Using a 128 Hz tuning-fork to check for sensory neuropathy ("on-off" method)
Why should you learn to use a tuning fork to check for sensory neuropathy?
Did you know that using a tuning fork is a more sensitive method to check for sensory neuropathy than the monofilament method? When we use a tuning fork, we are actually testing the patient's sense of vibration, which is also part of the sensory system. When a patient develops neuropathy due to diabetes or other systemic diseases, the sense of vibration diminishes earlier than the sense of touch. This means that we can detect whether the patient is developing sensory neuropathy at an earlier stage. Why is this important? When we see that the patient has an impaired sense of touch or vibration, there is an overhanging danger that the patient may develop foot ulcers because they may not feel discomfort when walking with tight shoes or stepping on sharp objects.
The tuning fork has become a somewhat forgotten tool in most clinics. However, you will always see a tuning fork in a neurological office because it is still a standard tool for doing a general neurological examination.
Using a tuning fork involves tapping on the fork so that it vibrates and then placing it on the bony prominences of the body. The patient is supposed to feel the vibration in the bone and not on the skin. We have a step-by-step guide further down in this chapter.
Several recent articles advocate strongly for the use of tuning forks to detect sensory neuropathy. Many neurologists also prefer this method to the monofilament method! So why is the monofilament method chosen as the standard of care internationally? The main reason for this is that a monofilament is usually readily available at most workplaces, and it is a cheap tool.
However, a tuning fork is much cheaper than using monofilaments in the long run. It is surprisingly difficult to buy monofilaments in some places, and in our chapter on monofilament, we have suggested some workarounds like using a nylon fishing line. So- if you can get hold of a tuning fork - buy it! We have seen that they are sometimes available for about 10 US Dollars!
What is a tuning fork?
A tuning fork is a tool designed to tune instruments. Tuning forks come in different frequencies, one for each tone. They are, for example, used when tuning a piano, although often digital tuning devices now have overtaken this. If you want to tune a guitar, you only need a tuning fork with the frequency of 440Hz - as a fun fact.
When it comes to using a tuning fork for testing the sense of vibration, this is standardized to tuning forks with a 128 Hz frequency. The sound you hear when you tap the fork is the equivalent of a middle-high C note on a piano - another fun fact :)
A step-by-step guide to using the tuning fork to test for sensory neuropathy. The " on-off" method
1. Just as when you would use a monofilament when you examine the patient, it is essential to have a relaxed atmosphere in the room. The patient should lie on an examination bench. Take off the shoes and socks on both feet. Always test both feet when you are checking for neuropathy.
2. Explain to the patient what you are about to do and that the test is entirely pain-free.
3. The patient should close their eyes during the examination
4. When we test for sensory neuropathy with a monofilament, we always demonstrate to the patient how the monofilament feels by touching the inside of the lower arms, for example, before we test the feet. This is done to give the patient a chance to get a feeling of what they can expect when we press the monofilament on the skin of the feet. The same principle applies when using a tuning fork, but we also need a bony prominence to demonstrate the vibrations to the patient. An easily accessible prominence is the lower end of the ulna bone outside the wrist. If you inspect your wrist on the top side, you can easily spot and feel this prominence. You could also use the olecranon ( elbow bone) or the sternum (chest bone) to demonstrate the tuning fork, but the wristbone is probably the most convenient.
5. Tuning forks for medical purposes come in two variations. One type has two thick round surfaces at one end. To get this type of fork to vibrate, it is usual to slap it in the palm of your hand. The other kind of tuning fork does not have these ends. To get this type of fork to vibrate, you slightly press the two arms of the fork together with your index finger and thumb and then release them quickly. The latter type requires a little more training, but it gives a more predictable vibration, and we prefer that type ourselves.
Regardless of which type of tuning fork you use - when you have " activated" it - the vibration should be so strong that it vibrates for at least 45 seconds.
Figure 1 There are two types of tuning forks - both can be used for testing for sensory neuropathy. The first variant (left) has two round flat surfaces in one end. This type is " activated" by slapping it gently in the palm of your hand. The other type ( right) is set into vibration by holding it at the bottom with your dominant hand, pressing the ends of the forks together with your left hand's thumb and index finger, and releasing them quickly.
