Deborah A. Edwards

The influence of intra-uterine growth retardation on brainstem development of preterm infants

This study examined brainstem function in 76 appropriate‐for‐gestational‐age (AGA) and 25 small‐for‐gestational‐age (SGA) infants born at less than 35 weeks gestation, using brainstem auditory evoked responses. During the preterm period the mean brainstem conduction time (BCT) of the 25 SGA infants was significantly shorter than that of AGA babies of the same gestation. The BCTs of the AGA infants decreased rapidly during postnatal development to term‐equivalent age; those of the SGA infants did not change significantly. It is likely that prenatal factors are responsible for the alteration of early development in the neural function of non‐asphyxiated SGA infants. This change in neural development may be important in determining later neurological performance.

Peripheral nerve conduction velocity and brainstem auditory evoked responses in small for gestational age preterm infants

Ulnar nerve conduction velocity (NCV) and brainstem auditory evoked responses (BAER) were measured in each of 11 preterm small for gestational age (SGA) infants born at less than 35 weeks gestation. The mean motor NCV in the SGA infants was similar to that reported for infants who were appropriately grown for their gestational age (AGA). However, the mean central conduction time of the BAER in SGA infants was significantly shorter than that of AGA infants of the same post-menstrual age. Thus, the precocious development of auditory brainstem neural function in preterm SGA infants is not accompanied by changes in functional maturation of the peripheral motor nerves.

A LONGITUDINAL STUDY OF EARLY LEG POSTURES OF PRETERM INFANTS

The prone and supine resting postures of the pelvis and legs of 106 preterm infants (25 to 34 weeks gestation) were studied longitudinally between 30 and 38 weeks post‐menstrual age. All the infants subsequently had normal motor development. Posture was graded in relation to three items: the angle of elevation of the pelvis and the weight‐bearing surface of the knee in prone, and the angle of rotation of the leg in supine. Although no grade was found to be exclusive to a particular post‐menstrual age, all three items showed a statistically significant progressive trend. Only <6 per cent of the infants regressed in grade during the study. Infants dependent on oxygen for more than 28 days had significantly lower grades for leg posture in both prone and supine. Posture was not significantly altered by time ex utero or by intra‐uterine growth retardation. The results show that these observation items are suitable measures of the progressive development of antigravity postures of preterm infants.

The early development of head control in preterm infants

This study documents the longitudinal development of head control in 104 infants born at 25-33 weeks gestation. Protective side turning of the head was found to have a developmental sequence of reducing spinal extension. In the 93 infants with normal motor outcome, individual differences in the rate of development correlated with caudo-cephalic muscle development (P less than 0.001, r = 0.5) but not with the length of extra-uterine experience. As a group, the 11 infants with later motor handicap showed a persistence of the early form of movement at 35-39 weeks post-menstrual age, without a significant correlation with the ratio of upper-lower limb muscle development. Head control, supine to sitting, in the infants with normal motor outcome showed variations in the rate of preterm development. The infants born at less than 31 weeks showed significantly higher scores at 33-35 weeks post-menstrual age than those born at 31-33 weeks (P less than 0.01). There was no difference at later ages. The spinal extension movement accompanying protective side turning of the head is age specific to the preterm infant. Individual rates of normal preterm development can be evaluated by longitudinal standardised examination. As a group, the infants with subsequent abnormal motor development showed delay at preterm age but this was not individually significant.

Screening for auditory dysfunction in high risk neonates.

Brainstem auditory evoked responses were recorded in 117 pre-term and 71 full-term infants from the general population of infants born at a referral obstetric unit. The threshold intensity required to evoke a reliable BAER was determined at different post-menstrual ages (PMAs) and in many cases at follow-up clinics. The BAER thresholds for 12 infants born and tested at less than 31 wk PMA were all greater than or equal to 50 dBHL. Sixty-two low-gestational-age infants who were tested between 31 and 36 wk PMA had BAER thresholds between less than or equal to 30 dBHL and greater than or equal to 80 dBHL. The majority of pre-term and term infants tested at term equivalent age had BAER thresholds less than or equal to 30 dBHL. Longitudinal studies also indicated that BAER thresholds can decline rapidly during the pre-term period. Follow-up studies showed that those pre-term and term infants with BAER thresholds less than or equal to 30 dBHL had normal auditory thresholds as determined using conventional behavioural testing at 4 or more months of age. Of those infants with BAER thresholds greater than or equal to 40 dBHL at the time of discharge or at term equivalent age, 67% (n = 16) were confirmed later as having a moderate to profound hearing deficit. The remaining 8 infants in this group had had BAER thresholds at term of 40 or 50 dBHL and had normal BAER and behavioural thresholds at follow-up. The cross-sectional and longitudinal data indicate that the majority of low-gestational-age infants who are at risk of hearing deficit achieve BAER thresholds less than or equal to 30 dBHL by term equivalent age. We recommend that auditory screening of infants in this group is best performed at the time of discharge from hospital or at term equivalent age, whichever is the later. Those infants with thresholds greater than or equal to 40 dBHL at that time should be encouraged to attend follow-up testing and, if high thresholds persist, they should then be referred on for behavioural testing and assessment for habilitative support.

Brainstem auditory evoked response development in preterm and term baboons (Papio hamadryas)

Brainstem auditory evoked responses were recorded longitudinally from 11 neonatal baboons (Papio hamadryas), 6 of which were preterm. Recordings were made in unsedated animals from day 161 to day 362 after conception (term = 182 days). The pattern of development of both waveform morphology and of wave latency was consistent with that seen in the human neonate, with a rapid maturation of the response during the perinatal period, and then a slower development to adult values. Brainstem conduction time was measured from the wave I to wave IV interval, and this demonstrated a similar pattern, with a rapid decrease in latency up to term, and then decreasing more slowly to reach adult values by 4 months of age in the baboon.

Neurological assessment of the preterm infant in the special care nursery and the diagnostic significance of the asymmetrical tonic neck reflex

Neurological assessment at preterm age of 105 infants born at < 34 weeks gestation is discussed. The development of the asymmetrical tonic neck reflex (ATNR) was studied. There was no consistent post-menstrual age at which the 7 allocated grades occurred. The diagnostic significance of the preterm ATNR response for motor development (until 2 years) was considered. The incidence of an imposable reflex dominating spontaneous movement was significantly different (p<0.001) in the normal (n = 89) versus the abnormal development group (n = 16;13 having spastic cerebral palsy). The inclusion of observation of the quality of movement for this reflex could provide useful information in neurological assessment of preterm infants.