'Thumb localizing test' for detecting a lesion in the posterior column-medial lemniscal system

A proprioception examination, called the 'thumb localizing test' (TLT), is described as a technique for testing 'limb localization'. With the patient's eyes closed, the examiner positions one of the patient's upper limbs (fixed limb) and asks him to pinch the thumb of that limb with the opposite thumb and index finger (reaching limb). The findings for 221 patients (423 limbs) show that: there were TLT deficits when the limb contralateral to the cerebral lesion or ipsilateral to the peripheral nerve lesion was the fixed limb, but not when the fixed limb became the reaching limb; that the deficits of limb localization found by the TLT were strongly correlated with deficits of deep or discriminative perceptions such as sense of joint position and movement and tactile cutaneous localization, but uncorrelated with sensory deficits of pain and temperature; that deficits of limb localization were apt to arise from lesions in the posterior column-medial lemniscal system at various levels in the peripheral nerves, cervical cord, brainstem, thalamus or parietal lobe; and that impairment of limb localization and other deep or discriminative sensations were occasionally dissociated. The TLT is easily done at the patient's bedside and can detect a lesion in the posterior column-medial lemniscal system. Moreover, it provides the means to examine the perceptual deficits using a motor task of the opposite limb.