6. Tell the patient that you will demonstrate the vibration on the arm. An accessible anatomic area to demonstrate this is the prominence of the distal ulna at the wrist of the hand. Ask the patient to tell you when they feel the vibration AND when they feel that the vibration has stopped. The last part is very important, and this is what we mean by the " on-off" method. The patient can mistakenly interpret the touching of the skin with the fork as a vibration, and therefore it is essential that we also ask the patient to tell us when the vibration is stopping. To stop the vibrations, the examiner simply holds the ends of the tuning fork, and the vibrations will cease immediately. Wait for about 10 seconds before you stop the tuning fork.
7. Once you have activated the tuning fork, place it on the bony prominence of the ulna - don't press very hard- a slight pressure is enough. The patient has their eyes closed and tells you when they feel the vibration. After about 10 seconds, you stop the tuning fork, and at that point, the patient( still with eyes closed) should tell you that they notice that the vibration has stopped. Do not ask the patient, " do you notice now that the vibrations have stopped? ". You have to inform the patient beforehand that they, on their own accord, should tell you when the vibrations have started and when they have stopped.
8. Once you have demonstrated this on the forearm, you can go to examine the feet. It is common first to test the big toe. Here we usually use the bone which is just behind the toenail. Keep the tuning fork at 90 degrees to the bone and use the " on-off" method.
Figure 2 Usually, we start by testing the sense of vibration on the bone just in front of the toenail of the greater toe. Note how the examiner supprots the great toe slightly with the index finger of the other hand to keep a good contact between the tuning fork and the toe. In the picture below the examiner stops the vibration by holding the tuning fork on the oppsite end. With eyes closed during the entire examination the patient has his/her eyes closed and syas when he/she notices the vibration start and when they stop. This is the so called " on-off" method.
9. If the patient has an intact sense of vibration in the greater toe, it is very unlikely that they have sensory neuropathy, and we do not need to test other areas of the foot. This is because the thinnest nerves- i.e., the nerves furthest down in the foot are first affected by neuropathy. If they appear healthy, then the bigger nerves further up the foot should definitely be in good shape.
On the other hand, if the patient has an impaired sense of vibration in the big toe, then we have to test other areas of the foot further up to map out how high the sensory impairment is. For this purpose, we usually use the bony prominence on the back of the foot and both malleoli of the ankle.
Figure 3 If the patient has an impaired sense of vibration in the greater toe, you should also test the sense of vibration on other foot areas to determine how widespread the neuropathy is. Does it affect the whole foot or is it only in the toes. If you find an impaired sense of vibration in the whole foot then this is a very high risk patient for developing foot ulcers in the future. To test the rest of the foot we usually use the bony prominence on the back of the foot and both malleoli over the ankle.
A slight digression- did you know that a tuning fork also can be used for other medical examinations?
Tuning forks are, for example, used by an ear specialist to diagnose the cause of hearing loss. This is called the Rinne test, where the doctor places the tuning fork on the patient's skull to differentiate between sound transmission by air and conduction through the bone.
Did you know that you can use a tuning fork to diagnose a fracture of a bone? This may be used in an off-the-grid clinic where no x-ray is available. The method involves checking if the patient has pain in the bone when a tuning fork is placed on the same bone which is fractured. Obviously, you shouldn't hold the tuning fork right over the suspected fracture- that will be painful anyway. Instead, find a bony prominence at least 10 cm away from the alleged fracture and place the activated tuning fork there.
Another method is to listen to the vibrations with a stethoscope on the other side of a suspected fractured bone. The fracture area does not transmit vibration well. You will need a very quiet room to perform this test. Obviously, none of these techniques are very reliable and are not really a substitute for an x-ray examination. But if there is no chance of obtaining an x-ray, this may be better than nothing. Anyway, this has nothing to do with wound care- it was just a digression to widen your horizons a little :)
Figure 4 A demonstration of how to use a tuning fork to check for a fracture in the fibula, for example. The examiner has placed the stethoscope at the upper end of the fibula bone and holds the activated tuning fork at the lower end ( lateral malleolus) of the same bone. If the examiner does not hear the sound of the tuning fork, then there may be a fracture present. Note that the vibrations may be painful for the patient as they pass through the bone to the fracture site. This itself is also an indication of a possible fracture.