Neck circumference, as a predicator of obesity, is a well-known risk factor for obstructive sleep apnea and cardiovascular diseases. However, little research exists on neck length associated with these factors. This study explored the association of neck length with sleep and cardiovascular risk factors by measuring midline neck length (MNL) and lateral neck length (LNL).
We examined 240 patients aged 30 to 75 years who visited a health check-up center between January 2012 and July 2012. Patients with depressive disorder or sleep disturbance were excluded from this study. MNL from the upper margin of the hyoid bone to the jugular notch and LNL from the mandibular angle to the mid-portion of the ipsilateral clavicle were measured twice and were adjusted by height to determine their relationship with sleep and cardiovascular disease risk factors.
Habitual snorers had shorter LNL height ratios (P = 0.011), MNL height ratios in men (P = 0.062), and MNL height ratios in women (P = 0.052). Those snoring bad enough to annoy others had shorter MNL height ratios in men (P = 0.083) and women (P = 0.035). Men with objective sleep apnea had longer distances from the mandible to the hyoid bone to the mandible (P = 0.057). Men with metabolic syndrome had significantly shorter LNL height ratios (P = 0.021), and women with diabetes, hyperlipidemia, and metabolic syndrome had shorter MNL height ratios (P < 0.05).
This study shows that a short neck by measuring the MNL is probably associated with snoring. In addition, MNL is related to cardiovascular disease risk factors in women.
With a high prevalence of 35% to 45% in men and 15% to 28% in women, snoring is a common health problem and a major symptom of sleep disordered breathing.
With the recently discovered associations between sleep disordered breathing and daytime sleepiness, decreased concentration and intellectual ability, common causes of secondary hypertension, insulin resistance, dyslipidemia, cardiovascular disease, and so on, there is an increasing interest in sleep apnea.
Under the approval of the institutional review board (IRB No. 2012-045), the initial number of subjects selected included 350 adults aged 30 to 75 years who visited a general hospital between January 2012 and July 2012, were informed of the purpose and procedure of the study, and gave their prior consent for participation. Patients with a history of depression or sleep disorder or those currently on related medications were then excluded from among these subjects, leaving 240 subjects to participate in the study.
Each subject's height and weight were measured using an automatic anthropometry device, and body mass index was calculated using the weight (kg)/height2 (m2) formula. For neck length measurement, the subject was seated on a chair with his or her back resting firmly against the back of the chair and facing front. As illustrated in
Investigators interviewed the subjects directly to complete a structured questionnaire on factors such as sleeping patterns, snoring, and sleep apnea. Questions concerning snoring included the degree of disturbance to others as well as the frequency and intensity of snoring. The questionnaire also categorized any subjects whose sleep apnea was witnessed by others sleeping with them 'at least once a week' as having sleep apnea. For sleeping patterns, the total sleeping duration, the time taken to fall asleep (sleep latency), and the degree of freshness after waking up were investigated.
The subjects' history of hypertension, diabetes, hyperlipidemia, stroke, smoking, alcohol consumption, physical exercise, and duration of physical exercise were investigated through self-administered questionnaires and investigator interviews. For blood pressure measurement, the subject was instructed to relax in a sitting position for at least 5 minutes and abstain from coffee and smoking for at least 30 minutes. Then, the subject's arm was placed on a desk at the same level as the heart, and the blood pressure was measured using a mercury sphygmomanometer. As stated in the JNC-7 (seventh report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure), subjects with systolic blood pressure (SBP) ≥140 mm Hg or diastolic blood pressure (DBP) ≥90 mm Hg or those on antihypertensive drugs were categorized as having hypertension. As for diabetes, subjects with blood glucose levels ≥126 mg/dL after 8 hours of fasting or glycosylated hemoglobin level ≥6.5% or those under treatment for diabetes were categorized as having diabetes. Subjects with fasting low density lipoprotein (LDL) cholesterol levels ≥130 mg/dL or those on hyperlipidemia medication were categorized as having hyperlipidemia. Metabolic syndrome was defined as any patients who met more than three of the five criteria according to the American Heart Association/National Heart, Lung, and Blood Institute Adult Treatment Panel III 2005 criteria. We used the following diagnostic criteria for metabolic syndrome: (1) SBP ≥130 mm Hg, DBP ≥85 mm Hg, or the current use of antihypertensive drugs; (2) fasting blood sugar ≥100 mg/dL or the current use of antidiabetic drugs; (3) triglyceride level ≥150 mg/dL or the use of treatment agents; (4) high density lipoprotein (HDL) cholesterol level <40 mg/dL in men and <50 mg/dL in women or the use of treatment agents; and (5) waist circumference ≥90 cm in men and ≥85 cm in women.
