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The oculocardiac reflex (OCR) is a trigeminal-vagal reflex characterized mainly by bradycardia, triggered by mechanical stimulation of the extraocular muscles, eyeball, or orbit. OCR was first proposed by Aschner and Dagnini in 1908, and it is generally accepted that a heart rate decrease of more than 20% from the baseline indicates the occurrence of OCR. Most OCR cases resolve spontaneously after stopping surgical manipulation, but severe OCR can lead to significant bradycardia, arrhythmias, or even cardiac arrest, and in rare cases, death[1,2]. Strabismus is a common ophthalmic condition, with a prevalence of approximately 0.8% to 6.8%[3]. Strabismus correction surgery is the most routine and effective treatment, and intraoperative traction on the eye muscles is an unavoidable step, with an OCR occurrence rate as high as 93%[4]. Therefore, it must be taken very seriously. This article reviews the factors influencing OCR during strabismus surgery and preventive measures.
1. Factors Influencing OCR During Strabismus Surgery
(1) Anesthetic Factors
1. Intravenous Anesthetics
A large observational study indicated that the use of opioids and dexmedetomidine increases the incidence of OCR during strabismus surgery[5], especially remifentanil[6,7]; dexmedetomidine increases the incidence of OCR whether administered nasally preoperatively[8] or intravenously[9], as both drugs reduce sympathetic nervous system stimulation and enhance parasympathetic nervous system stimulation[10,11]. Studies show that propofol can increase the incidence of OCR during strabismus surgery[12,13,14]. Additionally, muscle relaxants also have an impact on OCR during strabismus surgery. Research indicates that the use of rocuronium can reduce the incidence of OCR[15,16,17]; however, Arnold et al.[5] found that rocuronium increases the incidence of OCR during strabismus surgery, while succinylcholine and atracurium reduce the incidence; another small randomized study showed that atracurium significantly increased the incidence and severity of bradycardia, arrhythmias, and atropine usage compared to pancuronium[18].
2. Inhalation Anesthetics
Picker et al.[19] compared the effects of different inhalation anesthetics on canine heart rates and found that desflurane and sevoflurane had the least impact on heart rate, isoflurane and enflurane were intermediate, and halothane had the greatest effect. In strabismus surgery, inhalation anesthetics have a minor impact on the oculocardiac reflex, with halothane leading to a higher incidence of OCR than sevoflurane, likely due to sevoflurane’s superior suppression of vagal activity[20]. Desflurane is the only volatile agent that increases sympathetic activity[21]. Studies show that desflurane does not increase the incidence of OCR during strabismus surgery compared to sevoflurane[22].
3. Depth of Anesthesia
The depth of anesthesia is closely related to OCR. Systematic reviews indicate that light anesthesia is a risk factor for trigeminal cardiac reflex during surgery[23]. Studies show that in strabismus surgery, maintaining a BIS value of 60 significantly increases the incidence of OCR compared to maintaining a BIS value of 40-50[24,25]; further research confirms that deeper anesthesia results in a lower incidence of OCR, possibly due to the effects of anesthetics on the cerebral cortex and subcortical areas, where increased anesthetic can act on subcortical areas to suppress harmful and autonomic reflexes[26,27].
4. Others
To prevent postoperative nausea and vomiting, dexamethasone and ondansetron are often used prophylactically during strabismus surgery; however, some studies indicate that these can increase the incidence of OCR[5,28]. Other studies suggest that administering pentobarbital after anesthesia induction can reduce the incidence of both postoperative nausea and OCR[29], with no significant effects on heart rate[30]. Blanc et al.[31,32] indicated that hypercapnia is an important influencing factor for OCR during strabismus surgery. Arnold et al.[5] noted that an ocular respiratory reflex may occur during muscle traction, causing a decrease in respiratory rate or shallow breathing, which may lead to hypercapnia and hypoxemia, potentially resulting in OCR.
(2) Surgical Factors
The type of muscle involved in surgery, the strength of muscle traction, and the order of muscle traction are closely related to the occurrence of OCR. Most studies suggest that the medial rectus muscle is the most likely to induce OCR during strabismus surgery, while the lateral rectus muscle has the lowest incidence of OCR, likely due to the medial rectus being the thickest and strongest of all extraocular muscles, resulting in a particularly strong afferent reflex[25,33,34]; however, some studies point out that the occurrence of OCR is not related to specific eye muscles involved in surgery[4,35,36]. The greater the degree of muscle traction, the higher the incidence of OCR[37,38]; deeper positioned muscles, such as the inferior oblique, require greater traction, making OCR more likely[39]; Aletaha et al.[40] found that the incidence of OCR is higher during vertical muscle strabismus surgery compared to horizontal muscle surgery, possibly due to the more difficult exposure of vertical muscles requiring stronger traction. The order of muscle traction during surgery is also related to the occurrence of OCR. Studies indicate that when two or more muscles are involved in surgery, the incidence of OCR for the first muscle is higher than for subsequent muscles[4,35,41], possibly due to a compensatory process in the patient after the first OCR occurs[42]. Therefore, it is particularly important to monitor heart rate changes when tractioning the first muscle during surgery.
