Relations between obstructive sleep apnea syndrome and specific cephalometric measurements, body mass index, and apnea-hypopnea index

Anatomic Changes of Patients With Obstructive Sleep Apnea Syndrome at Different Stages

AIM

In this study, it was aimed to determine the changes in the anatomic structures of individuals with obstructive sleep apnea syndrome (OSAS) classified according to the apnea-hypopnea index (AHI).

MATERIALS AND METHODS

Individuals were divided into groups as group 1 (AHI=0, n=20), group 2 (AHI ˂5, n=20), group 3 (AHI=5-15, n=20), group 4 (AHI=16-30, n=20), group 5 (AHI ˃30, n=20). The individuals left lateral cervical vertebra radiographs were taken. Measurements were made from the morphometric points determined using the "Enlil PACS" program on the radiologic images (mm). These measurements are categorized into soft tissue, hyoid bone, airway size, and craniofacial variables.

RESULTS

As AHI increased, age, body mass index, neck circumference, and Epworth Sleep Scale scores increased in all groups. Soft palate and tongue length, hyoid-mandible, hyoid-posterior pharyngeal wall, and hyoid-spina nasalis posterior distances lengthened. An increase was found between spina nasalis anterior, spina nasalis posterior, distance between condylion and gnathion, and total facial length and lower face length in all groups.

CONCLUSIONS

It is thought that the results obtained due to the differentiation in anatomic structures may help the methods that will facilitate the diagnosis and treatment of OSAS.

Cephalometric Evaluation in Patients with Obstructive Sleep Apnea undergoing Lateral Pharyngoplasty

Introduction  Lateral pharyngoplasty (LP) has shown promising results. Craniofacial deformity reduces the pharyngeal space, contributing to the etiopathogenesis. The analysis of craniofacial features can be performed using cephalometry. Objective  To verify if craniofacial deformity is associated with worse polysomnographic data in patients with obstructive sleep apnea (OSA), and to verify if the preoperative cephalometric parameters can interfere with the surgical success of the LP. Methods  The study included 21 patients with OSA aged between 18 and 65 years who underwent LP in a university hospital from 2015 to 2019. Polysomnography was performed postoperatively, after a minimum period of 6 months from the surgical procedure. In addition, a cephalometric evaluation was performed to assess craniofacial deformity, and to correlate it with the polysomnographic results. Results  The mean and median of all polysomnographic respiratory parameters improved postoperatively, especially the apnea-hypopnea index (AHI), which went from 40.15 to 16.60 events per hour ( p  = 0.001). Of the 21 patients, 15 showed improvements in the AHI postoperatively. As for the cephalometric evaluations, we found that the longer the distance between the hyoid bone and the mandibular plane, the greater the patient's preoperative AHI ( p  = 0.011). When assessing whether cephalometric changes related to craniofacial deformities influence the surgical outcome of LP, no correlation was found for any cephalometric measurement. Conclusion  The longer the distance between the hyoid bone and the mandibular plane, the greater the preoperative AHI, and LP was an effective OSA treatment. Cephalometric variables are not able to modify or determine the success of LP in apneic patients in the population assessed.

The Link between Obstructive Sleep Apnea Syndrome and Cephalometric Assessment of Upper Airways and Hyoid Bone Position

Background and Objectives: To determine whether there are changes in the cephalometric characteristics of the upper airways and hyoid bone, in patients diagnosed with obstructive sleep apnea syndrome (OSAS) compared to a healthy control group. Material and Methods: This retrospective case−control study included 24 patients diagnosed with OSAS (apnea−hypopnea index (AHI) > 5 obtained after polysomnography) and 24 healthy subjects as a control group that completed the STOP-Bang questionnaire to determine whether they had OSAS. Lateral cephalometric examinations were recommended for all these patients. The software used for the cephalogram interpretation was CS 3D Imaging and CS Airway imaging from Carestream Dental. Results: The subjects with OSAS had a smaller superior posterior airway space (SPAS), with an average of 10.32 mm compared to a 12.20 mm mean in the control group (p = 0.03). Patients with OSAS, had a lowered middle airway space (MAS) with a mean of 7.96 mm in the OSAS group and a 10.96 mm mean in the control group (p = 0.00). All the measurements made for the hyoid bone, such as—H-MnP, H-C3, and H-B—showed increased values (means of 26.31 mm, 39.08 mm, 60.05 mm, respectively), for the OSAS group (p = 0.00). Conclusions: Patients suffering from OSAS had reduced dimensions of the SPAS and MAS values. The hyoid bone had a more inferior position in the study group (with increased values for H-MnP, H-C3, and H-B) compared to the control group.

