RELATIONSHIP BETWEEN LATENCY OF BRAIN-STEM AUDITORY-EVOKED POTENTIALS AND HEAD SIZE IN DOGS

Cranium and brainstem dimensions were measured in 32 post-mortem dog heads. Positive correlations were found between cranium length (CL) and brainstem length (BL) (r=0.87), between cranium width (CW) and brainstem width (BW) (r=0.83), and between cranium distance (CD = CL + CW/2) and brainstem distance (BD = BL + BW/2) (r=0.91). Positive correlation coefficients were also found between CL and CW (r=0.90), and between BL and BW (r=0.85). It was concluded that head size accurately reflected brainstem size. A least squares estimation of the brainstem distance (BD) from CL and CW values was BD = 10.9 + 0.16 (CL + CW/2) (BD, CL and CW in mm). Brainstem auditory evoked potentials (BAEPs) and cranium dimensions were measured in 43 dogs (86 ears) with different head size, body size, sex and age. Wave form, absolute and interpeak latencies and correlation coefficients, relating latencies to cranium dimensions and body weight, were analysed. CL, CW, and CD were positively correlated with body weight (r=0.93, 0.70 and 0.93, respectively), and CL, CW and CD were correlated with age (r=0.33, 0.52, and 0.40, respectively). BAEPs consisted of five distinct positive peaks (I to V). Secondary positive peaks following peaks I and II were seen in 60% (I') and 90% (II') of the recordings. Late waves were recorded in 90% (VI), 50% (VII), and 25% (VIII) of the recordings. Latencies increased with decreasing stimulus intensity level (from 90 dB to 10 dB hearing level HL), especially for peaks I, II, V and the I-V interpeak interval. Absolute and interpeak latencies were positively correlated with cranium distance and body weight. Correlation coefficients increased as wave latencies increased. At 90 dB HL, the highest correlation coefficients, relating cranium distance to peak V and the I-V interpeak latency, were 0.55 and 0.53 (P < 0.00001), respectively. Regression analysis showed that each 1 cm increase in cranium distance was accompanied by an increase of 0.006 ms in the latency of wave I, 0.03 ms for wave III, 0.05 ms for wave V and 0.05 ms for the I-V interpeak interval Regression analyst's showed that an increase of 1 kg in body weight was accompanied by an increase of 0. 001 ms in the latency of wave I, 0.005 ms for wave III, 0.011 ms for wave V and 0.01 ms for the I-V interpeak interval. It is concluded that head size, which accurately reflects brain size, is a relevant source (25%) of intersubject variance of BAEP latencies in the dog.