Development of Postural Asymmetry in Premature Human Infants

JUDITH GARDNER DAVID LEWKOWICZ

Biopsy chology Program Hunter College

City University o f New York New York, New York

GERALD TURKEWITZ Department of Pediatrics Rose F. Kennedy Center

Albert Einstein College of Medicine Yeshiva University Bronx, New York

and Department of Psychology

Hunter College City University of New York

New York, New York

The head position of 95 infants, born between 30 and 37 weeks gestational age, was observed every 15 min for 2 hr weekly from birth until 39 weeks conceptional age. At 35 weeks and increasingly thereafter, significantly more time was spent with the head to the right than to the left. By 39 weeks the infants were similar to term infants: almost 90% spent 80% or more time with their heads to the right. The increase appeared independent of intra- or extrauterine experience; neither age at birth nor time since birth was consistently related to head position. Unlike term infants, prematures were equally likely to turn left as right and have their heads left as right 15 min after release from a midline position. These findings suggest complex determinants of asymmetry including factors intrinsic to the fetus and those con- tributed by the uterine environment.

The current investigation is concerned with the development of the newborn human infant’s marked tendency to lie with his head turned to the right (Gesell & Amatruda, 1945; Turkewitz, Gordon, & Birch, 1965). Prior investigation has indicated that the asymmetrical posture may be basic to a variety of lateral dif- ferences in the infant’s response to stimulation. As a result of the infant’s asymmetrical posture environmental stimulation will be unequal at his 2 sides. The asymmetry of input together with the asymmetry of muscle tonus associated with the head-right

Reprint requests should be sent to Dr. Gerald Turkewitz, Department of Pediatrics, Albert Einstein College of Medicine, Bronx, NY 10461.

Received for publication 9 August 1976 Revised for publication 12 November 1976 Developmental Psyckobiotogy, lO(5): 471-480 (1977) @ 1977 by John Wiley & Sons, Inc.

47 1

472 GARDNER, LEWKOWICZ, AND TURKEWtTZ

posture may be at least in part responsible for the observed lateral differences in the neonate’s response to auditory, visual and somesthetic stimulation (Turkewitz, Moreau, & Birch, 1966; Turkewitz, Moreau, Birch, & Crystal, 1967; Turkewitz, Moreau, Davis, & Birch, 1969). Some evidence suggests that lateral differences observed early in life are related to subsequent lateral differences in functioning (Gesell & Ames, 1947; J. Viviani, C. Turkewitz, & F. Karp, unpublishecl observa- tionsj.

Because infants delivered at term exhibit the asymmetrical head posture even during the 1st 12 hr after birth (Turkewitz & Creighton, 1974), the antecedents of this position bias must be sought during the prenatal period. One potential souice for this bias was ruled out when we found that passage througli the birth canal was not essential for the appearance of the typical asymmetrical head position. Thus, infants delivered via cesarean section exhibit the same head-right postuic during the 1st 2 days after birth as d o infants delivered nornially (G. Turkewitz and S. Creighton, unpublished observations). The results indicate t h t factors operating prior to birth are responsible for the infant’s head-right posture. There- fore, examination of postural asymmetries prior to the usual time of birth iiiigiit help elucidate the developmental course of such a position bias. The current investigation was undertaken to examine developmental changes in the head position of prematurely born infants.

Methods

The subjects studied were all residents in the 1nten:;ive Care 1J!?it i-)f Jarobi Hospital of the Bronx Municipal Hospital Center, which serves as ;I rweivjiig ceiittr for thc northeast Bronx and southern Wcstchester Coirnty in ’dew kork. ‘The unit has 6 nurseries with approximately 6 babies accornriiodated in cach. Within each nurscry, inrants are maintained in open bassinettcs or errc1:)sed isolettes. ‘Thcy are placed in bassinettes after their medical condition ni ) longer requires the highly controlled thermal and oxygen enriched envirc.)nnient of the isolette. lnfm ts ii: both isolettes and bassinettes are frecjueiitly housed in the sane iilirseiy K I W ~ although those infants in good medical z ~ ~ n d i i i o n :crid 1 0 be segregated into bassinettcs in “growing rooms.” Approximatcly 4.00 ba: ) ia , half‘ 01‘ w l ~ o n i are preniature. are admitted into the unit each ycir . The popula- tion of the unit is comprisctl mainly of infanls from a lower socioeconotnk background arid contains a 1)reponderant number of inf‘ants froin b i , d rind Yuerto Ricari Carnilies.

