Introduction
The position of the eye, head and shoulders can be negatively influenced by many diseases of the nervous system, (particularly by visual and vestibular disorders) (Cerny R. et al, 2006). Disturbances of the cervical vertebral column are another frequent cause of abnormal head position. In this topic we describe advanced methods of measuring the precise position of the eye, head and shoulders in space. The systems and methods are designed for use in neurology to discover relationships between some neurological disorders (such as disorders of vestibular system) and postural head alignment. We have designed a system and a set of procedures for evaluating the inclination (roll), flexion (pitch) and rotation (yaw) of the head and the inclination (roll) and rotation (yaw) of the shoulders with resolution and accuracy from 1° to 2° (Hozman et al, 2007). We will also deal with systems designed for parallel measurement of eye and head positions and a new portable system for studying eye and head movements at the same time is described as well (Charfreitag et al, 2008). The main goal of this study is to describe new systems and possibilities of the present methods determined for diagnostics and therapy support in clinical neurology. Furthermore, we describe the benefits of each method for diagnosis in neurology.
Background and related works
The measurement of eye position is an important diagnostic instrument in both clinical and experimental examination of human vestibular system (Cerny R. et al, 2006). Also, the simultaneous measurement of head (Murphy et al, 1991) and shoulders position (Raine et al, 1997) could contribute to better definition of diseases affecting the vestibular system (labyrinthine) function in man.
Clinical significance of head posture measurement
Abnormal head posture (AHP) is an important clinical sign of disease in many medical specialities. AHP is a consequence of dysfunction of musculoskeletal, visual and vestibular systems (Brandt et al, 2003). AHP is of particular importance in childhood, when developmental abnormalities of different origin can manifest with AHP as a main clinical symptom. The differential diagnosis is broad and quantitative assessment of head position in space it is important for both treatment and evaluation of disease evolution. In an Italian study 73 children referred by paediatricians the most common cause of AHP was orthopaedic disease (congenital muscular torticollis, 35 cases) followed by ocular motor palsy (mostly superior oblique palsy, 25 cases). Neurological disease was found in 5 cases, in 8 cases no underlying disease was indentified (Nucci et al, 2005).
Most peculiar forms of AHP are due to cervical dystonia, a movement disorder due to the disturbance of motor control of cervical muscles. Exact pathophysiology of this disabling and hard to treat condition is not known and includes local, suprasegmental and psychological factors. It can be classified according to the abnormal positioning of the head and spine into ante/retrocollis (sagittal plane), laterocollis (frontal plane) and rotatocollis (horizontal plane), pure forms are rare, typical is combination (torticollis). The pattern of muscles involved in generation of the AHP can be inferred from the head position. Objective and quantitative measurement of head position is of great importance, as treatment with botulotoxin (nowadays first choice) requires exact identification of muscles involved in AHP generation and follow up of treatment efficacy with objective head positions recordings is important for choosing optimal long term treatment strategy. Standard assessment scales for torticollis use semiquantative clinical scores or simple goniometers with low precision (Galardi et al, 2003; Novak et al, 2010).
Blockades and disease of cervical spine due to spondylosis or trauma are very common cause of AHP in clinical practice. Here the quantitative head posture measurement is not imperative, but simple objective recording of abnormality evolution can be useful in chronic cases and when cervical spine surgery is considered.
AHP is a frequent and important sign in ophthalmology, particularly in childhood. It represents compensation of abnormal eye position and/or motility. Paralyses of eye muscles are compensated by a tilt of the head in direction of the weakened muscle. In congenital nystagmus the AHP tends to shift gaze direction in the null zone of the nystagmus. As a result of the compensatory head position, the vision acuity is enhanced or restored, but unbalanced muscle activation can lead to cervical spine disorders in the long term. Surgical procedures aimed at correction of the eyeball position are effective in repairing the AHP and are considered treatment of choice (artificial divergence, Kestenbaum surgery). The dosage of ocular muscle retroposition/resection depends on the angle of AHP with fixation of distant target. The reduction of abnormal head turn with 1mm muscle resection was 1.4° head turn on average in one study (Gräf et al, 2001).