Correlation between Apnea Severity and Sagittal Cephalometric Features in a Population of Patients with Polysomnographically Diagnosed Obstructive Sleep Apnea

Background and Objective: Obstructive sleep apnea (OSA) is a sleep-related breathing disorder featuring a repeated closure of the upper airway during sleep. Craniofacial anatomy is a potential risk and worsening factor for OSA. This study aims to assess the relationship between cephalometric features of craniofacial morphology and OSA severity in a population of patients with OSA. Material and Methods: A sample of forty-two patients (n = 42, M = 76%, mean age = 57.8 ± 10.8) with a polysomnographically (PSG) confirmed diagnosis of OSA were recruited and underwent cephalometric evaluation of 16 cephalometric variables. In addition, the apnea−hypopnea index (AHI), oxygen desaturation (SatMin), Epworth sleepiness scale (ESS), and body mass index (BMI) were assessed. Then t-tests were performed to compare the values of all cephalometric variables between two AHI severity-based groups (mild-to-moderate = AHI ≤ 30; severe = AHI > 30). Single- and multiple-variable regression analyses were performed to assess the associations between AHI scores and cephalometric features. Results: Mean AHI, SatMin, and BMI were 31.4 ev/h, 78.7%, and 28.1, respectively. The cephalometric variables were not significantly different between the two OSA-severity groups (p > 0.05). Multiple-variable regression analyses showed that gonial angle and nasopharynx space were negatively associated with AHI, explaining 24.6% of the total variance. Conclusion: This investigation reported that severity of AHI scores in patients with OSA showed a negative correlation with gonial angle and nasopharynx space. As a general remark, although maxillofacial anatomy can be a predisposing factor for OSA, disease severity depends mainly upon other variables.

Usefulness of Hard Palate Measurements in Predicting Airway Dimensions in Patients Referred for Cone Beam CT

Purpose:

The aim of the study was to evaluate the usefulness of hard palate measurements in predicting airway dimensions in patients referred for cone-beam CT (CBCT).

Materials and Methods:

Six hundred forty-three patients (239 males and 404 females) were examined by CBCT. Using dedicated CBCT software (Kodak CS 3D imaging version 3.8.6, Carestream, Rochester, NY, USA); different hard palate (palatal interalveolar length, palatal arch depth, maxillo-palatal arch angle, and alveolar width) and airway measurements (airway volume, minimum cross-sectional area, minimum anteroposterior distance, minimum right to left distance and airway length) were obtained and correlated using Pearson’s correlation coefficients and regression analysis.

Results:

Although the correlation between hard palate and airway measurements was weak (Pearson coefficient (r) < 0.40), there were significant (P < 0.05) additive effects for hard palate measurements in predicting airway dimensions. Maxillo-palatal arch angle was the only hard palate measurement that had no effect in predicting airway dimensions.

Conclusion:

Hard palate measurements are considered useful in predicting airway dimensions in patients referred for CBCT.

The Airway Volume Related to the Maxillo-Mandibular Position Using 3D Analysis

Objective

The aim of this research was to compare three cephalometric analyses and their correlation with the airway volume in subjects with different skeletal classes using 2D and 3D images. Study Design. Cross-sectional descriptive study. Material and Method. Steiner, McNamara, and Ricketts analyses and the airway volume were compared in 115 subjects who were candidates for orthognathic surgery under diagnosis using cone beam computed tomography (CBCT); 46 males (40%) and 69 females (60%) were included. The sagittal positions of the maxilla and mandible, the angulation of the mandibular plane, the sagittal positions of the upper and lower incisors, measurements of the largest or shortest airway area, and the volume were compared using Spearman's test considering a p value < 0.05.

Results

Differences were observed between the Steiner and McNamara techniques for the sagittal position of the maxilla (p = 0.01). For mandibular angulation, there was a greater difference between values for Steiner and Ricketts techniques (p = 0.001). In the upper incisor, the results for McNamara and Ricketts techniques were significantly different (p = 0.004). Analysing the airway, subjects with a class II skeletal pattern had a smaller volume than those with a class III pattern (p = 0.034).

