Vibratory Sensation Testing

In order to appreciate the clinical significance of this test it is necessary to understand the basic anatomy of the somatosensory pathways involved (see Figure 1). The afferent fibres arising from the cutaneous receptors responsible for the detection of vibration, together with those of touch and proprioception, synapse with the first order neurons located in the dorsal root ganglia of the spinal nerve root. From here they travel ipsilaterally the full length of the cord, in the posterior column-medial lemniscus system to synapse with second order dorsal column nuclei in the medulla. In the medulla the fibres cross and ascend as the medial lemniscus into the thalamus, synapsing with the third order neurons, and finally project into the anterior parietal cortex. Within the cord the fibres from the lower thoracic, lumbar and sacral segments ascend in the fasciculus gracilis, while those of the upper thoracic and cervical segments ascend via the fasciculus cuneatus. These fibres are laminated in such a way that on entering the cord they are positioned more lateral and superficial and then gradually proceed more medially. Thus in the cervical region the fibres of the fasciculus cuneatus are more lateral and those of the fasciculus gracilis are more medially situated in the cord (1). This anatomical arrangement is of clinical importance with respect to the localisation of a lesion, in that the superficial fibres are more vulnerable to the extrinsic compression and those situated deeper in the cord are more vulnerable to intrinsic compression (2).