Tuning fork in neurological examination

What it is

The tuning fork carries the two bedside hearing tests, the Weber test and the Rinne test, and the fork used is 512Hz. These tests are performed with tuning forks of different frequencies such as 128, 256, 512, 1024, 2048 and 4096 Hz, but for routine clinical practice a tuning fork of 512 Hz is ideal: forks of lower frequencies produce sense of bone vibration, while those of higher frequencies have a shorter decay time and are thus not routinely preferred. The Rinne test sets air conduction against bone conduction: the fork heard in the air against the same fork pressed on the mastoid bone. To test air conduction the vibrating fork is placed vertically in line with the meatus, about 2 cm away from the opening of the external auditory canal, and the sound waves are transmitted through the tympanic membrane, middle ear and ossicles to the inner ear, so air conduction tests the function of both the conducting mechanism and the cochlea. To test bone conduction the footplate of the vibrating fork is placed firmly on the mastoid bone and the cochlea is stimulated directly by vibrations conducted through the skull bones, so bone conduction is a measure of the cochlear function only. Normally hearing through air conduction is louder and heard twice as long as through the bone conduction route. A positive Rinne test is air conduction greater than bone conduction. Each ear is read on its own, so one ear can be Rinne positive while the other is Rinne negative, and a negative ear is one in which bone is heard better than air. The Weber test is done by placing the fork over the central forehead, and what is asked is where the sound is heard: in the midline, or loudest in one ear, which is what is meant when the sound lateralizes to that side. The hand comes before the fork. A rough hearing test is done first: the patient occludes one ear, the examiner gently rubs index finger and thumb together and moves the hand from peripherally towards that ear, and the patient says when the sound is heard; then the other side. The way each fork test is set out. For Weber the round base of the fork is placed on the forehead between the eyes, after the long ends are struck. For Rinne the round base is placed on the mastoid process and the patient says when the sound stops, and only then are the long ends held near the ear and the patient asked whether the sound is heard again. The same fork does the neurological work too. Vibration perception is tested with a tuning fork of 128 Hz, and in diabetic foot screening it stands beside sensory examination using a 10g monofilament, pinprick sensation, proprioception and the ankle reflex. The order of the diabetic foot examination places it step by step. The examiner starts with a hand wash, an introduction and consent, then both feet are inspected dorsal and plantar for skin changes, deformities such as Charcot, ulcers, scars and signs of infection; the shoes are inspected for wear patterns, fit or foreign objects; the dorsalis pedis and posterior tibial pulses are palpated and compared bilaterally, an absent pulse being a vascular concern; temperature is felt with the back of the hand and cap refill pressed on the toe pad, cold feet meaning poor perfusion and a CRT over 2s being delayed. Then the 10g monofilament is applied on the 1st, 3rd and 5th MT heads plus the heel and hallux, with the patient asked to close the eyes and callused areas avoided; then the 128 Hz tuning fork on the bony prominence of the great toe; then proprioception, moving the big toe up or down and asking the direction; then optionally the ankle jerk reflex, which is diminished in neuropathy. The last step is to thank the patient, summarize and document. The dermatological part looks for fungal infection between the toes, and the musculoskeletal part for Charcot joint and claw fingers.