Repeated antenatal corticosteroid treatments adversely affect neural transmission time and auditory thresholds in laboratory rats

Abstract

Antenatal corticosteroid (AC) treatment is given to pregnant women at risk for preterm birth to reduce infant morbidity and mortality by enhancing lung and brain maturation. However, there is no accepted regimen on how frequently AC treatments should be given and some studies found that repeated AC treatments can cause growth retardation and brain damage. Our goal was to assess the dose-dependent effects of repeated AC treatment and estimate the critical number of AC courses to cause harmful effects on the auditory brainstem response (ABR), a sensitive measure of brain development, neural transmission and hearing loss. We hypothesized that repeated AC treatment would have harmful effects on the offspring's ABRs and growth only if more than 3 AC treatment courses were given. To test this hypothesis, pregnant Wistar rats were given either a high regimen of AC (HAC), a moderate regimen (MAC), a low regimen (LAC), or saline (SAL). An untreated control (CON) group was also used. Simulating the clinical condition, the HAC dams received 0.2mg/kg Betamethasone (IM) twice daily for 6 days during gestation days (GD) 17-22. The MAC dams received 3 days of AC treatment followed by 3 days of saline treatment on GD 17-19 and GD 20-22, respectively. The LAC dams received 1 day of AC treatment followed by 5 days of saline treatment on GD 17 and GD 18-22, respectively. The SAL dams received 6 days of saline treatment from GD 17 to 22 (twice daily, isovolumetric to the HAC injections, IM). The offspring were ABR-tested on postnatal day 24. Results indicated that the ABR's P4 latencies (neural transmission time) were significantly prolonged (worse) in the HAC pups and that ABR's thresholds were significantly elevated (worse) in the HAC and MAC pups when compared to the CON pups. The HAC and MAC pups were also growth retarded and had higher postnatal mortality than the CON pups. The SAL and LAC pups showed little or no adverse effects. In conclusion, repeated AC treatment had harmful effects on the rat offspring's ABRs, postnatal growth and survival. The prolonged ABR latencies reflect slowed neural transmission times along the auditory nerve and brainstem auditory pathway. The elevated ABR thresholds reflect hearing deficits. We concluded that repeated AC treatment can have harmful neurological, sensory and developmental effects on the rat offspring. These effects should be considered when weighing the benefits and risks of repeated AC treatment and when monitoring and managing the prenatally exposed child for possible adverse effects.

Fig. 1

ABR P4 latencies as functions of treatment group and tone pip frequency (mean±SE). Group differences (p<0.05): * HAC>LAC, SAL and CON but not MAC, MAC=LAC=SAL=CON; † HAC>all others, MAC=LAC=SAL=CON. HAC=high antenatal corticosteroid, MAC=moderate antenatal corticosteroid, LAC=low antenatal corticosteroid, SAL=saline control, CON=untreated control. Stimulus intensity=100 dB.

Fig. 2

ABRs from the CON pups had normal results, whereas several rat pups in the HAC group had abnormal prolongations of one or more ABR waves (age = day 24). Dotted lines represent the expected position of each wave based on the CON group means. Arrowheads identify abnormally prolonged waves, defined as ≥2SD longer than the CON group means. Stimuli=16 kHz tone pips at 100 dB.

Fig. 3

ABR thresholds as functions of treatment group and tone pip frequency (mean±SE). Group differences (p<0.05): *HAC>all others, MAC>CON and LAC, LAC=SAL=CON; †HAC>all others, MAC>CON, LAC=SAL=CON; **HAC>all others, MAC>LAC, SAL and CON, LAC=SAL=CON; ‡HAC>SAL and CON, MAC=LAC>CON, SAL=CON.

Fig. 4

ABR P2 latency-intensity (L-I) profiles of the 10 HAC offspring with elevated ABR thresholds. Two HAC offspring (B7M1 and B5M2) had P2 L-I profiles suggestive of sensorineural hearing loss (SNHL) and 1 (B5M1) had a P2 L-I profile suggestive of conductive hearing loss (CHL). The remaining 7 animals with elevated ABR thresholds had P2 latencies that bordered the upper normal range and with no ABRs below 40 dB. The shaded region is the normal range. Stimuli=8 kHz tone pips.

Fig. 5

ABR P2 amplitudes as functions of treatment group and tone pip frequency (mean±SE). Group differences (p<0.05): *HAC and MAC

Fig. 6

ABR P2 amplitude-intensity (A-I) profiles (mean±SE). HAC and MAC treatments caused decreased P2 amplitudes. Group differences (p<0.05): *HAC and MAC