(3) Patient Factors
Patient age is closely related to the occurrence of OCR during strabismus surgery. Studies indicate that children are at higher risk for OCR during strabismus surgery[33,34,40,43,44]; Tramer et al.[45,46] found that prophylactic use of high doses of atropine in adults and children undergoing strabismus surgery still resulted in increased OCR incidence in children, possibly due to stronger vagal tone in children[40]. Arnold et al.[34] also found that race affects OCR incidence, with Caucasians experiencing more bradycardia than Blacks and Asians; additionally, the study found that iris color does not predict OCR occurrence[39], which contradicts Fry et al.[47] who suggested that patients with brown or light brown eyes are more prone to OCR than those with blue or gray eyes.
2. Preventive Measures for OCR During Strabismus Surgery
(1) Anesthetic Methods
Research shows that ketamine can reduce the incidence of OCR in children undergoing strabismus surgery[12,14], and its efficacy in reducing OCR incidence is even greater than that of atropine[48,49]. Ketamine is considered the only intravenous anesthetic that stimulates the sympathetic nervous system, selectively inhibiting the brain and thalamus while relatively stimulating the medulla and limbic system, leading to increased heart rate, cardiac output, and arterial blood pressure. Its excitatory effect on the circulatory system is believed to be due to sympathetic stimulation and vagal inhibition, thus ketamine is recommended for anesthesia in strabismus surgery to reduce OCR incidence[50] and to prevent adverse effects of arrhythmias during surgery[48]. Some studies indicate that there is no significant difference in OCR incidence between midazolam and ketamine during strabismus surgery[51], and benzodiazepines can reduce OCR incidence[17]. Additionally, intravenous anesthetics that suppress sympathetic nervous system activity or stimulate vagal activity, such as dexmedetomidine and remifentanil, should be avoided during surgery.
2. Inhalation Anesthetics
Studies indicate that sevoflurane and desflurane are safe and effective for strabismus surgery, with minimal impact on OCR[22]. When selecting inhalation anesthetics, it is recommended to use sevoflurane and desflurane while avoiding halothane, which significantly affects heart rate[19]. Several studies comparing inhalation anesthetics and intravenous anesthetics in strabismus surgery suggest that inhalation anesthetics have less impact on OCR than intravenous anesthetics, but may increase postoperative nausea and vomiting[13,50,52,53,54].
3. Nerve Blocks
Research shows that retrobulbar block[55], peribulbar block[56,57], Sub-Tenon’s block[58,59], and trochlear nerve block[60] can effectively reduce the incidence of OCR during strabismus surgery. Retrobulbar and peribulbar blocks may lead to complications such as vision loss, globe perforation, nerve damage, and retrobulbar hemorrhage[61,62], while Sub-Tenon’s block provides a safer anesthesia option and minimizes the risk of severe complications[58,63]. Misurya et al.[64] found that preoperative use of atropine combined with retrobulbar block is very effective in preventing OCR during strabismus surgery.
(2) Anticholinergic Drugs
Prophylactic use of the anticholinergic drug atropine can effectively reduce the incidence of OCR[65], and the sublingual and intravenous routes are more effective than oral and intramuscular injection[66]. However, atropine may cross the blood-brain barrier leading to side effects such as postoperative delirium and facial flushing, which are symptoms of anticholinergic syndrome[67,68,69]. Additionally, the potential arrhythmias caused by atropine may be more difficult to control[70,71], thus routine prophylactic use of atropine is not recommended, especially in pediatric ophthalmic surgery[1]. However, atropine remains a common medication for treating bradycardia when OCR occurs.
(3) Others
In addition to anesthetic management and prophylactic use of anticholinergic drugs, gentle intraoperative manipulation, avoiding excessive muscle traction and pressure on the eyeball and orbit are key to preventing and alleviating OCR. When tractioning muscles, it should be done gently, and necessary traction and compression should be performed intermittently, waiting for the patient’s heart rate to return to baseline before proceeding with surgery. However, once OCR occurs, the heart rate is unlikely to fully return to baseline at any point during surgery, making the clinical value of gentle surgical manipulation in reducing OCR potentially lower than that of pharmacological intervention[38]. Additionally, maintaining sufficient depth of anesthesia during surgery is crucial to avoid hypoxemia and hypercapnia, both of which help reduce the incidence of OCR.
3. Summary and Outlook
OCR is the most common complication during strabismus surgery, with factors influencing its occurrence primarily including the use of dexmedetomidine and opioids, light anesthesia (BIS value > 60), hypercapnia and hypoxemia, children, muscle type involved in surgery, and the strength of muscle traction. Currently, ketamine, sevoflurane and desflurane, nerve blocks, anticholinergic drugs, maintaining normoxia and normocapnia, and ensuring sufficient depth of anesthesia during surgery are the most common methods to prevent OCR. However, OCR remains unavoidable in strabismus surgery; the most effective way to interrupt OCR is to stop traction on the eye muscles, but repeated interruptions during surgery can increase operation time and cause anxiety among staff. Therefore, further exploration is needed on how to effectively block the occurrence of OCR during strabismus surgery.
OCR is a series of adverse reactions caused by nerve conduction; low temperatures can reduce nerve conductivity[72], and perhaps using low-temperature techniques to suppress nerve conduction to prevent OCR is a promising new preventive measure.
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Editor: Michel. Mi Xuan
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