Chi-square tests and t-tests were used to analyze baseline characteristics of subjects according to sex, and analysis of variance tests and t-tests were used for comparative analysis of anthropometrics against snoring, sleeping disorder, sleep apnea, daytime sleepiness, etc. All statistical analyses were done using PASW SPSS ver. 18.0 (SPSS Inc., Chicago, IL, USA). A P-value <0.05 was considered statistically significant.
Of all 240 subjects, 178 (74.2%) were men and 62 (25.8%) were women. The average age was 49.1 ± 8.5 years for men and 47.2 ± 9.7 years for women. All measurements of the MNL, measured twice for each subject, were around 8 cm with no significant differences between men and women. Women had slightly longer LNLs than men (P < 0.05). Blood tests showed no significant differences in C-reactive protein level, LDL cholesterol level, or history of diabetes or hyperlipidemia between men and women. However, there were differences between men and women in serum HDL cholesterol, triglycerides, and drinking and smoking status. While sleeping duration was similar between men and women, those habitually snoring 4 times or more a week numbered 48 men (27.0%) and 6 women (9.7%), and those exhibiting symptoms of sleep apnea numbered 41 men (23.0%) and 2 women (3.2%), which was significantly higher in men than in women (
Men who snored 4 times or more in a week had significantly shorter LNL height ratios (P = 0.011) and shorter MNL height ratios (P = 0.062), which was not statistically significant. There were no differences in the MHL. There were also no differences between the neck length indicators for intensity of snoring or snoring bad enough to annoy others.
Women who snored 4 times or more in a week had shorter MNL height ratios (P = 0.056), but there were no differences in LNL height ratio or MHL. While there were no differences in neck anthropometrics for intensity of snoring, those snoring bad enough to annoy others had shorter MNLs (P = 0.036) (
Men with sleep apnea had longer MHL than those without sleep apnea (P = 0.057) but without statistical significance. There were two women who had sleep apnea and their MNL height ratios were significantly shorter than that of other women without sleep apnea (P = 0.020) (
For sleeping patterns, our analysis of the relationship of sleep times, sleep latency, and waking refreshed to the MNL height ratio and the LNL height ratio revealed that there were no significant differences in neck lengths between men and women. Men with sleep times <6 hours had shorter LNL height ratios than men with sleep times ≥8 hours but without statistical significance, and women with sleep times of 6 to 8 hours had significantly shorter LNL height ratios than women with sleep times ≥8 hours (
In the relationship between neck length height ratios and hypertension, diabetes, hyperlipidemia, and metabolic syndrome among men, while there were no differences in the MNL height ratio, the LNL height ratio was found to be shorter only in the group with metabolic syndrome than in the group without metabolic syndrome (P = 0.021). Among women, the MNL height ratio was significantly shorter in the group with diabetes (P = 0.038), the group with hyperlipidemia (P = 0.013), and the group with metabolic syndrome (P = 0.015). However, no differences were found in the LNL height ratio regarding cardiovascular risk factors (
The purposes of this study were to explore the association of a short neck with sleep disordered breathing and cardiovascular risk factors and to evaluate the methods of measuring neck length with regard to the relationship. As a result, we learned that a short neck length has some correlation with snoring, with the MNL showing a greater correlation than the LNL in most of the categories examined. Short MNL also showed a significant correlation to cardiovascular risk factors among women but no correlation among men. Total sleep time, sleep latency, and waking up refreshed had no correlation with neck length.
