfentanil 20 g/kg).

A nondepolarizing muscle relaxant is used instead of succinylcholine because of the latter's influence on intraocular pressure.

Ketamine is best avoided as it can increase IOP. The LMA can be used for ophthalmic surgery and may be associated with less coughing in emergence.

Monitoring & Maintenance

Eye surgery necessitates positioning the anesthesiologist away from the patient's airway, making close monitoring of pulse oximetry and the capnograph particularly important for all ophthalmological procedures.

Kinking of the endotracheal tube, breathing-circuit disconnections, and unintentional extubation may be more likely. Kinking and obstruction can be minimized by using a reinforced or preformed right-angle endotracheal tube.

The possibility of arrhythmias caused by the oculocardiac reflex increases the importance of constantly scrutinizing the electrocardiograph and making sure the pulse tone is audible.

In contrast to most other types of pediatric surgery, infant body temperature often rises during ophthalmic surgery because of head-to-toe draping and insignificant body-surface exposure. End-tidal CO₂ analysis helps differentiate this from malignant hyperthermia.

Nitrous oxide presents a special problem in some vitriretinal procedures.N2O diffuses and cuases buble expansion with the potential of danagerous increase of IOP when sulfu hexafluoride agent is used.

N2O shold be shut off for 15min before placing sulfu hexafluoride buble and should be avoided for 7 -10 days thereafter.

Perfluoropropane is a new agent which can persist for weeks. N2O should be avoided at least for a month or until the buble is resorbed.

Extubation and emergence

Smooth emergence from general anesthesia is desirable to lessen the risk of postoperative wound dehiscence.

Coughing on ETT can be prevented by extubating patients during moderately deep level of anesthesia or administration of lidocaine 1.5 mg/kg 1 – 2 min before.

Emesis caused by vagal stimulation is a common postoperative problem, particularly following strabismus surgery. The Valsalva effect and the increase in central venous pressure that accompany vomiting can be detrimental to the surgical result and increase the risk of aspiration. Intraoperative administration of intravenous metoclopramide (10 mg in adults) or a 5-HT₃ antagonist (eg, ondansetron 4 mg in adults) decreases the incidence of postoperative nausea and vomiting (PONV). Antiemetics should generally be given to patients receiving opioids and those with a history of PONV.

Severe postoperative pain is unusual following eye surgery. Scleral buckling procedures, enucleation, and ruptured-globe repair are the most painful operations. Small doses of intravenous narcotics (eg, 15–25 mg of meperidine for an adult) are usually sufficient. Severe pain may signal intraocular hypertension, corneal abrasion, or other surgical complications.

Regional anesthesia for ophthalmic surgery

Eye surgery usually requires akinesia of the eye and profound anesthesia of the operative site.

Several regional anesthetic technique have been developed that satisfy the requirement of ophthalmic surery and are generally reliable and safe.

Regional anesthesia has several advantages over general anesthesia including:

• Significant postoperative analgesia
• Infrequent nausea and vomiting
• Early ambulation and dischare of the patient.

Contraindications:

• Young patient
• Uncooperative patient
• Extreme myopia
• Open eye surgery
• Coagulopathy or warfarin therapy

Retrobulbar block

Technique: local anesthetic is injected behind the eye into the cone formed by the extraocular muscles. A blunt tip 25 gauge needle penetrates the lower lid at the junction of middle and lateral third of the orbit ( 0.5 cm medial to lateral canthus). The needle is advanced 3.5 cm towards the apex of the muscle cone. After aspiration, 2-5 ml of local anesthetic are injected and the needle is removed. Local anesthetic used varies but the most common is lidocaine and bupivacaine e.g equal volumes of bupivacaine 0.5 -7.5% + lidocaine 2-4% is used.

Additives to local anesthetics:

Hyaluronidase, a hydrolyser of connective tissue polysaccharides is frequently added to enhance the spread of local anesthetic. 5 units for each one ml local anesthetic.

Epinephrine can be added to prolong the duration of action of local anesethetic and decrease the incidence of HE due to V.C.

A succeful retrobulbar block is accompanied by anesthesia, akinesia, and abolishment of oculcephalic reflex( a blocked eye dose not move during head turning).

Extrocular pressure application help in spreading the local anesthesia--------decrease the volume--------- decrease IOP.

It is done by: gentle digital pressure and massage fot at least 20 min.

Or a pressure reducing device such as Honan s ballon, it is applied for 20 min at a pressure of no more than 35 mmgh, then the ballon is removed just before the operation.

Complications of retrobulbar block

• Retrobulbar hge
• Globe perforation
• Optic nerve atrophy
• Frank convulsions
• Oculcardiac reflex
• Acute neurogenic pulmonary edema
• Trigeminal nerve block
• Respiratory arrest
• Postretrobulbar apnea syndrome: is probably due to injection of local anesthetic into the optic sheath with spread to the cerebrospinal fluid. The CNS is exposed to high concentration of local anesthetic leading to apprehension and unconcioueness. Apnea occurs in 20 min and resolve in an hour.

facial nerve block

facial nerve block prevents squiting of the eyelids and allows placement of a lid speculum.

There are several technique of facial nerve block:

Modified vanlint: the needle is placed 1 cm lateral to the orbital rim and 2-4 ml of local anesthetic are injected deep in the periosteum just lateral to supralateral and infralateral orbit rim.

