Fig. 24.1
Aqueous humor flow pathway. The aqueous humor is formed by the ciliary body. It then passes from the posterior into the anterior chamber and then eventually drains out of the trabecular meshwork into Schlemm’s canal where it exits. Any blockage along this pathway, particularly in the trabecular pathway or canal, leads to accumulation of this highly osmotic fluid and increases IOP, which can lead to glaucoma
The conjunctiva covers both the surface of the globe and eyelids. The surgeon can deploy various local anesthetics to these areas, and it is important for the anesthesiologist to tally those volumes along with any induction doses of lidocaine to prevent local anesthetic toxicity.
NOTE: when a local anesthetic such as lidocaine is applied topically to the conjunctiva, its absorption falls in between intravenous and subcutaneous administration [2].
Blood supply to the eye is primarily via the internal and external carotid arteries and the drainage is chiefly through the interwoven bridges between the superior and inferior ophthalmic veins which merge in one outflow tract from the central retinal vein [1].
The Physiology of Intraocular Pressure
What is the function of intraocular pressure? It maintains the spherical shape of the eye and that facilitates our vision. For most patients, small changes in intraocular pressure are of no major concern but for patients who have arteriosclerotic disease from diabetes, peripheral vascular disease, or other chronic conditions, small increases in IOP may be the nidus to retinal ischemia [2].
Normal intraocular pressure lies in the range of 10–22 mmHg, but if the contents inside the eye increase then so will intraocular pressure [3]. In fact, ocular hypertension is defined as any pressure reading higher than 21 mmHg [4]. The net difference between ciliary body production of aqueous humor and its elimination via Schlemm’s canal, the ebb and flow of choroid volume, extraocular muscle contraction, and volume of vitreous humor create a constant change of IOP flux [5]. If there is bleeding within the eye or if there is an obstruction in the trabecular network that allows aqueous humor to accumulate in this space, there can be an increase in total content in the eye and intraocular pressure. Other changes associated with anesthesia can directly affect intraocular pressure including arterial blood pressure, ventilation [6], oxygenation, and medications [2]. At all times, it is best to prevent coughing and gagging that can dramatically increase eye pressure (See Table 24.1).
Table 24.1
Factors affecting intraocular pressure (IOP)
Causes of increased IOP | Causes of decreased IOP |
|---|---|
Increased production of aqueous humor (or decreased drainage of aqueous humor) | Inhalational anesthetics |
Medications (e.g. steroid medications for asthma) | Decreased production aqueous humor |
Eye trauma | Increased flow/drainage aqueous humor |
Glaucoma | |
Factors that may be controllable under anesthesia | |
Succinycholine | |
Ketamine (+/−) | |
Increased CVP | Low CVP |
Increased arterial BP | Low arterial BP |
Increased PaCO2 (hypoventilation) | Decreased PaCO2 (hyperventilation) |
Decreased PaO2 (hypoxia) | Adequate PaO2 |
What cases most greatly threaten this balance? Patients with open globe traumas where the globe may need to be opened in surgery inevitably causing intraocular pressure to equilibrate with atmospheric pressure. These types of injuries may lead to vitreous humor extrusion or aqueous humor travelling through an open wound. In cases where the posterior segment may be involved, the result can be permanent blindness [2].
Preoperative Evaluation
Patients presenting for ophthalmologic surgery are often at the extremes of age and have a high incidence of concomitant systemic disease, so a careful preoperative exam and assessment is a must.
Ophthalmologic surgery is considered low-risk, as there are no major physiologic changes and no significant blood losses or fluid shifts.
A large multi-center trial showed that patients presenting for cataract surgery under local anesthetic with no acute medical conditions, no preoperative labs or EKGs are required [7].
If local/Monitored anesthesia care (MAC) is to be used the patient must be evaluated for their ability to lie flat, have no symptomatic gastroesophageal reflux, have no neuropsychiatric disturbances, no tremor of the head and neck, no uncontrollable cough, and be able to follow commands [8]
If general anesthesia (GA) is to be performed on patients with >4 METS, they require no further cardiac workup [9].
Lab tests and EKG should be obtained if indicated by history [8].
For cataract surgery, vitreoretinal surgery and oculoplastic surgery, patients may continue to take their aspirin, antiplatelet agents, and warfarin without increased risk for ocular hemorrhage [10, 11].