The subjects for the study were 95 inhnls, 42 males and 53 1;m:1lcs 411 \yere classified as premature because they were estimated to have bzcn biun a t 01- below 37 weeks gestational age (the htunan gestational period is

40 weeks). Gestational age was estimaled by the Dabowit/. method w iieiit o f 10 ncuro!ogic and 1 1 external bodily i.11 2 , & Chldberg, 1970). ‘ h e number of infBnr

eai,ij I:si~ina!cd CeatationaJ Age (EGA) arid rtic birth weight> of t1ic pitxa<:litcd In ‘Tablc 1.

POSTURAL ASYMMETRY IN PREMATURE INFANTS 473

TABLE 1. Characteristics of the Infants Studied.

EGA at birth Number of Mean birthweight (weeks) Males Females (8)

30 31 32 33 34 35 36 37

2 0 0 4 5 3 3 5

13 12 5 9 7 8 7 12

970 1160 1388 1421 1172 1971 2190 2588

To examine the development of lateral differences in posture, we observed head position once a week until the infant reached 39 weeks of conceptional age or until he left the unit.’ All observations were made while the infants were in their own bassinettes or isolettes. Each infant’s head position was noted at 15-min intervals for 2 consecutive hr during the late morning and early afternoon. All the premature infants on the unit, except those whose heads were maintained in a midline position for purposes of mechanically aided respiration, were observed regardless of medical condition. Because an attempt was made not to interfere with the routine of the nursery, we encountered occasions in which babies were being handled at the time they were scheduled for observation. These observations were therefore omitted.

Recognizing that the observed head posture may be influenced by the infant’s immediately preceding head position, which might have been determined either by the infant’s own behavior or by the position in which he had been placed, we performed a 2nd type of observation which assessed the infant’s bias following placement in a midline position. This observation, which required manipulation of the infant, was restricted to 19 infants (10 males and 9 females) who were in a medically stable condition at the time of observation. The infants were placed in the supine position with their head held in the midline by the experi- menter. The head was positioned with the bitemporal region between the ex- perimenter’s thuinb and forefinger. As soon as no differential pressure was felt on the restraining fingers, the head was released and the direction of the first head turn was noted. Fifteen minutes after release the head position was again noted.

Because premature infants change their head position while in the prone position with difficulty, only data obtained while the inrants were supine are included in the analysis.

Results

Due to the restriction that the infant be in a supine position, as well as the omission of data resulting from routine handling of the babies, an unequal number of usable observations were available for different subjects. Consequently, the head

474 GARDNER, LEWKOWICZ, AND TURKEWITZ

position of the infants is described in terms of the percentage of scoreable intervals in which the infant’s head was in a given position. Although the babies were scored as being in a particular position even if they showed only a moderate asymmetry, during almost all of the observations the head positions were sufficierltly extreme so that one or the other ear was in contact with the substrate. Because the head was almost never in the midline and the percent of observations in a head-left position is almost a perfect complement of the percent of observations in a head-right position, all data are presented solely in terms of the percentage of observations on which the infant’s head was to the right.

Because a number of gender differences with regard to lateral asymmetries have been found (Harris, 1973; Witelson, 1976), the data were analyzed to determine whether male and female infants differed with regard to head position. No significant differences were found and the data from males and females were therefore combined in all subsequent analyses.

