Abstract
Aim
Oral function in patients with schizophrenia has not been well‐characterized. To address this, we performed a cross‐sectional study of oral function in Japanese inpatients with schizophrenia.
Methods
We measured oral function, including occlusal force, tongue–lip motor function, tongue pressure, and masticatory function in 130 Japanese inpatients with schizophrenia. We then compared the frequency of clinical signs of oral hypofunction among 63 non‐elderly and 67 elderly inpatients with schizophrenia, as well as data from 98 elderly control participants from a previous Japanese study.
Results
The frequency of reduced occlusal force was significantly higher in the elderly inpatients (76.2%) than in the non‐elderly inpatients (43.9%) and elderly controls (43.9%). The frequency of decreased tongue–lip motor function in non‐elderly inpatients (96.8%) and elderly inpatients (97.0%) was significantly higher than that in elderly controls (56.1%). The frequency of decreased tongue pressure in non‐elderly inpatients (66.1%) and elderly inpatients (80.7%) was significantly higher than that in elderly controls (43.9%). Finally, the frequency of decreased masticatory function was highest in elderly inpatients (76.5%), followed by non‐elderly inpatients (54.8%) and elderly controls (15.3%).
Conclusion
Oral function was decreased in both non‐elderly and elderly Japanese inpatients with schizophrenia compared with elderly controls.
Keywords: masticatory function, occlusal force, schizophrenia, tongue pressure, tongue–lip motor function
Oral function was decreased in both 63 non‐elderly and 67 elderly Japanese inpatients with schizophrenia compared with 97 elderly controls.
1. INTRODUCTION
Although patients with schizophrenia have been found to have poor oral health, the factors related to this issue are not well‐understood. 1 Several studies have reported that patients with schizophrenia lose more teeth than those in the general population, 2 , 3 , 4 , 5 and systematic reviews have demonstrated that tooth loss is the most frequently reported risk factor for oral dysfunction. 6 , 7 Therefore, oral function may be indeed compromised in patients with schizophrenia. However, few studies have examined this topic. 8 , 9 , 10 One study found that occlusal force was lower in 20 Brazilian patients with schizophrenia compared with 20 control individuals. 8 Oral moisture, a measure of oral dryness, was lower in 70 Japanese patients with schizophrenia reporting xerostomia compared with 81 patients who did not report xerostomia. 9 Furthermore, oral hygiene tended to be poor in 249 Japanese inpatients with schizophrenia. 10 Despite these findings, additional studies are needed to comprehensively investigate oral function in patients with schizophrenia.
To address this in this study, we assessed oral hypofunction in Japanese patients with schizophrenia. Oral hypofunction is defined according to the presentation of seven clinical signs: poor oral hygiene, oral dryness, reduced occlusal force, decreased tongue–lip motor function, decreased masticatory function, and deterioration of swallowing function. 11 Individuals are diagnosed with oral hypofunction if they meet the criteria for three or more clinical signs. 11 Oral hygiene, oral dryness, and swallowing function are measured according to the number of microorganisms on the tongue dorsum, the results of an oral moisture checker, and responses to a self‐report questionnaire, respectively. 11 However, it could be difficult to reliably assess these three measurements in inpatients with schizophrenia. In addition, oral hygiene and swallowing function might not play a major role in the diagnosis of oral hypofunction. 12 Therefore, we did not examine oral hygiene, oral dryness, or swallowing function in this cross‐sectional study. Instead, we assessed oral function, including occlusal force, tongue–lip motor function, tongue pressure, and masticatory function, in 130 Japanese inpatients with schizophrenia. In addition, we compared the frequency of clinical signs of oral hypofunction among non‐elderly and elderly patients with schizophrenia, as well as elderly controls.
2. METHOD
2.1. Participants
A convenience sample of patients with schizophrenia aged 20 years and older who had been admitted more than 1 year prior to the start of the study were recruited from six psychiatric hospitals in Niigata Prefecture, as previously described. 3 All patients were diagnosed in accordance with the criteria for schizophrenia from the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition.
We excluded patients who were considered by their psychiatrist to be unable to give adequate informed consent. Of the remaining eligible candidates, we included patients who gave adequate written informed consent. Of 212 inpatients with schizophrenia in our previous study, 3 84 participated in this study. We also recruited an additional 46 inpatients with schizophrenia.
2.2. Measurements
A psychiatrist or nurse examined each patient and measured the number of remaining teeth. Psychiatrists or dentists measured oral function, including the maximum occlusal force, tongue–lip motor function, maximum tongue pressure, and masticatory function.