Special attention should be paid to ophthalmologic medications patients may be taking preoperatively, because adverse side effects may be observed under anesthesia. Such medications can be found in Table 24.2
Table 24.2
Ophthalmologic medications and their effects under anesthesia
Medication
Mechanism of action
Adverse effects under anesthesia
Timolol
Antagonizes beta receptors on the ciliary body which decreases aqueous humor production
Bradycardia, heart failure, asthma attacks, arrhythmia
Acetazolamide
Inhibits carbonic anhydrase which reduces aqueous humor production
Diuresis, hypokalemic hyperchloremic metabolic acidosis
Echothiophate
Irreversibly binds to & inhibits plasma cholinesterase (12)
Prolongation of action of succinylcholine & mivacurium, pupil dilation, bronchospasm
Phenylephrine (drops)
Induces mydriasis
Although possibility of HTN, tachycardia, and arrhythmia exist, a 2015 JAMA meta-analysis demonstrated no significant change in HR or BP & when changes were seen, they were short-lived (13)
Epinephrine (drops)
Induces mydriasis, can potentially decrease IOP
HTN, tachycardia, arrhythmias
Atropine/scopolamine
Induces mydriasis
Tachycardia, agitation, apnea, hypertension
Choice of Anesthetic Technique: Local Versus General for Ophthalmic Surgery
Local anesthesia has been associated with fewer complications than general anesthesia for ophthalmic surgery [8].
In one study comprising 81 patients with 2 or more risk factors for heart disease having cataract surgery under general or local anesthesia, there were significantly less intraoperative events in the local anesthesia group [14].
Many patients presenting for ophthalmic surgery are elderly, putting them at increased risk for cognitive dysfunction following general anesthesia [15].
There is significant reduction in incidence of nausea, sore throat and time to eat and drink with local/MAC vs. general anesthesia [16].
General Anesthesia
In certain cases, e.g., full stomach, open globe injuries, or in cases where patients have multiple comorbidities general anesthesia may be the desired anesthesia technique. Children, adults with psychiatric or mental deficits or those whose are unable to cooperate and communicate, individuals who have physiologic tremors or who cannot tolerate being supine may need general anesthesia [8]. In addition, if the procedure exceeds 3 hr or if the surgical field cannot be covered by local/regional techniques, then general anesthesia may be the only option. Relative indications for GA would include surgeon preference or concern for coagulopathy and bleeding. Even if one’s initial plan is to conduct the surgery under MAC/local, the potential to convert to general always exists. Whether elective or emergent, here are the goals for induction, maintenance, monitoring, and emergence to execute a successful anesthetic for ophthalmologic procedures.
Induction
During induction, do not place any extra pressure on the eye e.g., from mask during pre-oxygenation.
A smooth induction is of paramount importance especially for open globe injuries. The anesthesiologist must minimize coughing, gagging and therefore, escalation of already increased IOP during induction. This can be abated preemptively with premedication: IV lidocaine or opiates e.g., fentanyl, remifentanyl, or alfentanil [2].
In a randomized control trial which compared propofol 2 mg/kg with remifentanil 4 mcg/kg induction versus succinylcholine 1.5 mg/kg for induction, remifentanil was able to reduce IOP by 39 %, reduce MAP by 24–31 % and was effective in blocking the hemodynamic response to direct laryngoscopy and intubation [17]. Other studies have also shown that a combination of propofol with alfentanil has also counteracted the increase in IOP associated with airway manipulation [18]. These cases highlight the importance of adequate narcotic for smooth induction.
Make sure that enough time has passed from the time the narcotic and paralytic were administered to intubation in order to reduce the chance of coughing against the tube.
The question of profound paralysis with a modified rapid sequence induction via a nondepolarizing agent such as rocuronium or true rapid sequence with succinylcholine is at the discretion of the anesthesiologist. In the patient with a full stomach and open globe injury, the debate is whether the risk of aspiration or the risk of increasing IOP to the point of ocular expulsion is the priority. Most anesthesia providers carry a sense of relative or absolute contraindication to succinylcholine in these cases. However, the cases that reported increased IOP after succinylcholine administration in the 1950s are based on physiologic studies and not on any specific or documented case reports of vitreal extrusion [19].
