Extent of cognitive decline in traumatic brain injury based on estimates of premorbid intelligence

Global cognitive impairment following traumatic brain injury (TBI) is common, with some abilities more significantly affected than others. However, due to difficulties in estimating premorbid intelligence, there has been no systematic evaluation of the extent of decline in different cognitive abilities following TBI. Recent studies indicate that the Wide Range Achievement Test-Revised (WRAT-R) Reading subtest is an accurate estimate of premorbid intelligence, suggesting that post-TBI cognitive test scores can be compared to the WRAT-R to estimate the extent of decline that occurs in specific cognitive abilities. The current study estimated the extent of deficit in intelligence, memory, attention, speed of processing, and cognitive flexibility for 97 outpatients with TBI. Extent of decline was calculated by subtracting WRAT-R z-scores from cognitive test z-scores to determine a z-difference score (ZDiff) for each cognitive ability. The results suggest that intelligence is least declined following TBI (WAIS-R 3-4-point decline; VIQ ZDiff = -0.23: FIQ ZDiff = -0.27), followed by attention (WMS-R 5-point decline; ZDiff = -0.31), memory (WMS-R 6-9-point decline; Verbal Memory ZDiff = -0.41; General Memory ZDiff = -0.51; Delay Memory ZDiff = -0.57), speed of processing (Trails A 15-16 second decline; ZDiff = -1.90) and cognitive flexibility (Trails B 35-52 second decline; ZDiff = -2.65). Implications for provision of feedback to individuals with TBI and their families are discussed.