Use of a Microelectromechanical Systems Sensor for Objective Measurements of Abnormal Head Posture in Congenital Superior Oblique Palsy Patients

Abstract

Purpose: The purpose of this study was to design an objective method for measurement of head positions as achieved with use of a microelectromechanical systems (MEMS) sensor. In addition, to use this system to observe the abnormal head position (AHP) in patients with congenital superior oblique palsy (SOP) before and after their surgery.

Methods: An MEMS sensor was designed for recording of the pitch, roll, and yaw values of the head position in real time. The MEMS sensor was then fixed on the synoptophore from –30 degrees to +30 degrees positions horizontally and vertically to test the accuracy of these measurements. Then, we tested 13 participants with AHP using the MEMS method and the photographic method and compared their correlations. Finally, the pitch, roll, and yaw values of head positions were measured using this MEMS sensor in 31 patients with congenital SOP as performed before and after their surgery.

Results: The MEMS sensor (LPMS-B2; Alubi, Guangzhou, China; 400 hertz [Hz]), as based on the theory of a gyroscope, was designed and connected to a smartphone via Bluetooth. It was able to conveniently record the patient’s pitch, roll, and yaw head positions in real time, recordings which were consistent with the scales of the synoptophore (P > 0.05) and good correlations with the photographic method (P < 0.001). The main preoperative AHP in patients with SOP was roll (22/31, 71%). Pre- and postoperative vertical deviations were 16.4 ± 7.3 prism diopters (PD) and 4.1 ± 4.2 PD, respectively (P = 0.001). The AHP in patients with SOP was positively correlated with the angle of extorsion in the dominant eye (P = 0.01), rather than that of the vertical deviation.

Conclusions: The MEMS sensor described in this report is a simple, practical, and accurate objective device for use in head position measurements. In patients with SOP, the AHP is related to the angle of extorsion in the dominant eye.

Translational Relevance: The MEMS sensor was designed as a micro-wireless dynamic high-precision device for AHP measurement, which has the potential for use in a clinic.

Introduction

Abnormal head position (AHP), especially head tilts to the right or left shoulder, is often the first notable sign indicative of superior oblique palsy (SOP).^1,2 Patients with SOP adopt an AHP to maintain binocular function.^3,4 Being the most prevalent cause of ocular torticollis, congenital SOP accounts for 75% of these cases.^5,6 The severity of AHP is an important factor and indicator for surgical intervention.^7,8 Thus, accurate measurements of AHP represent a very important component in the evaluation of patients with strabismus. AHP is usually described as the head rotation about three axes in degrees such as face-turn, chin-up/down, and head tilts. However, the significance of this measurement is often ignored, due to the lack of suitable devices for conducting such measurements. Currently, a few devices have been developed for the measurement of AHP, such as a goniometer, digital head posture measuring system,^9,10 infrared optical head tracker,¹¹ and computer postural analysis fitting.¹² Unfortunately, these devices are inconvenient, time-consuming, expensive, and/or require extensive patient cooperation not always available with children. 

The ideal device for measuring AHP requires little patient cooperation and provides instantaneous reliable measurements. We found that a microelectromechanical systems (MEMS) sensor, which is based on the theory of a gyroscope, may provide a suitable device for AHP measurements. Gyroscopes, which have the physical characteristics of pin-point stability and accuracy, can be used for three-dimensional spatial positioning and are widely used in aerospace settings as well as in other fields. Accordingly, the MEMS sensor, as based on gyro positioning, can provide a simple, practical, accurate, and objective means for head position measurements in clinical settings. 