Currently, the main school of thought is that where prompt securing of the airway is important, succinylcholine can be used judiciously. The advantages of using succinylcholine are not trivial: It has an incredibly rapid onset, reliably hastens apnea and achieves dense muscle relaxation with an equally rapid recovery [19].
How much has succinylcholine been estimated to increase IOP?
Succinylcholine follows a “5–10” rule. It raises IOP an estimated 5–10 mmHg for duration of 5–10 min [2]. Is this clinically significant for us as providers? As stated earlier, the clinical outcomes have not proven it should always be avoided and its deleterious effect on IOP pales in comparison to airway manipulation [20].
The mechanism by which succinylcholine increases IOP has to do with its site of action on the extraocular muscles. The extraocular muscles have copious neuromuscular junctions. When those numerous junctions are activated by succinylcholine, the repeated depolarization leads to prolonged contraction and therefore higher IOP [2]. It has also been postulated that fasciculation of the orbicularis oculi, ocular venous congestion, and changes in venous return from abdominal fasciculation may also contribute to the rise in IOP [8].
Maintenance
One goal is to maintain a motionless surgical field whether the patient is under MAC or GA.
Most anesthetics, volatile and IV, reduce IOP with the exception of ketamine.
In a study performed by Wadia [21] that looked at IOP changes after administration of ketamine in sample size of 60 children, only mild increases in IOP were observed ranging from 0 to 8 mmHg with a median change of 3 mmHg. Only 15 children experienced a brief increase of 5 mmHg or greater increase in IOP. Antal’s study of the effect of ketamine on intraocular pressure demonstrated that on average ketamine increases IOP about 7 % [22].
If performing MAC/Local, small boluses of versed, fentanyl, or propofol may be necessary if the patient is uncomfortable and there is risk for movement.
However, the provider should avoid excess narcotic that can cause hypercarbia-induced intraocular hypertension.
If the patient is intubated, maintenance on volatile anesthetics or TIVA with propofol/remifentanyl is suitable.
There are three main physiologic changes created by anesthetic agents:
- 1.
Reduced MAP = less choroid volume
- 2.
Relaxation of extraocular muscles = less wall tension
- 3.
Pupil constriction = better flow of the aqueous humor
The net effect is either no change or a decrease in IOP.
- 1.
Nitrous is dangerous if the surgeon plans to administer intravitreal gas. In retinal detachments, the surgeon can employ a gas bubble (air, sulfur hexafluoride SF6, or octafluoropropane C3F8) to replace vitreous that has leaked out. N2O is 117 times more soluble than SF6 and therefore, if the patient is breathing it after gas bubble placement, the injected gas volume can as much as triple from its original size. Once, the nitrous is discontinued, the bubble will rapidly decrease, and this rapid rise and decline of IOP can culminate in complete retinal detachment [8].
As a rule, one should not use N2O less than 20 min from the time the surgeon plans to instill any intravitreal gas. If the patient is to have any other surgical procedures following intravitreal gas injection, they should avoid N2O for another 3–4 weeks [8]. Furthermore, patients should avoid flying or any activities with rapid pressurization above sea level for the same 3–4 week duration.
If there is any concern for choroid hemorrhage, MAP should be monitored closely to avoid hypertension and increased bleeding [8].
Monitoring
These cases usually involve field avoidance with the patient’s face turned towards the ophthalmologist and away from the anesthesiologist. Capnography and pulse oximetry can provide early signals of hypoxia, circuit disconnects, and airway obstruction when direct visualization of the head and neck may not be possible.
Tachycardia and hypertension may be harbingers of patient movement secondary to discomfort. Therefore, pain medications and anxiolytics should carefully be titrated to avoid any disastrous consequences of a patient moving amidst fine procedures performed on the eye.
EKG monitoring is of utmost importance to detect cardiac arrhythmias.
The Oculocardiac Reflex
The oculocardiac reflex is a key physiologic response we want to avoid in these surgeries. The reflex may be elicited by a number of things: pressure on the globe, surgical traction on the extraocular muscles, conjunctiva, or structures of the orbit, retrobulbar blocks, or the initial eye trauma itself [2].
The cardiac manifestations can range from bradycardia to ventricular arrhythmias or even cardiac arrest. Which patients are most vulnerable? The reflex is most often seen in the pediatric population undergoing strabismus repairs because they employ intermittent surgical traction. However, this risk remains present for any age population and for many different types of eye surgeries.Related posts:
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