背景與目的: 良性陣發型姿勢性眩暈症(Benign Paroxysmal Positional Vertigo)為一常見的周邊前庭疾病，起因為耳石從橢圓囊或球狀囊脫落而掉進其中一個半規管裡，當頭部姿勢快速變換時，會造成耳石滾動而引發頭暈、天旋地轉等症狀。雖然天旋地轉症狀的持續時間很短暫，但患者常因為害怕做出這一類的頭部動作會引發症狀，而避免做出有頭部姿勢快速改變的動作。然而，轉身的動作是日常生活中必需且包含有頭部轉動的動作，但目前對於良性陣發型姿勢性眩暈症患者在轉身時的平衡表現與姿勢控制是否會受到影響並不清楚，因此本研究的目的為探討良性陣發型姿勢性眩暈症患者在轉身時的平衡表現與肌肉控制，同時與健康控制組比較其間之差異，此外也會比較患者在接受耳石復位術前後之表現差異。
方法: 本研究徵召25位健康年輕人(22.6±2.0歲)，25位健康老年人(65.3±4.5歲)，以及25位良性陣發型姿勢性眩暈症患者(54.9±14.7歲)，其中良性陣發型姿勢性眩暈症患者之Dix-Hallpike 測試結果為陽性方能納入資料分析。所有受試者皆接受例行身體檢查，包含視力、視覺敏感度、下肢肌力及足底感覺。主要測試為轉身動作測試，受試者站在力板上執行轉身動作，並計算受試者執行動作過程時身體壓力中心(Center Of Pressure)移動情形，同時使用肌電圖記錄下肢的肌肉活動情形。其中良性陣發型姿勢性眩暈症患者在測試完成後，執行耳石復位術的介入，並在一週後再次測試。使用單因子變異數分析(One-way ANOVA)比較三組間的差異，以及配對T檢定分別比較三組轉向兩側之組內差異與患者在介入前後之差異。
結果: 良性陣發型姿勢性眩暈症患者不論是轉向患側(p=0.008)或轉向健側(p=0.041)時，轉身到達到身體穩定的時間都需要比年輕人長。且轉向患側時，其身體壓力中心往對側最大位移(p=0.022)、左右側位移範圍(p=0.025)、前後側移動軌跡長度(p=0.004)、左右側移動軌跡長度(p=0.010)、移動軌跡總長度(p=0.003)及改變晃動方向之次數(p=0.013)皆顯著大於年輕人，且同側大腿共同收縮指數顯著小於年輕人(p=0.006)，而對側大腿共同收縮指數(p=0.001)、對側小腿共同收縮指數(p=0.017)、對側大腿共同收縮期(p<0.001)和對側小腿共同收縮期(p=0.009)皆顯著大於年輕人。此外，良性陣發型姿勢性眩暈症患者在轉向患側時，其往後最大移動速度顯著小於轉向健側(p=0.032)，且對側大腿共同收縮指數(p=0.034)和對側小腿共同收縮指數(p=0.032)顯著大於轉向健側。再者，良性陣發型姿勢性眩暈症患者在接受耳石復位術之後其轉身達到穩定的時間明顯較治療前短(p=0.042)，且轉身過程中改變晃動方向之次數也顯著低於治療前(p=0.019)，同側小腿共同收縮指數(p=0.017)及對側小腿共同收縮期(p=0.033)皆顯著小於治療前。
討論與結論: 良性陣發型姿勢性眩暈症患者與年輕人相比，其執行轉身動作時身體壓力中心的移動範圍、移動軌跡長度及改變晃動方向之次數皆較健康年輕人大，而與老年人則無顯著差異。且患者對側下肢共同收縮指數及共同收縮期皆顯著大於年輕人。此結果差異，可能與其前庭系統損傷而引起的代償有關。然而，經過耳石復位術介入後，其轉身時間、改變晃動方向之次數、同側小腿共同收縮指數及對側小腿共同收縮期有顯著改善，顯示耳石復位術的介入可能改善患者轉身時之平衡及肌肉控制表現。

SUMMARY
Benign paroxysmal positional vertigo (BPPV) is a common peripheral vestibular disorder. Patients with BPPV usually suffer from positional vertigo during certain head movements. Turning is a common movement requiring head movements during activities of daily living. It is unknown whether the symptoms of BPPV would affect such movement involving head movements. Therefore, the purpose of this study was to investigate balance performance and muscle activity during turning in patients with BPPV. Twenty-five healthy young adults, 25 healthy elderly adults and 25 patients with BPPV, examined with Dix-Hallpike test, were recruited in the study. Balance performance during quick turning responding to an auditory cue was examined with a force plate. The muscle activities of bilateral lower extremities during turning were recorded using wireless surface electromyography. The BPPV patients received canalith repositioning treatment after the task. One-way analysis of variance (ANOVA) was used to compare differences of the center of pressure (COP) and muscle activity parameters among the three groups. Paired-t test was used to compare differences between before and after treatment in BPPV group. The BPPV patients showed longer duration, larger displacement range, longer sway length and more repetitions of changing directions to complete the turning task when turning to the affected side than the young adults. In addition, the co-contraction index and the co-contraction duration of the contralateral lower extremity were significantly larger in BPPV patients compared with the young adults. However, the balance performance during turning seemed to slightly improve in the BPPV patients after the treatment. The results provided new insight of turning strategy in patients with BPPV.