Methods

To Design a Device for an Objective Measurement of Head Position as Based on a Gyroscope

MEMS Sensor

MEMS (LPMS-B2; Alubi, Guangzhou, China) is a universal sensor widely used in the areas of human motion capture and analysis, medical rehabilitation training, and drone control, as well as in a number of other fields. It can dynamically measure the posture of three dimensions (3Ds) in real-time. In specific, the parameters/specifications of this device include: volume of 39 × 39 × 8 mm, weight of 12 g, with a maximal communication distance of 20 m. It is capable of 3.7v@230mAh using a rechargeable lithium battery, with a power consumption of 120 [email protected] v, enabling a continuous working period of > 6 hours. The MEMS continuously measures a head posture at a maximal rate of 400 hertz (Hz). This MEMS works with 3 axis measurement range of full 360 degrees and range with resolution of < 0.01 degrees, static accuracy of < 0.5 degrees and dynamic accuracy of < 2 degrees. The MEMS sensor measures spatial attitude by calculating the directional differences between the sensor coordinate system (S) and the global reference coordinate system (G), both of which are defined as right-hand Cartesian coordinate systems. When the directional calculation utilizes three types of data: acceleration, gyroscope, and geomagnetic field (sensor filtering mode set to acc+gyr+mag), the G system is defined as X–pointing forward toward geomagnetic north, Y–pointing forward toward the west of the geomagnetic field, and Z–pointing upward (gravity vertically downward, −1 g; Fig. 1A). The MEMS integrates complex mechanical and electronic functions into a miniaturized device. When measuring the space orientation of an object, the MEMS uses three different internal sensing units: a three-axis gyroscope (detecting angular velocity), a three-axis accelerometer (detecting the direction of the Earth’s gravitational field), and a three-axis magnetometer (measuring the direction of the Earth’s magnetic field). By integrating the angular velocity data of the gyroscope, directional data in three spatial axes can be obtained. Errors from the gyroscope measurements are generally quite limited. The MEMS utilizes the information of accelerometers (roll and pitch) and magnetometers (yaw) to correct the directional data of gyroscopes, and ensure high-precision and stable calculation of directional information in fast sampling rates. The system utilizes an Extended Kalman Filter (EKF) combined with the directional information of three sensing units to reduce measurement errors, especially in situations of regular motion, such as human gait analysis and vehicle vibration analysis. The internal sampling and filtering rate of the sensor is 400 Hz, and frequency of the data stream is independent of the sampling and processing rates. The selected communication interface can be adjusted as needed (Fig. 1B). We use the terms yaw, pitch, and roll for face turn, chin-up/ down, and head tilts to describe the head position in 3D. Different directions are marked with + or – (Fig. 1C). 

Figure 1.

 

The microelectromechanical systems (MEMS) sensor. (A) Appearance and coordinate system of the MEMS sensor. (B) Schematic diagram of the components of the MEMS sensor. (C) Schematic diagram of the head position.

Smartphone Application

Within the Android, we have developed an AHP measurement application (APP) software program version 1.0 (software registration number 2016SR316044), which receives real-time spatial orientation information from the MEMS sensor through a wireless Bluetooth 2.1 communication and generates dynamic curves of real time data in three orientations, as presented on the screen. In this way, those viewing the MEMS can continuously monitor measurement data, select reliable time periods for automatic analysis and generate reports. The mobile APP has customized a user interface (UI) for the input and storage of basic patient information and to manage queries, making it easy for examiners to apply clinical data. 

The AHP Measurement

The MEMS sensor and a smartphone can be connected via Bluetooth. With the patient in a sitting position, the MEMS sensor is fixed on his/her head to achieve a synchronization with head movements. The gaze target with the best corrected visual acuity in both eyes is recorded, and the target is placed at a distance of 5 m, at a position level with that of the subject’s eyes. Bright pictures as gaze targets can be used in children who may be unable to cooperate with these visual acuity examinations. A designation of 0 is noted when the patient’s head posture is in a straight position. The examiner allows the patient to stare at the target, while freely adjusting the head position. A measurement is completed followed a 3-second period of head stability. These measurements are repeated three times (Fig. 2). The APP can display the continuous pitch, roll, and yaw values of the MEMS sensor in real time and project these as curves. The curves include three main peaks and three flat baseline sections, each of which should persist for a certain length of time. The examiner moves the marker line on the touch screen and marks the stable baseline and peak of the curve as corresponding to the three measurements performed. Finally, the APP automatically calculates the average pitch, roll, and yaw values and outputs and presents a report of these data (Fig. 3). 

