Threshold (VPT, Vibro 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
Gray 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 a Vibrogram
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 threshold at 125 Hz with a marked increase at 500 Hz is not an uncommon finding, and might be indicative of early pathology.
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 Vibrametry 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 vibrametry test even if the temperature is low.
A vibrogram can be classified as follows (click on the links or vibograms for a large view) :
- Stage 0: SI > 0.8; all seven thresholds values (8 – 500 Hz) within the grey zone: normal vibrogram (examples A and B)
- Stage 1: SI > 0.8; One to three thresholds values below the grey zone: suspected early pathology (example C)
- Stage 2: SI < 0.8; low vibrogram curve, just one or a few thresholds values still within the grey zone: obvious pathology (example D)
- Stage 3: SI < 0.8, very low vibrogram curve, all thresholds values under the grey zone: severe pathology (example E, right hand)
A complete vibrogram test should include recordings from the index- and little fingers in both hands, i.e. recordings from 4 fingers. The vibration perception in the index- and little fingers reflects function in the median and ulnar nerves respectively.
The summarized result from all four fingers tells a lot about the etiology of a sensory disturbance in the hand(s). Usually a vibration injury results in pathological recordings from all four fingers.
A carpal tunnel syndrome results in a pathological recording from the index finger in the corresponding hand, usually with normal recordings from the other fingers (example F, right hand ).
A pathological recording from the little finger may indicate entrapment of the ulnar nerve, usually at elbow or wrist level (example G, right hand).