Ocular Anatomy and Blindness

Ocular Anatomy

• The eye has a spherical shape of about 24 mm in diameter, and sits in the pyramidal bony orbit
• The eye has three layers; the sclera, the uveal tract, and the retina
• The sclera is the outermost, fibrous, and tough white layer
• The transparent cornea is the most anterior part of the sclera
• The uveal tract is the middle layer, containing the choroid, iris, and ciliary body
• The choroid is a layer of blood vessels located posteriorly
• The retina is the innermost layer, containing retinal photoreceptors, which produce neural signals carried to the brain by the optic nerve
• The retina has no capillaries
• The choroid provides oxygen to the retina
• Retinal detachment from the choroid layer can lead to vision loss

Blindness

• Cataracts are the leading cause of blindness worldwide
• Diabetes is the leading cause of new cases of adult blindness in the USA
• Further causes of vision loss include macular degeneration, glaucoma, and diabetic retinopathy
• Glaucoma increases IOP due to obstruction of aqueous humor outflow
• Frequent comorbidities include hypertension, diabetes mellitus, and coronary artery disease

Intraocular Pressure (IOP)

• IOP maintains shape and optical properties of the eye
• Normal IOP is between 10-20 mm Hg
• Tight regulation of IOP is important
• CVP increase leads to IOP increase, significantly
• BP increase leads to IOP increase, mildly
• PaCO2 increase leads to IOP increase, moderately
• PaO2 increase has NO effect on IOP, but a fall can increase IOP
• Anesthesia gases, barbiturates, benzodiazepines, & narcotics all reduce IOP
• Ketamine may possibly increase IOP
• Suxamethonium increases IOP 5-10 mm for 5-10 min

Strategies to Prevent Increased IOP

• Avoid direct pressure on the eye via patching with a shield post-operation
• Avoid retro/peri bulbar blocks in trauma
• Use a careful mask technique
• Prevent coughing and avoid increases in CVP
• Ensure deep anesthetic and relaxation before intubating
• Laryngoscopy increases IOP by 10-20 mm Hg
• Avoid head-down position
• Extubate deeply
• Prevent vomiting
• Avoid ketamine and suxamethonium

Oculocardiac Reflex

• Use anticholinergic medications like IV glycopyrrolate, being cautious in CAD patients
• Traction on extraocular muscles or pressure on the eye can cause dysrhythmias, such as sinus arrest, bradycardia, or ventricular ectopy/fibrillation via the vagus nerve
• Most common during pediatric strabismus surgery
• Awake patients may experience nausea
• Retrobulbar block or deep inhalational anesthesia may help prevent the reflex
• Treatment includes stopping surgical stimulus, assessing ventilation, and administering atropine IV if resistant (10mcg/kg)

Systemic Effects from Ophthalmic Medications - Mydriatic Dilators

• Atropine drops may cause tachycardia
• Cyclopentolate, a strong dilator, may cause disorientation, psychosis, or seizures
• Adrenaline drops can cause hypertension, tachycardia, and ventricular ectopy that is worsened by halothane
• Phenylephrine causes hypertension, arrhythmias, and bradycardia
• Scopolamine may cause agitation and/ or disorientation

Systemic Effects from Ophthalmic Medications - Miosis/Constriction

• Acetylcholine can cause bronchospasm, bradycardia, hypotension and are placed after cataract surgery
• Echothiophate is an irreversible cholinesterase inhibitor used to treat high IOP in glaucoma
• Topical application causes a reduction in plasma cholinesterase activity, prolonging the effects of suxamethonium which can last 3-7 weeks after discontinuing drops
• Bradycardia on induction can be prevented with IV atropine

Systemic Effects from Ophthalmic Medications - Glaucoma Meds to Decrease IOP

• Acetazolamide can cause diuresis and should be avoided in patients with renal disease
• Timolol drops are non-selective beta-adrenergic blockers that reduce IOP via decreasing the production of aqueous humor. It may also cause atropine-resistant bradycardia, hypotension, and bronchospasm, but this is rare

Balanced Technique - IV Sedation/MAC

• Maintaining adequate ventilation is the most important
• Small doses at intervals is best with an effective regional or topical anesthetic
• Avoid deep sedation because of risk of apnea and patient movement with dysinhibition
• Small divided doses of propofol (30-100) mg work well during block placement while maintaining spontaneous ventilation
• Small doses of opioid (fentanyl 25-50 mcg) combined with small doses of benzodiazepine (midazolam 1-2 mg) and topical tetracaine may be all that is required/ sufficient for cataract surgery

Regional Anesthesia Blocks for Eye Surgery

• Regional anesthesia provides akinesia (loss of movement), analgesia, and abolished oculocardiac reflex using 2-5 cc lignocaine or bupivacaine, while aspirating before injection
• Retrobulbar block is administered using a needle through the lower lateral eyelid, placing the local anesthetic in the extraocular muscle cone behind the eye and usually requires temporary sedation
• Peribulbar block uses a needle that does not penetrate the cone, with a longer onset of 5-10 min, and less risk of puncture to structures
• Complications include hemorrhage, globe puncture, central spread through nerve sheath into CSF, causing seizure or apnea, and oculocardiac reflex
• Topical 0.5% tetracaine drops are used for anterior surgery, like cataract and glaucoma procedures