Snoring is the sound produced when breathing air passes through the upper airways and vibrates the structures in the pharynx region while sleeping. Snoring used to be considered a sign of sound sleep in the past, but recent findings have indicated that snoring is a symptom of sleep-disordered breathing, which not only interferes with one's family or community life because of the noise but also is a cardiovascular risk factor. The ratio of habitual snorers who snore at least 4 times a week was 27% and 9.7% for men and women, respectively, in this study. This was slightly more than the ratio of 19.5% for men found by Oh and Cho
Male habitual snorers were generally found to have shorter LNLs and MNLs, while female habitual snorers were found to have shorter MNLs. Those who snored bad enough to annoy their sleep partners were generally found to have shorter MNLs, and there were no differences in neck lengths regarding the intensity of the snoring noise. In other words, snoring was found to have some correlation to neck length, with a possibly close correlation to shorter MNLs. A study of residents in Ansan, Korea, reported a positive correlation between short neck length and snoring among women but no differences among men.
In this study, sleep apnea as reported by sleep partners was found to have no significant differences in all neck length height ratios among men. However, a report of a boy with Rubinstein-Taybi syndrome
In this study, we found that men with sleep apnea tended to have longer MHL, which is consistent with the lateral cephalometric radiograph findings of several studies.
In this study, we obtained the MHL by subtracting the MNL from the LNL instead of taking direct measurements. This method was chosen because it is not easy to directly measure this length with a measuring tape. Since the correlation analysis of the MHL and the MNL among the subjects of this study, with the values adjusted for age and sex, showed a highly negative correlation (r = -0.636, P < 0.001), applying the MNL, i.e., the distance from the upper margin of the hyoid bone to the jugular notch, reversely may be a viable method in the clinical context. In terms of anthropometrics, individuals with longer soft-tissue upper airways and shorter lengths from the upper margin of the midline hyoid bone to the jugular notch are thought to be more susceptible to sleep-disordered breathing.
Men generally showed little correlation between neck length and cardiovascular risk factors, but those with cardiovascular risk factors tended to have slightly shorter LNLs than MNLs. On the other hand, women with diabetes, hyperlipidemia, or metabolic syndrome had significantly shorter MNLs. It has been reported that neck circumference is an indicator of subcutaneous fat distribution in the upper body, is closely related to insulin resistance, and acts as an independent cardiovascular disease risk factor.
It is quite interesting that women exhibited a greater correlation of MNL not only to snoring or sleep apnea but also to cardiovascular risk factors than men. Such prominent differences are thought to be because women are less exposed to environmental factors such as smoking and alcohol consumption.
The limitations of this study include, firstly, that the study could not include a wide range of population groups, but the majority of the subjects were confined to relatively healthy middle-aged men because of the difficulties involved in measuring the neck circumference, waist circumference, MNL, LNL, etc., with a measuring tape. Secondly, although each measurement was taken twice to minimize the error that could be caused by the position or condition of the patient, it is thought that the degree of errors between individuals would not be negligible. Nonetheless, this study is a meaningful first attempt in measuring neck length. When conducting future studies on neck length, it might be interesting to measure MNL in the reclining position and in the sitting position and compare the results. Another idea would be to examine the relative condition of the neck length to the neck circumference with regard to sleep-disordered breathing and cardiovascular risk factors.
In conclusion, it was found that short necks, especially those with short MNLs, are related to snoring to some extent and short necks among women are related to cardiovascular risk factors.
No potential conflict of interest relevant to this article was reported.
Values are presented as mean ± SD or number (%). Sleep apnea means as stop breathing ≥ once a week.
*Calculated by t-test or chi-square test.
P-value was calculated by t-test.
MNL: midline neck length, LNL: lateral neck length, HML: length of mandible to hyoid bone.
*MNL height ratio is calculated by formula as (MNL/height × 100). †LNL height ratio is calculated by formula as (LNL/height × 100). ‡HML is calculated by LNL minus MNL.
P-value was calculated by t-test. Sleep apnea means the individual stops breathing during sleep ≥ once a week.
MNL: midline neck length, LNL: lateral neck length, HML: length of mandible to hyoid bone.
*MNL height ratio is calculated by formula as (MNL/height × 100). †LNL height ratio is calculated by formula as (LNL/height × 100). ‡HML is calculated by LNL minus MNL.
P-value was calculated by t-test or analysis of variance test.
MNL: midline neck length, LNL: lateral neck length.
*MNL height ratio is calculated by formula as (MNL/height×100). †LNL height ratio is calculated by formula as (LNL/height×100).
P-value was calculated by t-test.
MNL: midline neck length, LNL: lateral neck length.
*MNL height ratio is calculated by formula as (MNL/height × 100). †LNL height ratio is calculated by formula as (LNL/height × 100).