Conclusion

It may be concluded that skeletal class II patients have a significantly smaller airway volume than class III patients. The skeletal parameter does not always relate to the airway volume; however, the high mandibular angle could be related to the airway conditions.

The interaction of obesity and craniofacial deformity in obstructive sleep apnea

OBJECTIVE

Both obesity and craniofacial deformity are important etiologies of obstructive sleep apnea (OSA). The present research aimed to explore their interaction and different impacts on OSA severity.

METHODS

A total of 207 consecutive OSA patients (169 males, 38 females) were included in the research. Based on the body mass index (BMI) value, patients were divided into 77 normal-weight patients (BMI <24 kg m^-2), 105 overweight patients (24 ≤ BMI<28 kg m^-2) and 26 obese patients (BMI ≥28 kg m^-2). All accepted overnight polysomnography and standard lateral cephalogram. Cephalometric measurements involved 25 cephalometric variables. The correlations between these cephalometric variables, BMI and the apnea-hypopnea index (AHI) were evaluated.

RESULTS

For the whole sample after controlling for gender and age, stepwise regression analysis showed that the factors affecting AHI were increased BMI, narrowing posterior airway space, inferior displacement of hyoid and elongation of the tongue. When grouped by BMI, normal-weight group exhibited with more reduced maxillary length and mandible length, and steeper mandible plane than overweight and obese patients (p < 0.0167). Obese group showed least skeletal restriction and most prominent soft tissues enlargement (p < 0.0167). However, these skeletal indexes were not statistically correlated with AHI.

CONCLUSIONS

Obesity and skeletal malformations were both etiological factors of OSA, but obesity seemed to have a greater influence on AHI severity in all kinds of obese and thin OSA patients. Only in normal-weight group, it was affected by both cephalometric variables and BMI.

Validity of the STOP-Bang Questionnaire in Identifying OSA in a Dental Patient Cohort

Background and objectives: Obstructive Sleep Apnea represents a widespread problem in the population, but it is often not diagnosed and not considered a true pathology. Different diagnostic tools are available for the diagnosis of sleep apnea. This study aims to demonstrate the ability of the STOP-Bang (Snoring, Tiredness, Observed apnea, high blood Pressure, Body mass index, Age, Neck circumference, and Gender) questionnaire in identifying subjects with Obstructive Sleep Apnea (OSA) Syndrome, highlighting the role of dentists as epidemiological sentinels. Materials and methods: the STOP-Bang questionnaire was administered to a cohort of 1000 patients, assessing three private dental clinics in Italy. Excessive daytime sleepiness was measured using Epworth Sleepiness Scale (ESS) and defined as ≥ 10. Subjects were considered at risk of OSA if they had three or more positive items at STOP-Bang and were invited to undergo further examination with a type 3 polygraph. Presence of OSA was measured with the apnea-hypopnea index (AHI) and defined as AHI ≥ 5. Results: 482/1000 subjects (48.2%) had three or more positive items in the STOP-Bang questionnaire and were considered at risk for Obstructive Sleep Apnea Syndrome (OSAS). Excessive daytime sleepiness (EDS ≥ 10) was more frequent among subjects at risk for OSAS (73/482, 15.1%) vs. those not at risk for OSAS (30/518, 5.8%) (p < 0.0001). Moreover, 153/482 subjects at risk for OSAS (31.7%) accepted further examination with a type 3 polygraph. Presence of OSAS (AHI ≥ 5) was suggested in 121/153 subjects (79.1%, 95% CI 71.6% to 85.1%), with 76/121 subjects (62.8%) needing treatment (AHI ≥ 15). Conclusion: the high prevalence of OSAS highlights the role of dentists as “epidemiological sentinels”. The STOP-Bang questionnaire is a simple and efficacious instrument for screening sleep apnea patients.

Clustering-based characterization of clinical phenotypes in obstructive sleep apnoea using severity, obesity, and craniofacial pattern

OBJECTIVES

To identify and characterize the phenotypes of adult obstructive sleep apnoea (OSA) patients based on clustering using OSA severity, obesity, and craniofacial pattern.

