TWO-POINT DISCRIMINATION

Two-point discrimination



Abstract

This study investigated two point discrimination (TPD) perception in survivors of traumatic brain injury (TBI). Inpatient and outpatient survivors of severe TBI and age-matched healthy controls aged between 16 and 65 years were included in the study with a mean TPD of 3.61, 3.41 and 2.61 mm respectively. Significant group effects were seen in TPD between subjects with TBI and controls. TPD deficits did not appear to be influenced by GCS or PTA duration, nor did they show evidence of improvement over time. Similarly, CT scan data did not explain the observed TPD differences in TBI survivors. Admission functional independence measure (FIM), a global measure of functional independence, had a strong negative correlation with TPD. The lack of change in TPD over time mirrors other basic markers of neurological recovery but is at odds with TBI outcome literature reporting continuing improvements in function for at least 5 years post injury.

Two-point discrimination

Introduction

Two-point discrimination (TPD) is defined as the smallest separation between two stimuli placed on the skin that can be perceived as two separate points. The clinical test originated from Weber (1834) in order to demonstrate a tactile illusion. Accepted normal values for different body locations range from 1 mm for the tongue, 2 -5 mm for the fingertips, 8-12 mm for the palms and between 400-600 mm on the back.On the fingertip, longitudinal assessment of TPD gives values 1 mm higher than transverse assessment. TPD has been demonstrated to be a valid measurement of functional sensibility in the hand with good inter-rater and test -retest reliability(Moberg, 1990, pp. 127).

TPD thresholds have been measured in a number of disease states. In focal cerebral lesions, the larger the lesion in the mid-part of the postcentral gyrus, the greater the threshold increase. TPD has generally been found to be unaltered in more posterior parietal lesions. and In cerebral palsy, TPD was abnormal in 50-72% of cases (in hemiplegics, diplegics, and also on the healthy side). In people with epilepsy following parietal lobe tumours, 50% had impaired TPD in contralateral fingers (with abnormal imaging studies in 91% of cases).

Although TPD is primarily thought of as an anterior parietal lobe function, subcortical lesions can also modify TPD sensitivity. Lesions of the prefrontal cortex and the communication routes between the post-central gyrus and the prefrontal cortex have been found to interfere with transmission and analysis of information necessary for detection of weak somatosensory signals. These changes are postulated to relate to limited attention span, distractibility and reduced selective attention. Alterations in TPD threshold could also result from damage to non-sensory systems, such as defects in decision making, short-term memory or motor response. In particular, threshold experiments are very sensitive to fluctuations in attention(Ganong, 1999, pp. 130).

Traumatic brain injury (TBI) produces a complex mix of focal and diffuse lesions, often complicated by some degree of cerebral hypoxia and secondary brain injury. Due to the shape of the inner surface of the skull, focal cortical lesions are most common in the frontal and temporal ...