II. OCULAR PHYSIOLOGY

A. Formation and Drainage of Aqueous Humor

1. Aqueous humor is formed in the posterior chamber by the ciliary body in an active secretory process involving carbonic anhydrase as well as by passive filtration from the vessels on the anterior surface of the iris.

2. Drainage of aqueous humor is via a network of connecting venous channels (includes Schlemm’s canal) that empty into the superior vena cava. (Any obstruction between the eye and right atrium impedes aqueous drainage and increases intraocular pressure [IOP].)

B. Maintenance of Intraocular Pressure

1. IOP normally varies between 10 and 21.7 mm Hg but becomes atmospheric when the globe is opened. The major determinant of IOP is the volume of aqueous humor.

2. Any sudden increase in IOP when the globe is open may lead to prolapse of the iris and lens, extrusion of the vitreous, and blindness.

3. Straining, vomiting, or coughing (as during laryngoscopy and tracheal intubation) greatly increases venous pressure and IOP.

C. Glaucoma is characterized by increased IOP, resulting in impairment of capillary blood flow to the optic nerve.

1. Treatment consists of topical medication to produce miosis and trabecular stretching.

2. Atropine premedication in the dose range used clinically has no effect on IOP in patients with glaucoma. (Scopolamine may have a greater mydriatic effect, and its use may be avoided.)

III. EFFECTS OF ANESTHESIA AND ADJUVANT DRUGS ON INTRAOCULAR PRESSURE (Table 48-2

A. Equipotent paralyzing does of all the nondepolarizing neuromuscular blocking drugs directly lower IOP by relaxing the extraocular muscles.

B. Intravenous (IV) injection of succinylcholine (SCh) transiently increases IOP by about 8 mm Hg with return to baseline in 5 to 7 minutes (reflects the cycloplegic action of SCh and is not reliably prevented by pretreatment with nondepolarizing muscle relaxants or IV administration of lidocaine).

1. It is no longer valid to recommend that SCh be used only with extreme reluctance in ocular surgery (any SCh-induced increase in IOP would be dissipated before surgery is started), but SCh is not the ideal drug for penetrating ocular wounds.

2. SCh may interfere with interpretation of the force duction (FDT) test used to determine if strabismus is caused by muscle paresis or a restrictive force (need to wait about 20 minutes after administering SCh to perform the FDT).

3. In light of the boxed warning issued by the Food and Drug Administration stating that use of SCh in children may rarely be associated with hyperkalemia and cardiac arrest, it should be reserved for emergency intubation or when immediate airway control is needed (reason SCh is typically avoided in pediatric strabismus surgery).

IV. OCULOCARDIAC REFLEX

A. This reflex manifests as bradycardia (and occasionally cardiac dysrhythmias) that is elicited by pressure on the globe and by traction on the extraocular muscles (strabismus surgery), especially the medial rectus.

B. Monitoring of the electrocardiogram is useful for early recognition of this reflex.

C. Atropine given intravenously within 30 minutes of surgery is thought to lead to a reduced incidence of the reflex (controversial). Atropine as administered intramuscularly for preoperative medication is not effective for preventing this reflex.

V. ANESTHETIC RAMIFICATIONS OF OPHTHALMIC DRUGS

A. Anticholinesterase Agents. Echothiophate is a long-acting anticholinesterase miotic that decreases IOP and prolongs the duration of action of SCh.

B. Cyclopentolate is a mydriatic agent and may produce central nervous system toxicity.

C. Epinephrine as a topical has proved useful in some patients with open-angle glaucoma, but systemic side effects may occur. The prodrug of epinephrine, dipivefrin, produces similar ocular benefits (reduces aqueous productions and augments outflow) with fewer side effects.

D. Phenylephrine is a mydriatic agent that may produce cardiovascular effects.

E. Timolol lowers IOP, but systemic absorption may result in cardiac depression and increased airway resistance. Betaxolol may be more oculo specific and has minimal systemic effects.

F. Sulfur hexafluoride (SF6) is injected into the vitreous to mechanically facilitate retinal reattachment. Nitrous oxide (N₂O) (blood/gas solubility 0.47) should be avoided for 10 days after intravitreous injection of SF₆ (blood/gas solubility 0.004). A Medic-Alert bracelet might be helpful to warn against administration of N₂O during the period when this gas could cause expansion of the SF₆ bubble, risking optic nerve ischemia secondary to central retinal artery occlusion.

VI. PREOPERATIVE EVALUATION

A. Establishing Rapport and Assessing Medical Conditions

1. Preoperative testing should be based on the history and physical examination.

a. Many elderly adult candidates for ophthalmic surgery are on antiplatelet or anticoagulant therapy owing to a history of coronary or vascular pathology. Continuing warfarin therapy for cataract extraction may be associated with an increased risk of bleeding (self-limiting and not clinically relevant).

b. The risk of thrombotic complications in patients with drug-eluting stents appears to outweigh the risk of bleeding complications (continue dual antiplatelet therapy in the perioperative period). A recommendation is to delay elective surgery for at least 4 to 6 weeks after placement of a bare metal stent and for at least 12 months after drug-eluting stent placement.

c. Despite the possibility of eye injury from patient movement in the event of implanted cardiac defibrillator activation, there are no reports of activation during ophthalmic surgery, and magnets to inactivate the device before surgery are rarely used.

d. Perioperative movement is a possible cause of patient eye injury and potential anesthesiologist liability. Inadequate sedation during monitored anesthesia care (MAC) may be associated with unpredictable movement that results in blindness or poor visual outcome. Intraoperative movement during general anesthesia may also result in adverse visual consequences.

2. Anesthesia Options. A commonly selected regional anesthetic technique for cataract surgery is peribulbar block (which has a better safety profile than retrobulbar block). Topical anesthesia is also effective for cataract surgery.

3. Side of Anesthesia and Surgery. Ophthalmologic surgery and regional anesthesia confer greater risk than many other surgical procedures owing to the potential for laterality errors.

VII. ANESTHESIA TECHNIQUES. Most ophthalmic procedures in adults can be performed with either local or general anesthesia. Data have failed to demonstrate a difference in complications between local and general anesthesia for cataract surgery (Table 48-3).

A. Retrobulbar and Peribulbar Blocks. Retrobulbar block may be associated with significant complications, emphasizing that local anesthesia does not necessarily involve less physiologic trespass than general anesthesia (Table 48-4 and Fig. 48-2).

B. Topical analgesia can be achieved with local anesthetic drops or gels.

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