The maximum occlusal force was measured using pressure‐sensitive film (Dental Prescale II, GC Corporation, Tokyo, Japan). The participants were asked to bite the film as hard as they could for 3 s. 13 Decreased occlusal force was defined as that less than 500 N. 12 , 14
Tongue–lip motor function was measured via an automatic measuring device (Kenko‐kun Handy, Takei Scientific Instruments Co., Ltd., Niigata, Japan). The participant was asked to repeat the syllable /pa/, /ta/, or /ka/ as fast as possible for 5 s. 15 A frequency of less than 30 (less than 6 times per second) for any these was considered to indicate decreased function. 11
We evaluated tongue pressure using the following method. A tongue pressure probe connected to a tongue pressure measuring device (JMS tongue pressure measuring device TPM‐01, JMS Co., Ltd., Hiroshima, Japan) was placed between the tongue and palate of the participant, who was asked to crush the probe with maximum tongue pressure for 7 s. 16 Low tongue pressure was that less than 30 kPa. 11
Masticatory function was evaluated via the masticatory performance test, in which participants chewed a gummy jelly (UHA Mikakuto Co., Ltd., Osaka, Japan) 30 times and spit it into a white polystyrene box. 17 The surface area was then calculated via image analysis. A gummy jelly with a surface area of less than 1775 square millimeters was considered to indicate decreased masticatory function. 11 , 18
The control participants were 98 community‐dwelling independent individuals aged 65 years and older from a previous Japanese study. 14 For the control participants, we obtained data regarding the age, sex, and frequency of clinical signs of oral hypofunction, including occlusal force, tongue–lip motor function, tongue pressure, and masticatory function.
2.3. Statistical analysis
We performed the Pearson's correlation analysis between variables (age, sex, number of remaining teeth, occlusal force, tongue–lip motor function, tongue pressure, and masticatory function) in inpatients with schizophrenia. Subsequently, the participants were assigned to 1 of 2 groups according to their age: non‐elderly inpatients (less than 65 years) and elderly inpatients (65 years and older). We used Fisher's exact tests to compare the frequency of clinical signs of oral hypofunction, including reduced occlusal force, decreased tongue–lip motor function, decreased tongue pressure, and decreased masticatory function between non‐elderly and elderly inpatients, non‐elderly inpatients and elderly controls, and elderly inpatients and elderly controls. We used the Benjamini–Hochberg correction for multiple testing. The significance level was set at <0.05, and false discovery rate detection was applied.
We conducted statistical analyses using the Statistical Package for the Social Sciences (SPSS) version 28 (IBM Japan, Tokyo, Japan).
3. RESULTS
The study participants comprised 130 inpatients with schizophrenia (79 men and 51 women; mean age 62.7 [standard deviation; SD: 12.2] years; Table 1). Table 1 also shows the number of remaining teeth, occlusal force, tongue–lip motor function, tongue pressure, and masticatory function in 130 inpatients with schizophrenia.
TABLE 1.
Characteristics of inpatients with schizophrenia.
| Characteristics | All | Non‐elderly | Elderly |
|---|---|---|---|
| Age (years) | 62.7 ± 12.2 (N = 130) | 52.4 ± 8.3 (N = 63) | 72.5 ± 5.2 (N = 67) |
| Sex (men/women) | 79/51 | 39/24 | 40/27 |
| Number of remaining teeth | 16.3 ± 11.1 (N = 128) | 21.7 ± 8.6 (N = 63) | 11.0 ± 10.8 (N = 65) |
| Occlusal force (N) | 516.1 ± 446.5 (N = 83) | 645.3 ± 438.8 (N = 41) | 390.0 ± 421.8 (N = 42) |
| Tongue–lip motor function (counts of /pa/ per second) | 4.2 ± 1.7 (N = 128) | 4.5 ± 1.7 (N = 62) | 3.9 ± 1.6 (N = 66) |
| Tongue–lip motor function (counts of /ta/ per second) | 4.3 ± 1.8 (N = 128) | 4.7 ± 1.6 (N = 62) | 3.8 ± 1.8 (N = 66) |
| Tongue–lip motor function (counts of /ka/ per second) | 3.5 ± 1.6 (N = 128) | 3.6 ± 1.6 (N = 62) | 3.3 ± 1.5 (N = 66) |
| Tongue pressure (kPa) | 19.7 ± 12.8 (N = 119) | 23.3 ± 11.8 (N = 62) | 15.7 ± 12.8 (N = 57) |
| Masticatory function (mm2) | 1441.4 ± 1268.0 (N = 113) | 1654.7 ± 1318.7 (N = 62) | 1182.1 ± 1164.0 (N = 51) |
Note: Data are expressed as mean ± standard deviation.
