Higher-order aspects of sensation, or cortical sensation, should be tested as well. To test graphesthesia, ask the patient to close their eyes and identify letters or numbers that are being traced onto their palm or the tip of their finger. To test stereognosis, ask the patient to close their eyes and identify various objects by touch using one hand at a time. Test also for tactile extinction on double simultaneous tactile stimulation. Note that graphesthesia, stereognosis, and extinction cannot reliably be tested for unless primary sensation is intact bilaterally.
Somatosensory deficits can be caused by lesions in peripheral nerves, nerve roots, the posterior columns or anterolateral sensory systems in the spinal cord or brainstem, the thalamus, or sensory cortex. Recall that position and vibration sense ascend in the posterior column pathway and cross over in the medulla, while pain and temperature sense cross over shortly after entering the spinal cord and then ascend in the anterolateral pathway (see Neuroanatomy through Clinical Cases, Figures 2.13, 2.18 and 2.19). Intact primary sensation with deficits in cortical sensation such as agraphesthesia or astereognosis suggests a lesion in the contralateral sensory cortex. Note, however, that severe cortical lesions can cause deficits in primary sensation as well. Extinction with intact primary sensation is a form of hemineglect that is most commonly associated with lesions of the right parietal lobe. Extinction can also be seen in right frontal or subcortical lesions, or sometimes in left hemisphere lesions causing mild right hemineglect.
The pattern of sensory loss can provide important information that helps localize lesions to particular nerves, nerve roots, and regions of the spinal cord, brainstem, thalamus, or cortex (see Neuroanatomy Through Clinical Cases, Key Clinical Concept 7.3).