Key words: benign paroxysmal positional vertigo (BPPV), balance performance, postural control, turning, canalith repositioning manoeuvre

INTRODUCTION
Benign paroxysmal positional vertigo (BPPV) is a common peripheral vestibular disorder, resulting from free floating debris moving in the semicircular canals. Whenever there are head movements, the resultant endolymphatic flow caused by the moving debris in one of the canals would lead to inappropriate excitation of the vestibular system, and thus inducing symptoms such as vertigo and nystagmus. Therefore, patients with BPPV usually suffer from positional vertigo and nystagmus during specific head movements such as rolling in bed, lying down, sitting up from bed or bending forward. Despite the episodes of vertigo usually lasting less than one minute, patients still tend to refrain from performing head movements to avoid provoking vertigo. In addition to the vertigo symptom, many patients complain of lightheadedness, imbalance, and walking disturbances when performing daily activities. Turning is common movement in our activities of daily living, which requires head movements. However, few studies investigated balance performance during turning in BPPV patients. The purpose of this study was to investigate the balance performance and muscle activity during turning in patients with BPPV compared to healthy adults. Differences in balance performance between turning to the affected side and non-affected side in BPPV patients were also investigated. In addition, the short-term effect of repositioning manoeuver on dizziness symptoms, balance performance and muscle activity of lower extremities during turning in BPPV patients was examined.

MATERIALS AND METHODS
Twenty-five healthy young adults (22.6±2.0 years), twenty-five healthy elderly adults (65.32±4.45 years) and twenty-five patients with BPPV (54.9±14.7 years), examined with Dix-Hallpike test, were recruited in the study. Patients whose Dix-Hallpike test result was negative were excluded from analysis. All subjects completed a set of physical examination including visual acuity, muscle strength and cutaneous sensation of the feet. Subjects were instructed to stand on a force plate (Kistler model 9286A, Winterthur, Switzerland) and perform a quick turn either to the right or left responding to an auditory cue without moving the feet. The center of pressure (COP) parameters were calculated from the force plate. The muscle activities of bilateral lower extremities were recorded using wireless surface electromyography (Trigno™ Wireless Systems, Delsys Inc., Boston, USA,). Patients with BPPV received canalith repositioning treatment after the tests, and repeated the turning test one week after the treatment. One-way ANOVA was used to compare differences of the COP and muscle activity parameters among the three groups. Paired-t test was used to compare differences between turning towards the affected and non-affected side, and between before and after treatment in BPPV group.

RESULTS AND DISCUSSION
The BPPV patients showed significantly longer duration (p=0.008), larger maximum contralateral displacement (p=0.022), larger medial-lateral range (p=0.025), longer anterior-posterior sway length (p=0.004), longer medial-lateral sway length (p=0.010), total sway length (p=0.003) and more repetitions of changing directions (p=0.013) to complete the turning task when turning to affected side than the young adults. The co-contraction index of the ipsilateral thigh (p=0.006) was significantly smaller in the BPPV patients than the young adults, whereas the other muscle co-contraction parameters, including the co-contraction index of the contralateral thigh (p=0.001) and shank (p=0.017), and the co-contraction duration of the contralateral thigh (p<0.001) were significantly greater in the BPPV patients than the young adults. Comparing the performances of turning to the affected and non-affected side in the BPPV patients, the maximum posterior sway velocity was significantly smaller, and the co-contraction index of the contralateral thigh (p=0.034) and shank (p=0.032) were significantly greater when turning to the affected side than to the non-affected side (p=0.032). One week after the repositioning maneuver, the patients showed a significantly shorter duration (p=0.042) and less repetitions of changing directions (p=0.019) to complete a turn comparing to those before the treatment, as well as significantly smaller co-contraction index of the ipsilateral shank (p=0.017) and shorter co-contraction duration of the contralateral shank (p=0.033).

CONCLUSION
The BPPV patients showed longer duration, larger displacement range, longer sway length, and more repetitions of changing directions to complete a turn when turning to the affected side than the young adults did. No significant difference was found between the BPPV patients and elderly adults. The co-contraction index and the co-contraction duration of the contralateral lower extremity were significantly larger in the BPPV patients compared with the young adults. The results indicated that balance performance and postural control during turning in active BPPV patients was worse than young adults. The balance performance of the BPPV patients during turning seemed to slightly improve after the repositioning maneuver. The results provided new insight of turning strategy in patients with BPPV.

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