MATERIAL AND METHODS

The samples consisted of 89 adult OSA patients whose polysomnography and lateral cephalogram were available. With cluster analysis using apneahypopnea index (AHI, events/hour), body mass index (BMI, kg/m2), ANB (degree), and mandibular plane angle (MPA, degree), three clusters were identified. Cephalometric variables including craniofacial, soft palate, hyoid bone, and pharyngeal space compartments were compared among clusters by one-way analysis of variance or Kruskal-Wallis test. Multivariable linear regression analysis was performed to find contributing factors to OSA severity within each cluster.

RESULTS

Cluster-1 (obesity type; 49.4 per cent) exhibited moderate OSA, obesity, and normal sagittal and vertical skeletal pattern (AHI, 22.4; BMI, 25.5; ANB, 3.2 degrees; MPA, 26.3 degrees) without significant upper airway abnormality. Cluster-2 (skeletal type; 33.7 per cent) was characterized by moderate OSA, severe skeletal Class II hyperdivergent pattern with narrow pharyngeal airway spaces, without obesity (AHI, 27.9; BMI, 23.5; ANB, 7.5 degrees; MPA, 36.6 degrees). Cluster-3 (complex type; 16.8 per cent) included severe OSA, obesity, skeletal Class II hyperdivergent pattern (AHI, 52.8; BMI, 28.0; ANB, 4.5 degrees; MPA, 32.2 degrees), with posteriorly displaced hyoid and retroclined soft palate. The main contributing factors to AHI were obesity in Cluster-1; hyperdivergent vertical pattern with narrow pharyngeal space in Cluster-2; and hyperdivergent pattern, obesity, displaced hyoid, and soft palate in Cluster-3.

CONCLUSION

Three OSA phenotypes resulted from this study provide a clinical guideline for differential diagnosis and orthodontic intervention in the interdisciplinary treatment for OSA patients.

Relationship between cephalometric parameters and the apnoea-hypopnoea index in OSA patients: a retrospective cohort study

OBJECTIVE

The purpose of this study was to assess the relationship between cephalometric parameters and apnoea-hypopnoea index (AHI) controlling for the effect of gender, age, and body mass index (BMI) on a large sample of patients with obstructive sleep apnoea (OSA).

METHODS

This retrospective cohort study was conducted on the lateral cephalograms of 253 Caucasian adult OSA patients. Cephalometric analyses were performed using 14 parameters for skeletal and soft tissue morphology, including antero-posterior and vertical jaw relationships, hyoid bone position, soft palate length and thickness, airway space, and tongue length and height. A hierarchical regression was run to examine the amount of variability in AHI that cephalometric variables explained after controlling for patients' general characteristics (gender, age, and BMI).

RESULTS

After controlling for gender, age, and BMI, the increase in AHI variance accounted for by cephalometric parameters was equal to 0.103. Among the cephalometric variables, only MP-H and PNS-P were statistically significant (P < 0.05).

LIMITATIONS

Given the retrospective nature of the study, it is difficult to assess whether other confounding variables not considered in the present study could have influenced the relationship between cephalometric parameters and AHI.

CONCLUSIONS

This study revealed the existence of a relationship between OSA severity and some cephalometric parameters. Indeed soft palate length and vertical position of the hyoid bone were significant predictors of AHI in adult Caucasian OSA patients.

Gender-specific cephalometric features related to obesity in sleep apnea patients: trilogy of soft palate-mandible-hyoid bone

Background

The aim of this study is to investigate the relationship between gender-specific and obesity-related airway anatomy in patients with obstructive sleep apnea (OSA) by using cephalometric analyses.

Methods

We retrospectively evaluated 206 patients with suspected OSA undergoing polysomnography and anthropometric measurements such as body mass index, neck circumference, and waist-hip ratio. We checked lateral cephalometry to measure tissue landmarks including angle from A point to nasion to B point (ANB), soft palate length (SPL), soft palate thickness (SPT), retropalatal space (RPS), retrolingual space (RLS), and mandibular plane to hyoid (MPH).

Results

Male with OSA showed significantly increased SPL (P = .006) compared with controls. SPL and MPH had significant correlation with apnea-hypopnea index (AHI) and central obesity. Female with OSA showed significantly increased ANB (P = .013) and SPT (P = .004) compared with controls. The receiver operating characteristic curves revealed that SPT in male and ANB and SPT in female were significant in model 1 (AHI ≥ 5) and model 2 (AHI ≥ 15). MPH was also significant for male in model 2.

