## 4 Discussion  
Postoperative delirium after spinal surgery in elderly patients has been a recent concern, and several attempts to achieve effective and objective diagnoses of delirium have been reported. However, to the best of our knowledge, there has not been a study of delirium after spinal surgery based on EEG signals from a compact wearable device. Using EEG signals from such a device, we found greater changes in H-beta, gamma, and theta waves and tension index within 1 week after surgery from baseline in the delirium group compared to the non-delirium group. Based on these results, EEG analysis using a wearable device appears to be an effective diagnostic tool and may be applicable during the early diagnosis of delirium after spinal surgery. While postoperative delirium has been an increasing concern for spinal surgeons recently, it remains an elusive concept and difficult to diagnose [32]. A diagnosis of delirium is based on the patient's clinical history, behavioral observations, and cognitive assessments [1]. It is not easy for spinal surgeons and nurses to recognize delirium, and even psychiatrists may overlook or misdiagnose this disorder. Although there are clinical assessment tools for delirium, novel diagnostic methods--such as biomarkers and wearable accelerometer devices--are being actively studied [16; 33; 34; 35]. The association between EEG changes and delirium has been studied, but there are few published reports. EEG has been mainly used to differentiate delirium from non-convulsive status epileptics or other psychiatric conditions [36; 37]. Bispectral index monitoring and adjustments of the depth of anesthesia appear to correlate with reduced postoperative delirium [38; 39]. Since [PERSON] et al. first reported reduced alpha waves and increased theta and delta waves in patients with delirium [40], other studies have followed. In a study of 28 patients with delirium who underwent cardiothoracic surgery, an increase in delta waves from the frontal and parietal lobes was the only difference between patients with and without delirium [18]. [PERSON] et al. reported high mean global field power--mainly driven by delta wave activity--in septic patients with delirium and  
Figure 3: Mean percentage changes in (**A**) H-beta waves, (**B**) gamma waves, (**C**) tension index, and (**D**) theta waves within 1 week and 3 months after surgery compared to baseline in patients with and without delirium. The error bars shown are standard errors of the mean. proposed continuous EEG monitoring for its diagnosis [41]. However, [PERSON] et al. pointed out that the routine use of EEG monitoring for delirium screening is time-consuming and inefficient [17]. In the present study, we found differences in specific EEG waves between patients with and without delirium after spinal surgery. H-beta and gamma waves, which are known to be related to stress [24], increased in patients with delirium. Moreover, the tension index--which consists of alpha and H-beta waves--increased, whereas theta waves decreased, in patients with delirium. These results are somewhat different from those of published studies. Most studies of delirium were performed in ICUs [18; 41; 42]; however, our patients did not require ventilators or ICU care. We believe that patients in the ICU are more likely to show subdued and hypoactive features than those in the general ward, possibly due to the severity of their illness [43]. Therefore, slow waves such as delta and theta waves are more likely to be predominant in ICU patients. On the other hand, restless and agitated states, which are commonly seen in hyperactive delirium, correlate well with features of H-beta and gamma waves [44; 45]. Confused, anxious, and irritable behaviors are commonly seen in patients with delirium in the general ward and are a concern for most spinal surgeons and nurses. We believe that our results accurately reflect these aspects of patients with delirium.