Ocular tilt reaction is a well established symptom of dysfunction of the graviceptive pathways starting from the otholithic maculae of the inner ear to the vestibular nuclei and paramedian thalamus. This syndrome is defined by the triad of signs – head tilt, ocular globe rotation a deviation of the subjective visual vertical. All deviations directs towards the weak labyrinth, or to the contralateral side after crossing at the pontine level, in the case of brain stem lesions. Head tilt in the frontal plane is usually quickly compensated, after the acute phase is over, but more subtle signs (ocular rotation and subjective vertical) can last for weeks and months. Horizontal eyes alignment is precisely regulated within narrow range of several degrees (Halmagyi et al, 1991), (Brandt & Dieterich, 1994). Deviations in the
horizontal plane are also easily appreciated even by naked eye during examination. Little is known about head turn in vestibular syndromes. This type of deviation is hard to assess by observation only, indeed, only gross deviations in cases of ocular torticollis are used in clinical practice and regularly cited in literature. Vestibular imbalance due to unilateral labyrinthine failure causes vestibulospinal deviations towards the weaker labyrinth (Hautant reaction, Romberg deviation in standing with closed eyes etc.). It is reasonable to expect head turns of several degrees due to the functional imbalance between the activity horizontal channels. In contrast to the tilt reaction such a finding was not well described until now. Probably, this type of vestibular rotatocollis is compensated by spatial visual clues with open eyes and can be easily overlooked. In this situation, precise technique for head rotation measurement would be of paramount importance.
Last, but not least, precise 3D head position measurement has many potential implications for physical medicine and rehabilitation, particularly in the management and diagnosis of disorders affecting cervical spine. Head position in the sagittal plane is very variable and influenced by many factors, particularly habitual holding of the spine as a whole. Habitual head anteflexion with chronic overload of cervical and upper thoracic spine and muscle imbalance is typical consequence of uncompensated sedentary way of life, starting already in school age. Main reference for sagittal plane is so called Frankfort horizontal (line connecting meatus acusticus with the orbital floor or line connecting tragus with the outer eye canthus), see Figure 1. In most subjects this line is inclined forward bellow the space horizontal, in the extensor type of cervical positions is reclined backwards. The real position of Frankfort horizontal can vary more than 20° in the normative population, in comparison, the position of the eyes in frontal plane is held tightly within several degrees only (Harrison & Wojtowicz, 1996).
Fig. 1. Anatomical Frankfort horizontal and axis.
Precise measurement of head position in rehabilitation and physical medicine is important not only for objective diagnosis of the cervical spine abnormalities, but also as a means of cervical kinesthesia assessment. In this test the ability of the tested subject to assume exact position in space without visual clues is examined (Palmgren et al, 2009). Normal subjects are able to attain desired position with precision of several degrees. Again, these differences are below the discrimination capacity of simple observation or protractor measurement. It is hypothesized, that abnormal setting of cervical proprioception can play important role in many conditions like whiplash injury syndrome, chronic tension headache, cervicogennic vertigo, anteflexion headache etc. (Raine & Twomey, 1997). Evidence of abnormal cervical proprioception would be an important step in better understanding of these common clinical problems.
Monitoring head and shoulders movements
At present, an orthopedic goniometer is the widely used and standard way to simply and rapidly measure angles in clinical practice. However, there are some limitations, especially in case of head and shoulder posture measurement. Due to the combination of three movement components (in the three dimensional space), the measurement using only one goniometer is clearly insufficient. The following overview serves as enumeration of the applications related to the technology available during the last years. This enumeration is not exhaustive but the most important works in the area are included. The methods are typical by using some tools or technology.
Young, 1988, designeda new method to study head position by mirrors. The main principle of new approach is based on using three mirrors and special head markers. The resulting images are taken by one camera. After this, a set of vertical or horizontal lines is drawn with respect to the reference points i.e. markers. The last step is measurement of the relevant angles by a protractor. Head tilt (inclination), head turn (rotation) and chin elevation or depression (flexion/ extension) is evaluated. One drawback is the evaluation method based on vertical or horizontal lines defined by reference points, i.e. markers and thus wide variation in cranial configuration found between patients and associated with age.