• 1. To test the accuracy of the MEMS sensor method using synoptophore, the MEMS sensor is fixed on one of the arms of the synoptophore. When the arm is at 0 degrees, it is recorded as the initial position, 0. By horizontally pushing the arm, the yaw values changed synchronously with the scale of the synoptophore. By rotating the arm vertically, the pitch values changed synchronously (Fig. 4). Similarly, changing the fixed direction of the MEMS sensor to align its roll axis with the vertical rotation axis of the arm, the roll values can be tested. The MEMS sensor position is recorded 3 times at each 5 degrees from –30 degrees to +30 degrees horizontally and vertically as the graduation of the synoptophore (see Fig. 4).
• 2. To analyze the correlations of the MEMS sensor method and photographic method for AHP measurements. Thirteen adult participants with AHP were selected. A marker line on the subject’s face and a marker rod on the head were used to mark the body axis. The lines on the wall next to the participants served as the reference frame in space. The MEMS sensor was fixed on his/her head to achieve a synchronization with head movements. They were seated and held their head at an initial position (0 degrees in all 3D) at the beginning of each trial (Fig. 5A). The participants were allowed to stare at the target freely and presented AHP. The participants were asked to hold their heads stable. Taking photographs from three different axes, an analysis of these photographs of AHP were used to measure the roll, pitch, and yaw values (Fig. 5B). Meanwhile the APP displayed the continuous roll, pitch, and yaw values of the MEMS sensor in real time as curves and automatically output the average roll, pitch, and yaw values as described previously. These measurements are repeated three times.
• 3. Patients with typical unilateral congenital SOP performed surgery in our hospital from January 2022 to January 2023 were included in this report. All patients had AHP since childhood, positive Parks three step tests and obvious extorsion as demonstrated with fundus photography. Patients with acquired SOP, bilateral SOP, and SOP with previous strabismus surgery were excluded from this study. All the participants were not patients with amblyopia.
• 4. Deviations and AHP were measured at 1 day before surgery and at 2 months after surgery:  
  • (1) Objective measurements of torsional deviation: As described in a previous protocol,¹³ subjects were positioned in front of a non-mydriatic retinography (TOPCON) in a dimly lit room. They were instructed to maintain their head in a straight position, as achieved with use of a chin and forehead rest. Fundus pictures were recorded, transferred to a computer, and subsequently analyzed with use of Adobe Photoshop Elements software. Torsional angles were calculated as the disc-fovea angle on the pictures¹⁴ and consisted of recording the angle between the horizontal line crossing the middle of the optic disc and the line crossing it and the center of the fovea. The average value from three torsional angle measurements was then recorded.
  • (2) AHP measurement: We used the MEMS sensor measurement, as in the previously described method. AHP measurements were conducted when refractive errors were corrected by eyeglasses.

Figure 2.

 

The abnormal head position measurement. (A) Hold the head in a straight position. An initial position of 0 is noted (A1, A2). (B) The patient stares at the gaze target, inducing abnormal head position during fixation (B1, B2).

Figure 3.

 

Dynamic curves and mark lines of the three baselines and peaks of head position measurement.

Figure 4.

 

Test the accuracy of the MEMS sensor method using synoptophore. Adjustment of the synoptophore at each 5 degrees from −30 degrees to +30 degrees horizontally to test the yaw values (yellow marking). Adjustment of the synoptophore at each 5 degrees from −30 degrees to +30 degrees vertically to test the pitch and roll values (blue marking).

Figure 5.

 

Use the photographic method and MEMS sensor method for AHP measurements. (A) Subjects were seated and held their head at an initial position (0 degrees in all 3 dimensions) at the beginning of each trial. Taking photographs from three different axes. (B) Analysis the photos of AHP from three different axes and get the roll (B1), pitch (B2), and yaw (B3) values (yellow marking). Meanwhile, the MEMS sensor method record the AHP in real-time and output the values of roll, pitch, and yaw automatically.