The correlation analysis revealed that age was significantly negatively correlated with the number of remaining teeth, occlusal force, tongue pressure, and masticatory function (r = −0.533, −0.397, −0.341, and −0.300, respectively; Table 2). Tongue pressure was significantly lower in women than in men (r = −0.229). The number of remaining teeth was significantly correlated with the occlusal force, tongue–lip motor function (/ta/), and masticatory function (r = 0.485, 0.210, and 0.497, respectively). Occlusal force was significantly correlated with masticatory function (r = 0.607). There were significant correlations between tongue–lip motor function for syllables /pa/ and /ta/ (r = 0.744), /pa/ and /ka/ (r = 0.699), and /ta/ and /ka/ (r = 0.732). Tongue–lip motor function for syllables /pa/ and /ta/ were significantly correlated with tongue pressure (r = 0.231 and 0.351, respectively).
TABLE 2.
Correlation coefficients and p values between variables in inpatients with schizophrenia.
| Variable | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|---|---|---|---|---|---|---|---|---|---|
| 1. Age | – | 0.079 | −0.533 | −0.397 | −0.123 | −0.201 | −0.036 | −0.341 | −0.300 |
| 2. Sex | 0.370 | – | 0.038 | −0.067 | 0.003 | 0.054 | 0.150 | −0.229 | 0.002 |
| 3. Number of remaining teeth | <0.001 | 0.673 | – | 0.485 | 0.183 | 0.210 | 0.183 | 0.134 | 0.497 |
| 4. Occlusal force | <0.001 | 0.545 | <0.001 | – | 0.087 | 0.142 | −0.042 | 0.214 | 0.607 |
| 5. Tongue‐lip motor function (/pa/) | 0.168 | 0.970 | 0.039 | 0.439 | – | 0.744 | 0.699 | 0.231 | 0.143 |
| 6. Tongue–lip motor function (/ta/) | 0.023 | 0.542 | 0.018 | 0.206 | <0.001 | – | 0.732 | 0.351 | 0.095 |
| 7. Tongue–lip motor function (/ka/) | 0.687 | 0.091 | 0.040 | 0.713 | <0.001 | <0.001 | – | 0.172 | 0.080 |
| 8. Tongue pressure | <0.001 | 0.012 | 0.149 | 0.057 | 0.012 | <0.001 | 0.063 | – | 0.204 |
| 9. Masticatory function | 0.001 | 0.984 | <0.001 | <0.001 | 0.134 | 0.320 | 0.407 | 0.034 | – |
Note: Correlation coefficients and p values are provided above and below the diagonal line, respectively. Bold values indicate the statistical significance after applying the Benjamini–Hochberg correction for multiple testing.
Of the 130 participants, 63 were not elderly persons (39 men and 24 women; mean age 52.4 [SD: 8.3] years) and 67 were elderly persons (40 men and 27 women; mean age 72.5 [SD: 5.2] years; Table 1). We obtained data regarding the frequency of clinical signs of oral hypofunction, including occlusal force, tongue–lip motor function, tongue pressure, and masticatory function in 98 elderly controls (33 men and 65 women; mean age 74.8 [SD: 6.3] years) from a previous Japanese study. 14 We compared the frequency of clinical signs of oral hypofunction between non‐elderly and elderly inpatients, non‐elderly inpatients and elderly controls, and elderly inpatients and elderly controls (Table 3). Reduced occlusal force was significantly more common in elderly inpatients (76.2%) than in non‐elderly inpatients (43.9%) and elderly controls (43.9%). Decreased tongue–lip motor function in non‐elderly inpatients (96.8%) and elderly inpatients (97.0%) was significantly higher than that in elderly controls (56.1%). The frequency of decreased tongue pressure in non‐elderly inpatients (66.1%) and elderly inpatients (80.7%) was significantly higher than that in elderly controls (43.9%). The frequency of decreased masticatory function was highest in elderly inpatients (76.5%), followed by non‐elderly inpatients (54.8%) and elderly controls (15.3%).
TABLE 3.
Frequency of clinical signs of oral hypofunction in non‐elderly and elderly inpatients with schizophrenia and elderly controls.
| Clinical signs | Group | p | ||||
|---|---|---|---|---|---|---|
| Non‐elderly inpatients (NI) | Elderly inpatients (EI) | Elderly controls (EC) | NI vs. EI | NI vs. EC | EI vs. EC | |
| Reduced occlusal force | 18/41 (43.9%) | 32/42 (76.2%) | 43/98 (43.9%) | 0.004 | 1.000 | <0.001 |
| Decreased tongue–lip motor function | 60/62 (96.8%) | 64/66 (97.0%) | 55/98 (56.1%) | 1.000 | <0.001 | <0.001 |
| Decreased tongue pressure | 41/62 (66.1%) | 46/57 (80.7%) | 43/98 (43.9%) | 0.098 | 0.009 | <0.001 |
| Decreased masticatory function | 34/62 (54.8%) | 39/51 (76.5%) | 15/98 (15.3%) | 0.019 | <0.001 | <0.001 |
Note: Bold values indicate the statistical significance after applying the Benjamini–Hochberg correction for multiple testing.