Murphy et al, 1991, described a system for measuring and recording cranial posture in a dynamic manner. Measurement of the declination and inclination was performed by inclinometers. Inclinometers are widely used instruments for measuring angles of elevation or inclination of an object with respect to gravity based on the accelerometers. Inclinometer was attached to the spectacle rims. Processing of the inclinometer voltages was performed by a modified universal data logger. The inclinometer was calibrated by plastic visor and a perpendicular spirit level. However, principle of the inclinometer does not provide measurement of head rotation.
Ferrario et al, 1994, integrated a method based on the photographic technique, radiographic technique, cephalometric measurements and photographic measurements. The measured subjects were photographed and X-rayed in the same room. The set of standardized marks was traced on all the records. On all photographs, the soft tissues were traced, and the angle between the soft tissue marks and true vertical was calculated. The same angle was calculated on the cephalometric films, and the difference between the two measurements was used to compute the position of the soft and hard tissues. These new values were compared with the values previously observed. The main drawback is exposition of patients to X-ray and relatively time consuming procedures.
Ferrario et al, 1995, developed a new method based on television technology that was faster than conventional analysis. Subject’s body and face were identified by 12 points. All subjects were pictured using a standardized technique for frontal views of the total body and lateral views of the neck and face. After 20 seconds of standing, two 2-second films were taken of each subject. On the basis of an image analysis program, the specified angles were calculated after digitizing the recorded films.
Galardi et al, 2003, developed an objective method for measuring posture and voluntary movements in patients with cervical dystonia using Fastrack. Fastrack is commercial widely used electromagnetic system consisting of a stationary transmitter station and four sensors placed on patient’s head. The head position in space was reconstructed based on sensor signals and exact values of angles were observed from the axial, sagittal and coronal planes. The drawback is its inaccuracy in determining the exact position in space because of relatively large sensors placed on the patient’s head and therefore inaccurate determination of the anatomical axes. Second drawback is the negatively affected accuracy of an electromagnetic system by other laboratory systems.
Hozman et al, 2004, proposed a new method based on the application of three digital cameras placed on a stand and appropriate image processing software. The method was designed for use in neurology to discover relationships between some neurological disorders (such as disorders of vestibular system) and postural head alignment. The objective was to develop a technique for precise head posture measurement or, in other words, for measuring the native position of the head in 3D space. The technique was aimed at determining differences between the anatomical coordinate system (ACS) and the physical coordinate system (PCS). Pictures of the head marked on tragus and outer eye canthus are taken simultaneously by three digital cameras aligned by laser beam. Head position was measured with precision of 0.5° in three planes (rotation-yaw, flexion-pitch and inclination-roll). Hozman et al, 2005, described the new modified system and results are shown and measured on normal subjects. The disadvantage is complicated calibration and the impossibility of a frontal view of the measured subject (Hozman et al, 2007).
Cerny et al, 2006, described second advanced generation of the system. Head position was measured with precision of 0.5° in three planes. Mean values of the head position (100 healthy controls) are : retro flexion 21.7°; inclination to the right 0.2°; head rotation to the left 1.7°.
Meers et al, 2008, developed accurate methods for pinpointing the position of infrared LEDs using an inexpensive USB camera and low-cost algorithms for estimating the 3D coordinates of the LEDs based on known geometry. LEDs are implemented in the frame of eye-glasses. The system is accurate low-cost head-pose tracking system. Experimental results are provided demonstrating a head pose tracking accuracy of less than 0.5° when the user is within one meter from the camera. However, the system does not define the anatomical axis of the head and the adaptation of the system is impossible for measurement of anatomical angles.
Recently a number of instruments and tools based on commercial systems have been developed for evaluating the position of the head and shoulders. An example is Zebris motion analysis system (zebris Medical GmbH). Special instruments primarily allow studying ranges of motion of the head, ranges of motion of the spine and coordination of movement. The modified Zebris also allows studying the movement of the jaw. Zebris detects small misalignment of the lower jaw. The three-dimensional measuring coordinates of the ultrasonic markers can be recorded with an overall scanning rate of 200 measurements per second. The modified system consists of a face bow with integrated receiver module and an optimally balanced mandible which measures sensor close to the mandible joint. Unfortunately, the system also does not define the anatomical axis of the head and the adaptation of the system is complicated.