Results

Accuracy of MEMS Sensor Head Position Measurement on Synoptophore

The 95% limits of agreement (LoA) of deviation measurements (means ± SDs and ranges) on synoptophore as measured at intervals of 5 degrees for pitch angle = 0.66 ± 0.76 degrees (0.0–2.43 degrees), yaw angle = 0.30 ± 0.18 degrees (0.11–0.6 degrees), and roll angle = 0.77 ± 0.66 degrees (0.11–2.27 degrees; Table 1). There were no statistically significant differences among these three measurements (P = 0.12). The correlation coefficients between the deviation confidence interval (CI) and the measurement angle for these three measures were −0.39 (P = 0.19), −0.05 (P = 0.88), and −0.13 (P = 0.67), respectively, with none of these correlations achieving a statistically significant difference. 

Table 1.

 

Accuracy of MEMS Sensor Head Position Measurement on Synoptophore

 

Correlations of the MEMS Sensor Method and Photographic Method for AHP Measurements

The differences (means and 95% CI) of the MEMS sensor method and photographic method for AHP measurements for the roll angle = −0.05 degrees (−1.79 degrees to 1.68 degrees), the pitch angle = −1.19 degrees (−0.74 degrees to 3.12 degrees) and the yaw angle = 0.97 degrees (−0.60 degrees to 2.53 degrees). The correlations between the MEMS sensor method and photographic method for AHP measurements were 0.98, 0.98, and 0.99, respectively (P < 0.001; Table 2). 

Table 2.

 

Correlations of the MEMS Sensor Method and Photographic Method for AHP Measurements

 

Clinical Characteristics of the Patients With SOP

A total of 31 patients (10.03 ± 10.19 years old) with unilateral congenital SOP were included in this study. There were 18 (58.1%) male patients and 13 (41.9%) female patients (Supplementary Material). All 31 patients underwent strabismus surgery including inferior oblique myectomy/anterior transposition with or without superior oblique tucking. 

Repeatability of Head Posture Measurements in Patients With SOP

Three consecutive AHP measurements were taken within each of the 31 patients with unilateral congenital SOP and the intra-class consistency coefficient (ICC) was calculated for repeated measurements in 3 axes. The ICC for preoperative repeated measurements of pitch, yaw, and roll were 0.898, 0.887, and 0.954, respectively, with postoperative values being 0.846, 0.794, and 0.918, respectively, indicating a high degree of repeatability in these measurements (Table 3). 

Table 3.

 

Repeatability of Head Posture Measurement in Patients With Unilateral SOP

 

Pre- Versus Postoperative Clinical Characteristics in Patients With SOP

Respective pre- and postoperative values for vertical deviations were 16.4 ± 7.3 PD and 4.1 ± 4.2 PD (P = 0.001) and the rotation angles of fundus photography were 23.3 ± 10.2 degrees and 16.5 ± 8.1 degrees (P < 0.001). The main deviation axis in preoperative AHP was roll, which was observed in 22 cases (71%). There was a statistically significant difference in the composition of the main deviation axis between pre- versus postoperative AHP, with roll and yaw being the main deviation axis, respectively (P = 0.03). Pre- versus postoperative differences in roll deviation angles were statistically significant (P = 0.008), with a reduced AHP being observed postoperatively (Table 4). 

Table 4.

 

Pre- and Post-Operation Clinic Characteristics in Patients With Unilateral SOP

 

Possible Factors Associated With AHP in Patients With SOP

Correlation coefficients among vertical deviation, total angle of extorsion in both eyes, unilateral angle of extorsion in the dominant and non-dominant eyes, and AHP values were 0.151 (P = 0.52), 0.526 (P = 0.01), 0.538 (P = 0.01), and 0.278 (P = 0.21), respectively. The AHP of patients with SOP was positively correlated with the total angle of extorsion in both eyes and unilateral angle of extorsion in the dominant eye. 