Conclusion

Male and female with OSA had distinct anatomic features of the upper airway and different interactions among soft palate, mandible, and hyoid bone.

Evaluation of upper airway volume and craniofacial volumetric structures in obstructive sleep apnoea adults: A descriptive CBCT study

OBJECTIVE

The aim of this investigation was to assess, with a descriptive three-dimensional evaluation, the volume of upper airway (UAWV) and the volume of craniofacial structures in adult patients suffering from obstructive sleep apnoea (OSA) and compare them to the corresponding findings in adults with no sleep disorders.

MATERIALS AND METHODS

The sample consisted of 54 adult males, 27 suffering from OSA diagnosed by means of the Apnoea Hypopnea Index and 27 with no history of sleep disorders. All subjects had a cone beam computerized tomography scan performed with the same head position. UAWV was assessed with the Amira® software, and craniofacial volumes by means of a specially developed data-processing program, which allowed the construction of tetrahedrons using anatomical landmarks. Assessed volumes were naso-maxillary, cranium upper anterior, oral cavity, post-oral cavity, hyoid to mandible, and post-hyoid. SPSS (version 19.0) was used for the statistical analysis. The Levene's test for Equality of Variance, the t-test for Equality of Means and the Mann-Whitney test were used to evaluate the variables. The level of significance was set at P ≤ 0.05.

RESULTS

The mean value of UAWV was smaller in the OSA group. The post-hyoid volume, the calculated posterior volume, and the ratio of posterior to total volume showed differences between the groups.

CONCLUSIONS

Craniofacial structures did not show significant differences between the groups, but in the OSA group the posterior space released for upper airway was significantly bigger and UAWV was significantly smaller.

Relations Between Hyoid-Related Cephalometric Measurements and Severity of Obstructive Sleep Apnea

OBJECTIVE

To evaluate the relationship between the hyoid-related cephalometric measurements and the apnea-hypopnea index (AHI) in patients diagnosed with obstructive sleep apnea (OSA).

METHODS

A total of 56 subjects were evaluated by lateral cephalometric radiography and polysomnography (PSG). The OSA diagnosis was made according to the patients' AHI. Included were 13 primary snoring, 16 mild OSA, 10 moderate OSA, and 17 severe OSA. C3-hyoid distance and mentum-hyoid distance were measured on lateral cephalogram. Cephalometric measurements and PSG parameters were compared among the different OSA groups.

RESULTS

The distance between the mentum and hyoid was significantly longer in the severe OSA group than in the primary snoring, mild OSA, and moderate OSA groups (P = 0.029). There was a significant positive correlation between the AHI value and the distance of the mentum hyoid (r = 0.368, P = 0.005). The C3-hyoid distance among the groups was not statistically significant different (P = 0.889).

CONCLUSION

The mentum-hyoid distance of patients with severe OSA was longer compared to the other OSA groups. These patients might have more benefit from the surgeries that have an impact on the position of the hyoid bone compared to other patients with OSA.

Correlational Study of Sleep Apnea Patient Characteristics With Discharge Locations

PURPOSE

To determine if a correlation exists between OSA patient characteristics and the PACU discharge location; and the characteristics of the patients at-risk for low saturation levels, increased number of desaturations, and longer length of stay in the PACU.

DESIGN

Retrospective, correlational study design.

METHOD

Chart review of OSA patients ≥ 18 years old. Correlational analysis was performed between 15 high risk patient variables and the PACU discharge disposition: home or monitored bed. Complications resulting in monitored bed admission were reviewed.

FINDINGS

153 patients' charts were reviewed. The results showed that age>60, ASA classification, anesthesia type and narcotics use in the PACU were significantly correlated (p≤.05) with a patient's discharge disposition.

DISCUSSION

The findings are consistent with other OSA research except BMI was not significant in this study.

CONCLUSION

The results highlighted areas for future research and implications for clinical practice that would enable the perioperative care team to deliver safe care based on evidence.

Correlation between cephalometric data and severity of sleep apnea

INTRODUCTION

Obstructive sleep apnea syndrome has a high prevalence among adults. Cephalometric variables can be a valuable method for evaluating patients with this syndrome.

OBJECTIVE

To correlate cephalometric data with the apnea-hypopnea sleep index.