There are other modified commercial diagnostic systems based on ultrasonic measurement method (sonoSens Monitor), a camera method (Vicon motion systems, LUKOtronic AS100/ AS200), a gyro-accelerometer sensors (Xsens motion trackers), etc. But the systems have the similar disadvantages such as complex preparation, very large sensors or the inability to accurately define the anatomical coordinate system.
Monitoring eye and head movements
Monitoring eye movements and plotting their trajectories have a long tradition in medical practice. The measurement of eye position is an important examination tool in understanding human vestibular system. It is used as a diagnostic tool in neurology and psychology (Brandt et al, 2003). Eye tracking is a widely used method of measuring the point of gaze or the motion of an eye relative to the head. An eye tracker is a device for measuring eye position and movement. There are number of methods of measuring eye movement. Eye trackers fall into three categories:
One type uses an attachment to the eye, such as a special contact lens with an embedded mirror or magnetic field sensor. Measurements with contact lenses have provided extremely sensitive recordings of eye movement. However, mechanical elements attached to the eye can negatively influence patient’s eye.
The second category uses electric potentials measured by electrodes placed around the eyes. The eyes are the origin of a steady electric potential field. The electric signal that can be derived using two pairs of contact electrodes placed on the skin around eye is called Electrooculogram (EOG). This EOG is sensitive to the saccadic spike potentials from the ocular muscles. The electric potential field can also be detected in total darkness and if the eyes are closed.
The third category uses non-contact, optical method for measuring eye motion. The method is called Videooculography (VOG). Optical methods are widely used for gaze tracking and are favoured for being non-invasive and inexpensive. By looking to the eye we can see its elements – outer filamentous layer with title sclera, further is cornea, iris and eye pupilla. Light, typically infrared, is reflected from the eye and sensed by a camera. Video based eye trackers usually use the corneal reflection and the centre of the pupil as subjects to track over time. The videooculography based on IR spectrum usually uses the infrared light created by a LED (light emitting diode) diode with a wavelength approximately λ=880-940nm. The VOG method in the IR spectrum detects the pupil using an appropriate light that makes it completely black. The Advantage of this method is relatively easy pupil detection and good quality reflection, most often using an IR LED diode. Disadvantage and limitation is a need to make measurements without access of visible light, i.e. in conditions that do not correspond with patient’s real situation.
Eye analysis in the visible light spectrum is far more complicated. The method is called passive, because the eye is scanned in the visible light spectrum due to the diffused visible light. The method without the IR supplementary light is not only safer for the patient (undesirably warms up the eye), but also much more preferable, because it does not necessarily need suppression of background light. Detection can be done due to the sclera and iris interface. Disadvantages of these methods are the uncontrolled lighting from scattered sources, considerable luminous artefacts and high computational power. Also accuracy of these methods is rather poor, because in contrast to the pupil of the eye which is visible during measuring, the interface between sclera and iris is often hidden.
For parallel measurement of head and eye position (Eui et al, 2007) the best way is to use a VOG method that is based on the principle of scanning the eye (Ruian et al, 2006) using a mobile set of video cameras and consequent data post-processing to a different result in IR (infra red) or visible light spectrum (e.g. nystagmogram, fixing the eye to the projected area etc.). The mobile set is then attached to the head position measurement system based, for example, on gyro-accelerometer sensors. This new parallel measurement method has not been systematically studied and bothmentioned measurement methods have been examined only separately.
Precise advanced eye, head and shoulders position measurement
Numerous systems for evaluation of eye and upper body parts positions are currently offered on the market, but their wider application is impeded by high financial demands and inaccuracy, because these universal systems are not usually designed for application to study a particular body part – head and shoulders. In the following part of the topic we will describe the specialized systems designed at CTU Prague and the other labs, to precisely measure the eye, head and shoulders posture at the same time.
