**3. Epidemiology of the trigeminocardiac reflex**

The TCR occurs with mechanical/thermal manipulation around any of the branches of the trigeminal nerve [9, 10, 19–23], with specific prevalence in specific anatomical locations [1]. The OCR being a sub-variant of the peripheral TCR is studied extensively earlier and was said to occur in up to 67% after ophthalmic surgery [24]. Because of publication bias, the real prevalence may be substantially smaller, even the peripheral TCR has generally a higher prevalence than the central TCR. According to the senior author Schaller's experience, the central TCR occurs in up to 10–18% of the patients [1, 25–27]. In a retrospective time-series review of 125 patients operated for tumours of the cerebello-pontine angle, Schaller et al. noticed the TCR occurrence in 11%. [4] Three of these patients in this series developed asystole which lasted from 30 to 70 seconds [1]. In contrast to earlier studies on OCR, Schaller – for the first time – took into consideration both the heart rate and blood pressure and defined TCR as heart rate and mean arterial blood pressure (MABP) 20% lower than the baseline [1].

In another retrospective time-series study, Schaller also showed the TCR occurrence during microvascular decompression of the trigeminal nerve for trigeminal neuralgia [27]. In this review on 28 patients, the prevalence of TCR was up to 18% with the same definition used as in his prior above-mentioned study [27]. TCR was also reported during transsphenoidal surgery for pituitary adenoma [11, 26, 28]. Among 117 patients who underwent transsphe‐ noidal surgery for pituitary adenoma, 10% developed a TCR during the surgical procedure [26]. Peripheral stimulation of the nasopharynx may also lead to (peripheral) TCR [11].

In several other neurosurgical procedures, there exists only case reports or small case series, so that a robust prevalence of TCR occurrence is not (yet) known.