4. DISCUSSION
To the best of our knowledge, this is the first study to evaluate oral function in patients with schizophrenia using a multifaceted strategy. We found that the number of remaining teeth was significantly associated with occlusal force, tongue‐lip motor function, and masticatory function in inpatients with schizophrenia. Our results are in line with previous studies showing that tooth loss is a risk factor for oral dysfunction in the general population. 6 , 7 The number of remaining teeth has been proposed as an alternative method for testing occlusal force. 11 However, this was only weakly or moderately correlated with oral function in our inpatients with schizophrenia. Therefore, we evaluated not only the number of remaining teeth but also oral function in patients with schizophrenia.
In the current study, we found that elderly inpatients with schizophrenia had clinical signs of oral hypofunction with a higher frequency compared with elderly controls. Age was significantly correlated with oral function in our inpatients with schizophrenia, as previously observed in outpatients at a dental hospital clinic. 12 Nevertheless, all clinical signs of oral hypofunction with the exception of reduced occlusal force were more frequent in inpatients with schizophrenia, including non‐elderly participants, compared with elderly controls. It is possible that poor oral self‐care behaviors and extrapyramidal symptoms induced by antipsychotics lead to decreased oral function in both non‐elderly and elderly inpatients with schizophrenia. Previously, age, smoking, infrequent toothbrushing, and the severity of tremor were associated with poor dental condition in 523 Japanese inpatients with schizophrenia. 19 Furthermore, a prospective study demonstrated that poor oral status could predict sarcopenia and mortality in 2011 community‐dwelling elderly Japanese individuals. 20 A meta‐analysis indicated that a high proportion of Japanese inpatients with schizophrenia were underweight, 21 and patients with schizophrenia have been found to have higher mortality compared with the general population. 22 Taken together, these data indicate that oral hypofunction may be a risk factor for underweight status and increased mortality in patients with schizophrenia. Oral function could be maintained by ensuring a high quality of oral care from an early stage of life, 23 , 24 and this may prevent the observed decrease in life expectancy in patients with schizophrenia.
Our study had some limitations. First, instead of dentists, psychiatrists measured the oral function and number of remaining teeth in the participant group. However, dentists provided instruction to the psychiatrists regarding how to measure oral function. Therefore, we cannot ensure the reliability of these measurements. Second, we were unable to obtain all measurements of oral function in the 130 inpatients with schizophrenia. For example, data regarding occlusal force were obtained from 83 inpatients who were able to follow the task instructions. Third, we did not evaluate potential risk factors (e.g., the frequency of toothbrushing, extrapyramidal symptoms, physical illness, and use of antipsychotics or anticholinergics) for oral hypofunction.
In conclusion, our results demonstrate that oral function is decreased in both non‐elderly and elderly Japanese inpatients with schizophrenia compared with elderly controls.
AUTHOR CONTRIBUTIONS
Y.W., M. Otake, K. Murakami, K.H., and T.S. designed the study. M. Otake, M. Ootake, K. Murakami, K.K., K. Matsuzawa, and H.K. collected the data. M. Otake and S.O. performed the statistical analyses. Y.W. and M. Otake wrote the first draft of the manuscript. All authors contributed to the revision of the manuscript, and approved the final manuscript.
FUNDING INFORMATION
This work was supported by a grant from the Medical Association of Niigata City (to M. Otake). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.
ETHICS STATEMENT
Approval of the Research Protocol by an Institutional Review Board: This study was approved by the Ethics Committee of Niigata University (Approval No. 2020‐0191) and was conducted in accordance with the Declaration of Helsinki.
Informed Consent: Written informed consent was obtained from all participants.
Registry and Registration Number of the Study/Trial: N/A.
Animal Studies: N/A.
ACKNOWLEDGMENTS
The authors greatly appreciate the involvement of all participants and staff at the participating psychiatric hospitals: Shirone Midorigaoka Hospital, Kohdo Hospital, Seki Hospital, Niigata Psychiatric Center, Arita Hospital, and Saigata Medical Center. We thank Sydney Koke, MFA, from Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.
Watanabe Y, Otake M, Ono S, Ootake M, Murakami K, Kumagai K, et al. Decreased oral function in Japanese inpatients with schizophrenia. Neuropsychopharmacol Rep. 2024;44:356–360. 10.1002/npr2.12443
DATA AVAILABILITY STATEMENT
All relevant data are provided in the manuscript. We are not able to make the underlying data available to readers, because we do not have permission from the participating institutions to do so.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
All relevant data are provided in the manuscript. We are not able to make the underlying data available to readers, because we do not have permission from the participating institutions to do so.