O’ Brien block: the mandibular condyle is palpated inferior to the posterior zygomatic process and anterior to the tragus of the ear. The needle is inserted perpendicular to the skin about 1 cm to the peripsteum. As the needle is withdrawn 3ml of anesthetic is injected.

Nadbath Rehman block: 12 mm, 25 gauge needle is inserted perpendicular to the skin between the mastoid process and the posterior border of the mandible. The needle is advanced its full lenghth and after careful aspiration,3 ml of anesthetic is injected. It has been associated with vocal cords paralysis, dysphagia, and respiratory stress due to close proximity to vagus and glosspharyngeal nerve.

Peribulbar Blockade

In contrast to retrobulbar blockade, with peribulbar blockade the needle does not penetrate the cone formed by the extraocular muscles. Both techniques achieve akinesia of the eye equally well. Advantages of the peribulbar technique may include less risk of eye penetration, optic nerve and artery, and less pain on injection. Disadvantages include a slower onset and an increased likelihood of ecchymosis.

A blunt 23-gauge7/8   -inch Atkinson needle is placed at the junction of the middle and lateral thirds of the lower lid just above the inferior orbital rim; 1 mL of local anesthetic is put just below the orbital septum, 3 mL at the equator, and 2 mL posterior outside the muscle cone. If no bulging is noted at the superior nasal lid area, a second injection of 2 to 3 mL is administered inferonasally.

Sub-Tenon Block

Tenon's fascia surrounds the globe and extraocular muscles. Local anesthetic injected beneath it diffuses into the retrobulbar space. A special blunt 25-mm or 19-gauge curved cannula is used for a sub-Tenon block. After topical anesthesia, the conjunctiva is lifted along with Tenon's fascia in the inferonasal quadrant with forceps. A small nick is then made with blunt-tipped Westcott scissors, which are then slid underneath to create a path in Tenon's fascia that follows the contour of the globe and extends past the equator. While the eye is still fixed with forceps the cannula is inserted and 3–4 mL of local anesthetic is injected. Complications with the sub-Tenon blocks are significantly less than with retrobulbar and peribulbar techniques, but rare reports of globe perforation, hemorrhage, cellulitis, permanent visual loss, and local anesthetic spread into cerebrospinal fluid exist.

Intravenous Sedation

Several techniques of intravenous sedation are available for eye surgery. The particular drug used is less important than the dose. Deep sedation should be avoided because it increases the risk of apnea and unintentional patient movement during surgery. On the other hand, retrobulbar and facial nerve blocks can be quite uncomfortable.

As a compromise, some anesthesiologists administer a small dose of propofol (30–100 mg slowly) or a short-acting barbiturate (eg, 10–20 mg of methohexital or 25–75 mg of thiopental) to produce a brief state of unconsciousness during the regional block. Alternatively, a small bolus of an opioid (remifentanil 0.1–0.5 g/kg or alfentanil 375–500 g) allows a brief period of intense analgesia.

Other anesthesiologists, believing that the risks of respiratory arrest and aspiration are unacceptable, limit doses to provide only minimal relaxation and amnesia. Midazolam (1–2 mg) with or without fentanyl (12.5–25 g) or sufentanil (2.5–5 g) is a common regimen. Doses vary considerably among patients and should be administered in small increments. Moreover, concomitant use of more than one type of drug (benzodiazepine, hypnotic, and opioid) potentiates the effects of other agents; doses must be reduced accordingly. An antiemetic should probably be administered if an opioid is used. Regardless of the technique employed, ventilation and oxygenation must be carefully monitored, and equipment to provide positive-pressure ventilation must be immediately available.

Anesthesia for Pediatric Ophthalmologic Procedures

Anesthesia for pediatric eye surgery can be considered a subspecialty of its own. Small children may need examination under anesthesia. Intramuscular ketamine sometimes can be a good choice; it can be used when intravenous access may be problematic. Some ophthalmologists prefer ketamine because it does not reduce IOP as barbiturates and deep inhaled anesthetics do.

The most common eye surgery in children is for strabismus, or misalignment of the eyes. There is generally no severe postoperative pain, but nausea and vomiting are significant 50% to 80% of the time without treatment. Droperidol 5 to 75 µg/kg seems to decrease nausea and vomiting significantly without undue delay of discharge. Ondansetron has similar effects without sedation. If forced duction testing is used to assess the muscle tightness, the surgeon should be notified if succinylcholine is used. Succinylcholine causes a tonic increase in eye muscle tone, which resolves in about 20 minutes.

Strabismus is a very common condition, and most children are otherwise healthy. There is a higher incidence of strabismus in trisomy 21 or Down syndrome, cerebral palsy, and hydrocephalus. Malignant hyperthermia and myotonic dystrophy also have been associated with strabismus. Myotonic dystrophy also is seen in patients with ptosis and cataracts.

Cataracts can be seen in children with Pierre-Robin syndrome and phenylketonuria. Patients with Marfan syndrome have a high incidence of subluxation or dislocation of the lens. Aniridia, the congenital absence of the iris, is associated with Wilms’ tumor and hypertension. Congenital glaucoma also can be seen with Sturge-Weber syndrome and with seizures and angiomas of the mouth and larynx.