Multi Frequency Vibrometry
Multi Frequency Vibrometry (MFV) is an objective medical measurement method, used to support diagnosis of damage to nerve trunks, nerves and mechanoreceptors in the hands and feet, so-called Peripheral Sensory Neuropathy.
MFV measures and quantify the ability to perceive vibrations on the skin at fingers and feet. The method is specified in the ISO standard 13091-1/2. Multi Frequency Vibrometry can detect very early signs of sensory impairment due to neuropathy or nerve entrapment, such as carpal tunnel syndrome. Research at Lund University and Skåne University Hospital in Malmö, show that Multi Frequency Vibrometry can detect changes earlier than other methods.
Research behind the method
Multi Frequency Vibrometry is developed by professors Göran Lundborg and Lars Dahlin et.al at the university hospital in Malmö and Lund. MFV can be described as a hearing test on the skin since the method is strongly influenced by audiometry
A fingertip, or an area on the sole of the foot, is subjected to a controlled mechanical vibration of varying strength and frequency. The patient responds by pressing a response button when she/he can perceive a vibration of the skin.
Possibility to take preventive measures
Multi Frequency Vibrometry is an integral part of the VibroSense Meter II which can detect pre-symptomatic or subclinical changes. This means that preventive measures can be put in place to prevent delay or mitigate permanent damage. MFV can also be used to monitor neurological changes over time.
Several patient and occupational groups are at risk
MFV can be used in several contexts where neuropathy may occur, for example in diabetes, chemotherapy, vibration damage, alcoholism, drugs and more.
Peripheral neuropathy often evolves slowly when working with vibrating tools where vibrations are transferred from the tool to fingers, hands and arms. Workers in vehicle workshops, on construction sites, in the engineering industry, with dental and foot care are at high risk of getting this type of damage.
Physiological measurement principle
The vibration sensitivity is part of the neural sensory system. In the skin there are a number of mechanoreceptors which are activated by different stimuli, including static pressure and vibrations. Different receptors react to different frequencies. Multi Frequency Vibrometry utilizes this fact, giving the physician valuable information at diagnosis.
The vibration sensitivity deteriorates with age
The vibration sense has a maximum sensitivity between 200-250 Hz, which probably involves several types of receptors. The vibration sensitivity deteriorates with age, just like the hearing. In particular, the ability to perceive higher frequencies deteriorates.
The mechanoreceptors are connected to myelinated nerve fibres which are collected in larger nerve trunks. These lead up to the brain via various "connections" passing the spine. If a patient does not perceive a particular frequency, it may be due to several reasons, ranging from a thick fibre neuropathy in e.g. the foot or fingers to a nerve compression along path to the brain.
Automatic comparison with age and gender matched reference data
An examination of the sensitivity with VibroSense Meter II works in the same way as a hearing test. The VibroSense Meter II records objective Vibration Perception Thresholds (VPT) at different frequencies and compares them with the reference data from 1100 healthy subjects, age 8-70. Thus the comparison is also made with an automatic age- and gender matching. The latter is important, since the vibration perception capacity (as the hearing) deteriorates with age.
Fast and easy examination
During an examination, the patient first place the foot or a fingertip on a small "rod" which then starts vibrates with different intensity and frequency. The patient responds by pressing a response button (and holding it down) as long as she or he can perceive a vibration. The examination is fast (about 3-4 minutes per examined point) and very easy to perform.
Vibrogram disclose deviations
The result is recorded in a graphical curve, a Vibrogram, which shows whether the sensitivity is within a normal range or if there is a risk of incipient injury. A full examination with VibroSense Meter II includes measuring of two points per foot: under the big toe and under the little toe (metatarsal head 1/2). Two fingers are examined on the hand (index finger and little finger).
A questionnaire for different types of indications is integrated into the VSM PC software, which can be used to supplement the examination. The VibroSense Meter II software also contains multiple modes for screening examinations. Then a simplified and faster procedure is used.
Reports neurological sensory status
VibroSense Meter II measures the ability to perceive fine-tuned vibrations in the skin at seven different frequencies between 4 and 500 Hz. For each frequency, a so-called Vibration Perception Threshold (VPT) is recorded. Various mechanoreceptors in the skin react within specific frequency ranges. Hence, it is not enough to just measure the sensitivity at one frequency, in order to get a total picture of the vibration sensitivity,
Age and gender-related reference curve
The Vibrogram is a graphical curve that shows the neurological sensory status (vibration sensitivity) in relation to an age- and gender related reference data from healthy subjects.
A recorded Vibrogram has normally a characteristic shape. An impaired sensitivity of vibration is associated with different changes to the shape
Result reported in different units
Impaired vibration sensitivity results in a clear change in the Vibrogram curve. The Vibrogram shows the patient's curve in relation to age- and gender- related reference data (shaded pink area) on the screen. The patient's Vibrogram can therefore be directly compared to a population of healthy individuals.
