Medial Rectus Manipulation Most Frequently Triggers the Oculocardiac Reflex
Manipulation of the medial rectus muscle is the most common trigger of the oculocardiac reflex during strabismus surgery, though all extraocular muscles can elicit this response. 1, 2, 3
Evidence for Medial Rectus as Primary Trigger
The medial rectus muscle demonstrates the highest reflexogenic potential among extraocular muscles:
The medial rectus has a lower threshold for triggering bradycardia compared to other extraocular muscles, requiring less tension to elicit the oculocardiac reflex. 2
Asystole has been specifically documented during medial rectus traction, representing the most severe manifestation of this reflex. 3
The oculocardiac reflex occurs in 67.9% of cases during extraocular muscle traction or suture adjustment in strabismus surgery, with the medial rectus being the most frequently manipulated muscle. 1
Comparative Reflexogenic Potential
While the medial rectus is most commonly implicated, the hierarchy of muscle involvement includes:
The medial rectus and inferior oblique muscles consistently trigger the reflex in all patients when stretched, though the medial rectus requires less tension. 2
The lateral rectus muscle elicits bradycardia in only 7 of 15 patients (47%) even with tensions up to 600g, demonstrating lower reflexogenic potential than the medial rectus. 2
One older study using controlled traction techniques found no significant difference between medial rectus and other extraocular muscles, though this contradicts more recent quantitative analyses and may reflect methodological limitations. 4
Clinical Characteristics of the Reflex
The oculocardiac reflex demonstrates specific patterns during muscle manipulation:
The reflex is a graded phenomenon—as tension increases on the extraocular muscle, bradycardia occurs more rapidly and becomes more profound. 2
Abrupt and sustained traction (square wave stimuli) is significantly more reflexogenic than gradual, progressive traction (slow slope stimuli). 4
Patients experiencing oculocardiac reflex during manipulation of the first muscle in multiple-muscle surgery are significantly more likely to experience it again during subsequent muscle manipulation (χ² = 36.681, P < 0.001). 5
Important Clinical Caveats
Several factors modify the risk and severity of this reflex:
Profound bradycardia occurs in almost 10% of strabismus surgery cases without anticholinergic preventive measures, emphasizing the clinical significance of this reflex. 6
The absence of fine stereopsis preoperatively and surgery on a larger number of muscles are the best predictors of oculocardiac reflex occurrence. 5
Hypercapnia is an important adjuvant factor that potentiates the oculocardiac reflex, making controlled ventilation preferable to spontaneous ventilation. 4
Systemic atropine administration completely prevents bradycardia from occurring, though routine prophylactic use remains debated. 2