METHODS

We performed a retrospective and cross-sectional study that analyzed the cephalometric data of patients followed in the Sleep Disorders Outpatient Clinic of the Discipline of Otorhinolaryngology of a university hospital, from June 2007 to May 2012.

RESULTS

Ninety-six patients were included, 45 men, and 51 women, with a mean age of 50.3 years. A total of 11 patients had snoring, 20 had mild apnea, 26 had moderate apnea, and 39 had severe apnea. The distance from the hyoid bone to the mandibular plane was the only variable that showed a statistically significant correlation with the apnea-hypopnea index.

CONCLUSION

Cephalometric variables are useful tools for the understanding of obstructive sleep apnea syndrome. The distance from the hyoid bone to the mandibular plane showed a statistically significant correlation with the apnea-hypopnea index.

Surface cephalometric and anthropometric variables in OSA patients: statistical models for the OSA phenotype

STUDY OBJECTIVE

We used statistical modelling to probe the contributions of anthropometric and surface cephalometric variables to the OSA phenotype.

DESIGN

The design is prospective cohort study.

SETTING

The setting is community-based and sleep disorder laboratory.

PATIENTS OR PARTICIPANTS

Study #1-Model development study: 147 healthy asymptomatic volunteers (62.6 % Caucasian; age, 18-76 years; 81 females; median multivariable apnea prediction index=0.15) and 140 diagnosed OSA patients (84.3 % Caucasian; age, 18-83 years; 41 females; polysomnography [PSG] determined apnea-hypopnea index >10 events/h). Study #2-Model test study: 345 clinic patients (age, 18-86 years; 129 females) undergoing PSG for diagnosis of OSA.

INTERVENTION

We measured 10 anthropometric and 34 surface cephalometric dimensions (calipers) and calculated mandibular enclosure volumes for study #1 and recorded age and neck circumference for study #2. Statistical modelling included principal component (PC), logistic regression, and receiver-operator curve analyses.

MEASUREMENTS AND RESULTS

Model development study: A regression model incorporating three identified PC predicted OSA with 88 % sensitivity and specificity. However, a simplified model based on age and NC alone was equally effective (87 % sensitivity and specificity). Model test study: The simplified model predicted OSA with high sensitivity (93 %) but poor specificity (21 %).

CONCLUSION

We conclude that in our clinic-based cohort, craniofacial bony and soft tissue structures (excluding neck anatomy) do not play a substantial role in distinguishing patients with OSA from those without. This may be because craniofacial anatomy does not contribute greatly to the pathogenesis of OSA in this group or because referral bias has created a relatively homogeneous phenotypic population.

Lateral cephalometric analysis and the risks of moderate to severe obstructive sleep-disordered breathing in Thai patients

PURPOSES

This cross-sectional study was aimed to compare lateral cephalometric parameters among patients with different severities of obstructive sleep-disordered breathing and to determine if there are radiographic variables that increase risk of moderate to severe obstructive sleep apnea (OSA).

METHODS

Ten linear and 5 angular parameters of lateral cephalometry in 188 adult subjects which included 47 controls and 141 patients with OSA classified by apnea-hypopnea index (AHI) from level-I polysomnography, were analyzed. All radiographs were done under standardized processes and measured twice on separate occasions.

RESULTS

There were statistically significant differences between controls and patients with AHI ≥ 15 in parameters such as a distance from mandibular plane to hyoid (MP-H), posterior airway space (PAS), skull base angle (NSBA), a distance from posterior nasal spine to posterior pharyngeal wall (PNS-PP), and soft palate length (PNS-P), (p < 0.01). However, no significant difference was found between controls and mild OSA. Logistic regression analysis showed that only MP-H with cutoff point of ≥ 18 millimeters, NSBA ≤ 130°, and PAS ≤ 10 millimeters were independent cephalometric variables that increased risk of having AHI ≥ 15 compared to controls with adjusted odds ratio of 17.1, 8.3 and 4.2, respectively. Gender did not significantly associate with OSA severity in this analysis.

CONCLUSIONS

Patients who had longer MP-H, narrower PAS and narrower NSBA than specific cutoff points significantly increased risks of moderate to severe OSA. Treatments that effectively improve these parameters, particularly at tongue base level (MP-H and PAS), may decrease the severity of disease.