General Anesthetic Management for Ophthalmic Cases

• General anesthetic technique is required for children and uncooperative patients, as well as for delicate posterior capsule surgery, where an immobile patient is critical
• Smooth induction, deep anesthetic, and profound paralysis prevents coughing to control IOP
• Laryngoscopy, intubation, airway obstruction, coughing, pressure from poor mask fit, & Trendelenburg position all increase IOP
• Succinylcholine increases intraocular pressure 5-10 mm Hg for 5-10 min through prolonged contracture of extraocular muscles, so non-depolarizing muscle relaxants are preferred
• Careful monitoring of sedation, ventilation, EtCO2, and SaO2
• Positive ventilation capability must always be at the ready

Maintenance Technique for GA

• Low cardiovascular stimulation from eye surgery combined with a requirement for deep anesthesia to prevent movement may lead to hypotension, especially in elderly patients
• Adequate hydration improves this situation
• Small doses of ephedrine (2-5 mg IV) may counter low BP
• Nondepolarizing muscle relaxants may allow lighter anesthesia

Emergence from GA

• Prevention of cough is key: consider deep extubation
• Suction the oropharynx early, before reversal of muscle relaxants
• Consider redose of lignocaine 1.5 mg/kg IV 1-2 min prior to extubation
• Do not extubate deep if aspiration risk
• Post-op pain is minor, except for scleral buckle, enucleation and ruptured globe repair
• Small divided doses of narcotics may suffice
• Severe pain may indicate a surgical complication

Vitreous Surgery for Detached Retina

• Treatment options: vitrectomy or retinal buckling
• An air bubble may be injected into the posterior chamber to flatten the retina for five days to allow healing to occur
• Sulfur hexafluoride may be used, and will last 10 days
• Avoid Nitrous, which is more soluble than nitrogen in air and will expand the size of the bubble in either case

Strabismus Surgery and Pediatric Techniques

• Muscle repair or resection restores motor function to the lateral rectus muscle (crossed or lazy eyes)
• Common comorbidities- congenital musculoskeletal problems/syndromes, MH
• Emergence delirium risk in peds can be as high as 30%
• The oculocardiac reflex is most common in pediatric strabismus surgery

Malignant Hyperthermia

• Strabismus may be associated with underlying myopathy, which increases MH risk
• Avoid triggers like succinylcholine & volatile agents
• Closely observe for rapid rise in EtCO2, tachycardia, muscle rigidity, and/or rise in temp
• Treatment: stop trigger, administer 100% O2, hyperventilation, cooling, Dantrolene 2.5 mg/kg IV (max 10 mg/kg), bicarb, and follow ABG

Trauma & Penetrating Injury

• Open eye injury requires strict control of IOP with smooth, rapid sequence induction
• Coughing must be avoided by rapid deep induction and paralysis
• All patients are treated as a full stomach due to delayed gastric emptying from pain and anxiety
• Metoclopramide (10 mg IV) increases gastric emptying, lowers gastric volume, and offers an antiemetic effect
• Ranitidine (50 mg IV), Cimetidine (300 mg IV), and Famotidine (20 mg IV) are H2 histamine receptor antagonists that inhibit gastric acid secretion, although its value is limited in an emergency surgery
• Antacids, like sodium citrate, may be given immediately prior to induction (15-30 ml)

Trauma & Penetrating Eye Injury

• Premedicate with midazolam and/or fentanyl
• Premedication and Lignocaine 1.5 mg/kg IV can blunt increase in IOP due to laryngoscopy
• Propofol is the preferred induction agent for intubation
• Avoid Etomidate and Ketamine, as they may increase intraocular pressure and cause nystagmus or myoclonus
• Use rapid sequence induction with cricoid pressure
• Avoid positive pressure ventilation until airway is secured

PONV and Opthalmic Surgery

• Many eye patients are diabetic (retinopathy) and will need blood sugar monitoring; low BGL may cause nausea
• Emesis caused by vagal stimulation is common post-operatively, especially after strabismus surgery
• 48% to 85% of pediatric strabismus cases result in PONV
• Valsalva increases IOP and may stress surgical sites, increasing the risk for aspiration.
• Antiemetics should be given to all eye surgery patients, especially those with known PONV and those receiving opioids
• Metoclopramide, 5-HT3 antagonists, & dexamethasone are common choices
• Preventative measures include IV hydration, minimizing opioids, and using propofol or local anesthetics

Skills: Positioning

• Turn the table 90 degrees
• BP cuff opposite limb from IV
• ECG leads
• Pulse Oximetry on limb with IV ideally next to anesthetist
• Head taped in place with drape bar
• Blow by oxygen or nasal cannula- Fire Triangle?
• EtCO2 monitoring, especially in peds
• Arm restraints