Discussion

An accurate and reliable measurement of AHP in patients with strabismus represents an essential component for the diagnosis, surgical protocol, and assessment of surgical correction. It is critical that a simultaneous measurement involving the 3 axes in a 3D space be conducted to reflect an objective position of the head posture. As AHPs mostly occur in children in the early stages of their functional visual development,^15,16 the measurement device used needs to be lightweight and easily/rapidly applied. Moreover, when wearing the device, it is necessary to minimize adverse stimuli and fears in these children in order to obtain a valid and reliable measure of the AHP. 

Many head position measurement devices have been developed and applied for clinical use. For example, Hald et al.⁹ used a protractor installed on the top of a helmet to measure head tilt, while using a laser pointer on the helmet to project the position onto a screen to measure face rotation and up/down tilt angles. Although this device can simultaneously measure the angles within 3 axial directions of the head, the error can be as high as 8 degrees. Moreover, the need to wear a helmet and related physical devices on the head and carefully coordinate the initial position of the laser indicator with the center position of the screen imparts a considerable degree of inconvenience and discomfort, especially for uncooperative children. As an approach to improve measuring devices for children, the Hald et al.¹⁰ team upgraded and developed a head mounted motion tracker that included a magnetometer, accelerometer, and gyroscope. This device was connected to a specific display and recording program on a desktop computer through a cable. Consistency of the deviation angle measured with this device can achieve 0.99 and the error of measurement in children was < 10 degrees. However, the relatively large size of the device and cables required to connect the head to the computer can interfere with the head posture. Other previous methods have included an infrared optical head tracking apparatus¹¹ and a two-dimensional photography calculation of means.¹² However, these methods are difficult to apply in clinical practice due to their inability to dynamically record changes in head position posture in three directions without disturbing the patient’s gaze. 

Gyroscopes, with their capacity for stability and precision, are widely used in the field of aerospace, measuring such variables as spatial altitude and flight direction of aircraft. With a gyroscope stably and firmly adhered to the head and rotating with the head, it is possible to record real-time positions and postures of the head in a 3D space. The MEMS head position measurement device developed for use in this study does not require a headband or cable connection. In addition, it exerts a minimal impact on the patient’s head position, especially in young children, making it easier for them to cooperate with the recording of their daily AHPs. Its lightweight, wireless connection, dynamic curves, automatic data analysis, and ability to generate reports add to the assets of this technique. Compared to the standard position of the synoptophore, the maximal deviation of measurement accuracy is within 3 degrees. The ICC was calculated for repeated measurements in three axes and demonstrated a high degree of consistency (good to excellent ICC) as observed with repeated measurements of pitch, yaw, and roll. The head position outputs from the MEMS posture measuring system are almost identical to the photographic method. The correlation coefficient of the roll, pitch, and raw output between the MEMS and photographic method were 0.98, 0.98, and 0.99, respectively. The output of the roll, pitch, and roll varied within 4 degrees. This photographic method requires patients to maintain their head position for several minutes to take photographs from three axes. The patients need to be very cooperative so it is hard to apply on children. For their data analysis is time-consuming, and the results are not promptly available. The MEMS measurement is quick and easy, and each measurement takes only a few seconds. The APP automatically calculates the average pitch, roll, and yaw values and outputs. A goniometer is another instrument with graded markings, commonly used to measure AHP in the clinic, but obviously the head position of the subject is easy to be affected and difficult to maintain when measuring from three axes. Collating this information, it is clear that the MEMS sensor measurement has the advantage of real-time quantitative recording, brief measurement times, a high degree of accuracy, and consistency. Accordingly, MEMS can provide an effective means for AHP measurements in the clinical setting. 