The result is reported in various measurement units such as Z-score or SI (Sensibility Index, for hands only). The Vibrogram shows directly whether the vibration sensitivity is normal or if there are any signs of early impairments or a suspected injury. Click on the links to compare a normal (no injury) curve with a curve showing impairments (pathological).
A Z-score is calculated for each Vibration Perception Threshold (VPT). The Z-score is related to the mean (average) of the internal reference data and it is measured in terms of standard deviations from the mean value. Hence, a Z-score = 0 indicates that the VPT is identical to the mean score. A Z-score of 1.0 indicate a VPT value that is one standard deviation above the mean. Z-scores may be positive or negative. The Z-score is also known as the Altman Z-score
As a rule of thumb a Z-sore > 2 for one frequency should be interpreted as a pathologic value in a Vibrogram.
The Zavg-score is also calculated as the mean value of all Z-scores. A Zavg > 1,65 is regarded as pathologic value based on a Bonferroni correction.
Sensibility Index, SI
The Sensibility Index (SI) is a quantitative measure of the vibration sensibility, measured by the VibroSense Meter. In the skin, there are different types of mechanoreceptors that respond to vibrations of different frequencies. SI is calculated by testing the vibration sensibility at seven different frequencies between 8-500 Hz, which taken together yield a curve - a Vibrogram. Changes in the shape of the curve give an early indication of impaired sensibility.
The Sensibility Index (SI) is calculated as the ratio between the integrated areas under the patient curve and the age-matched normal curve. SI below 0.8 is considered pathological and indicative of nerve damage.
Patient curve is compared with reference data
The Vibrogram curve is automatically compared with a comprehensive age-matched normal population data. Therefore, it is possible to assess how the patients curve relates to the normal curve within the age span. Thus, the SI-value is inherently age-independent.
The SI should primarily be regarded as an average of the vibration sensibility that gives the physician a fast indication of the sensory status. Besides looking at the SI value, the shape of the Vibrogram curve should also be analysed since early sensory disturbances normally appear when the SI still is above 0.8. See also Vibrogram Interpretation below.
Threshold (VPT, Vibration Perception Threshold); The minimum applied vibration level (intensity) with which the patient can sense a vibration in the finger. The threshold is graded in decibel (dB).
Threshold values are recorded as curves at seven different frequencies between 8-500 Hz in a Vibrogram. High threshold values are positioned low in a Vibrogram (with the y-axis pointing downwards) and indicate impaired sensibility.
Grey zone; The shaded area at each frequency in a Vibrogram, showing corresponding threshold values for healthy persons, i.e. the normal population with the mean value +/- 1 standard deviation.
The area is age-matched meaning that the patient’s curve is always compared to normal values for a population of the same age. Just like with hearing, the sensibility deteriorates with age.
Interpretation of SI and Vibrogram for hands
Sensibility Index, SI, is a quick and useful indicator of whether the Vibrogram as a whole is normal or indicative of pathology. As a general rule, a SI > 0.8 should be regarded as normal, while SI < 0.8 indicates pathology.
However, SI is based on a summary of thresholds from all seven frequencies, (8-500 Hz) and in some cases there may be increased vibration thresholds at one or several frequencies, even though SI remains > 0.8. Therefore, it is important to study the shape of the curve, and to evaluate possible pathology at separate frequencies.
Usually, increased thresholds are first seen at the highest frequencies, i.e. 250 and especially 500 Hz. Normal vibration thresholds at 125 Hz with a marked increase at 500 Hz is not an uncommon finding, and might be indicative of early pathology.
Validate the patient's skin temperature
Even if SI is normal (> 0.8) increased thresholds at 250 or/and 500 Hz can justify follow-up testing. Cold hands (patient finger temperature below 27 °C) can also result in increased thresholds at 250 and 500 Hz without being indicative of early pathology.
It is therefore important to consider the temperature, and if necessary re-administer the Vibrometry test if in the case of the patient’s fingers being too cold. Finger temperature is automatically registered by the VibroSense Meter, but it is possible to perform a Vibrometry test even if the temperature is low.
A Vibrogram can be classified as follows (click on the links or vibrograms for a large view):
Outcome for all four fingers
A complete Vibrogram examination on the hand should be performed on four fingers - index finger and little finger on each hand. The sensitivity in the index and little finger reflects the function of the medianus and the ulnaris nerve, respectively. The outcome for all four fingers may tell a lot about the cause of the sensory disorder. A vibration injury usually renders a pathological outcome for several, usually in all four fingers.
A Carpal Tunnel Syndrome in one hand normally results in a pathological Vibrogram curve for the index finger (Dig II), but with normal outcome for the other fingers (example F, right hand). A pathological curve for one hand's little finger may indicate a nerve compression on the hand's ulnar nerve, usually at the elbow or wrist level (example G, right hand).