In this study, we found that older age, smoking, less frequent tooth brushing and a greater degree of tremor were associated with a higher DMFT score. While it is well acknowledged that schizophrenia patients often present co-morbidity such as metabolic derangements [3
], the clinical relevance of their dental hygiene is still under-recognized [4
]. It is self-evident that poor dental condition could lead to insufficient digestion; moreover, dental decay has been shown to be significantly associated with aspiration pneumonia [24
]. Furthermore, de Oliveira et al. demonstrated an association between poorer oral hygiene and a higher risk of cardiovascular disease in the general population in Scotland (N
]. In addition, poor dental health itself is associated with a lower quality of life as such patients no longer fully enjoy meals [26
]. These findings emphasize the need for attention to dental check-ups for patients with schizophrenia. In fact, as Ponizovsky et al. demonstrated that regular dental examinations and treatment for inpatients in psychiatric hospitals substantially improved the dental health of this population [29
]; consequently, the implementation of on-site dental services needs to be considered.
One of the novel findings in this cross-sectional study is a close association between more severe tremor and a poorer dental condition. This finding seems attributable to the fact that tremor is very likely to result in impaired fine motor movement, which would be expected to hamper smooth tooth brushing that consists of various elaborate movements. Almost all of schizophrenia patients are receiving antipsychotic medications that have a potential to cause parkinsonian symptoms [30
]. Therefore, it is critically important to try to find the lowest possible therapeutic dose of antipsychotics to maintain a patient’s fine motor function that would be needed for smooth brushing skills although antipsychotic dose-reduction clearly needs thorough caution in light of potential clinical worsening [31
More than half the patients brushed their teeth as few times as once or less a day in the present study. Furthermore, as expected, these patients showed higher DMFT scores than those who more frequently brushed their teeth. Poor dental hygiene has been found to be associated with changes in life-style and preferences that were frequently seen in patients with schizophrenia [32
]. Previous findings have demonstrated a lack of sufficient motivation in self-care in patients with schizophrenia [12
]. For example, Jovanovic et al. investigated behaviors and interests associated with dental care in 372 psychiatric inpatients and found that, when compared to healthy people, they visited a dentist less frequently, brushed their tooth for shorter periods and less often, and failed to acknowledge the adverse effects of oral diseases on their general health condition [36
]. These findings are compatible with our results and highlight the importance of regular tooth brushing by patients or a reminder to do so given by their caregivers. In addition, a high smoking rate is another concern in patients with schizophrenia [38
], which was also true for our sample (32.9
%). In the present survey, there was a positive association between smoking and the DMFT score; this is a consistent finding in the literature. Smoking can contribute to poor dental health as it is associated with an increased pocket depth, loss of periodontal attachment and a higher rate of tooth loss [39
]. Thus, it should be noted that smoking not only causes a variety of serious physical illnesses, but it also has the potential to result in poor oral-dental hygiene. These findings are particularly clinically relevant in patients with schizophrenia in light of their high smoking rate.
The results of our study must be interpreted in light of a number of limitations. First, the R squared value of 0.42 indicates that approximately half the variability is still not fully explained by the variables that are contained in our model. It would have been ideal to evaluate other clinical characteristics such as financial condition, nutritional status, and brushing technique that are likely to influence a patient’s dental condition. Second, psychotropic drugs were not included in the models in this study although their potential overall impacts on dental condition such as dry mouth and tremor were taken into consideration. Considering that psychotropic regimens are very unlikely to be constant over years [40
], this important issue clearly warrants further investigations in a longitudinal fashion. Third, a possibility of selection bias cannot entirely be rejected. Patients who did not agree to take part in this survey were more likely to be reluctant to have a dental check-up because of their poor dental health that they were already aware of. However, even if this holds true, the actual DMFT score in patients with schizophrenia would be worse than the results of this study, which would further emphasize the need for dental care in this population. Fourth, some variables would have been better to be addressed longitudinally. For instance, only current smoking status was evaluated, which precludes any speculation on the plausible cumulative dose–response analysis. Likewise, tremor is expected to take some time to finally translate into observable changes in the teeth. Fifth, we approached only inpatients with schizophrenia in the units whose consultant psychiatrists agreed to this survey; therefore, the sample did not always represent the general population with schizophrenia. Finally, due to the nature of the cross-sectional study design, a causal relationship between clinical characteristics associated with a higher DMFT score and a poor dental condition cannot be unequivocally established. This needs to be addressed in prospective trials.