AHP due to SOP can result in an asymmetric development of skeletal muscles in the neck and face,^17,18 symptoms which represent surgical indications for SOP. The aim of surgery for SOP is mainly to avoid diplopia, eliminate AHP, and/or to improve the general appearance of the patient. In our study, results of MEMS sensor measurements revealed that the main deviation of AHP in patients with congenital SOP is tilt (71%), with the average tilt angle being 14.16 ± 9.57 degrees. Khorrami-Nejad et al.¹⁹ analyzed the AHP of patients with congenital and acquired SOP using head photography, and reported that tilt was the main AHP symptom, accounting for 48.9% of their patients, with an average tilt angle of 15.1 degrees. These tilt angles are similar to that as observed in our present study. Here, we also report that the average vertical deviation in patients with unilateral SOP was 16.4 ± 7.3 PD, the average total angle of extorsion was 23.3 ± 10.2 degrees, inferior oblique weakening and/or superior oblique strengthening can correct at an average of 12 PD vertical deviation and 7 degrees extorsion and 10 degrees head tilt. Huang et al.²⁰ corrected Knapp V-type congenital SOP by performing a contralateral inferior rectus muscle recession. The vertical deviation in primary position decreased from an average of 6.33 PD preoperatively to 0.75 PD postoperatively, with the average AHP decreasing to 6.9 degrees. These results were less than the changes in correcting head position as obtained in our study. In the Huang et al.²⁰ study, no analyses were performed on changes in extorsion. We believe that vertical rectus muscle surgery may have less of an impact on extorsion than that of oblique muscle surgery, and therefore less of an effect in correcting AHP. Akbari et al.²¹ studied patients with unilateral SOP receiving an inferior oblique myectomy. The success rate of this surgery was 89.7%, with an average correction of 14.2 PD hypertropia in primary position. The amount of hypertropic correction was related to the preoperative vertical deviation and the average correction for AHP was 9.7 degrees. These results are similar to that as obtained in our current study. 

It is generally believed that patients with SOP adopt AHP to reduce their vertical deviation and maintain binocular vision. However, our analysis indicates that AHP is not related to vertical deviation, but to extorsion in the dominant eye and the total extorsion in both eyes. Nabie et al.²² studied the surgical effects of an anterior transposition of the inferior oblique muscle on unilateral SOP with a hypertropia of > 25 PD. The success rate was 72%, AHP was reduced from a preoperative value of 80% to 70% postoperatively and the average correction of extorsion was 5 degrees. It was the preoperative fundus extortion, and not the preoperative vertical angle, that was negatively correlated with the surgical success rate. Chen et al.²³ evaluated the changes in the Bielschowsky’s head tilt test before and after superior oblique muscle tucking in patients with unilateral congenital SOP. They reported that the total rotation angle of fundus photography decreased from an average of 15.62 degrees preoperatively to an average of 11.25 degrees postoperatively, with 77.3% of these patients showing a negative postoperative head tilt head test and the difference in bilateral quantification of the tilt head test decreased from 8.68 PD preoperatively to 3.77 PD postoperatively. The difference in vertical deviation at 1 day after surgery and at the last follow-up, when the head was tilted to both sides, was correlated with the sum of their binocular rotation. Therefore, both torsional and vertical changes in AHP may play a role in reducing diplopia and maintaining binocular vision in patients with SOP. A complete understanding of the pathophysiological mechanisms of AHP in patients with SOP remains unknown. The findings from Pansell et al.²⁴ and Groen et al.²⁵ indicated that the otolith–ocular system acts to stabilize the eye position in space and a close relationship exists between eye rotation and eye roll through the otolith–ocular system. 

This preliminary study provides considerable evidence demonstrating that the MEMS sensor with its high-precision and real-time head positioning can be used to quantitatively measure head posture. In patients with unilateral SOP, AHP is mainly related to extorsion of the dominant eye. Oblique muscle surgery can improve the extorsion and aid in correcting AHP. This micro-wireless dynamic high-precision device for head posture measurement has the potential for use in evaluating the efficacy of different surgical methods for strabismus, analyze the mechanisms of AHP resulting from factors, such as nystagmus, vestibular reflex, and strabismus, and enables a personalized plan for the treatment of this condition. 

Acknowledgments

Supported by grants from the University Science and Technology Achievement Service and Industrial Development Support Project, Foshan City, Guangdong, China, 2024 (No. 2024SWYY02). 

Disclosure: X. Qiu, None; Z. Wang, None; L. Pan, None; T. Shen, None; D. Deng, None; Q. Chen, None; J. Yan, None