At each conceptional age the infants spent more time with their heads to the right than to the left (Fig. 1). To ascertain whether this position bias was reliable we performed Wilcoxon tests comparing the number of observations on which the head was to the right with the number of observations on which the head was to the left. The observed differences were significant at each age. Week 33-3s2: z = 2.21, df = 35, p < .01; Week 36: z = 4.41, df = 31, p <: .001; Week 37: z = 2.41, df = 29, p < .01; Week 38: z = 3.36, d f =33 , p < .001; Week 39: T = 0, d f = 16, p < .005 (one tailed p’s). To determine the earliest age for a significant right position bias we analyzed the data for Weeks 33, 34, and 3 5 , separately. These analyses indicated a significant position bias at WI:ek 35 ( z = 1.80, df = 26, p < .05) but none at Weeks 33 and 34. In order to provide a larger sample at the younger ages we combined the data for Weeks 33 and 34. Even under these circumstances we found no significant right position bias in infants of this age. As can also be seen in Figure 1, the maintenance of a head-right posture increased with increases in conceptional age: a t 33-35 weeks the infants

33-35 36 37 38 3 9

CONCEPTIONAL AGE (weeks)

(n l (40 ) ( 3 3 ) ( 3 2 ) ( 3 7 ) (16)

Fig. 1 . Relationship between conceptional age and percent of observations in which infants had their heads to the right.

POSTURAL ASYMMETRY IN PREMATURE INFANTS 475

Fig. 2. Relationship between postnatal age and percent of observations in which infants had their heads to the right.

had their heads to the right during 63.9% of the observations and by 39 weeks the percentage was 95.6. The difference between infants at these ages was highly significant (t = 4.66, df= 49, p < .0013).

When data are ordered in terms of age at time of observation, conceptional age is confounded with age at birth. Therefore, the obtained increase in the strength of the head position bias may be due to the increased representation of later born infants at the later ages of observation. That is, if infants born at earlier ages had weaker position biases whereas infants born at later ages had stronger position biases, the observed increment could merely represent the increased contribution of later born infants at the older ages. To examine this possibility we grouped the data according to the infant’s age at birth. The per- cent of observations on which the head was to the right in infants born between 30-33 weeks and those born between 34-35 weeks were compared at different conceptional ages. None of the differences between infants in these 2 groups were significant at any of the ages of observation (t tests with dj’s ranging from 12 to 27). This absence of any significant differences in the head position bias of infants born at different ages suggests that neither extrauterine nor intra- uterine events are of primary importance.

To examine further the role of extrauterine influences, we considered head position in relation to postnatal age. As may be seen in Figure 2, with the excep- tion of an increase in the occurrence of a head-right posture during the 7th post- natal week, no marked relation existed between the strength of the head position bias and time since birth. The head-right bias increased significantly from the 1st to the 7th postnatal week (t = 3.58, df = 26, p < .Ol) but the differences between the 1st week and any of the other weeks were not significant (t test^).^ In view of the unique appearance of the difference at 7 weeks with no sign of a develop- mental trend we suspect that this difference probably represents a sampling error.

In order to determine whether the head-right bias was a function of a number of infants exhibiting a very strong position bias or was a general characteristic of

476 GARDNER, LEWKOWICZ, AND TURKEWITZ

the infants, we analyzed the data with regard to the proportion of infants exhibit- ing the right position bias at each conceptional age. When the relatively stringent criterion of 80% of the observations with the head turned to the riglit was used for designating a baby as one having a head position bias, the proportion of infants showing such a bias increased as a function of conceptional age with almost 90% of the oldest infants showing such a bias (Fig. 3) . (Using other criteria such as 50, 60, or 70% affected the level but not the shape of the curve.) Note that the percent of infants who had their heads to the left more frequently than to the right declined from 26.8% at Weeks 33-35 to 16.2% at Week 38 and .OO% at Week 39.

Finally, because of the possible relation between birthweight and maturity of the nervous system, birthweight may have been associated with head position. However, examination of the relationship between birthweight and head position for those babies seen during the first week after birth failed to indicate any such association (Y = -.28, df= 22).

Self-Determined Head Position

In addition to the other factors thus far considered, the infant’s head posi- tion may have been influenced or even determined by its position prior to observa- tion. To determine the extent to which the head position bias is independent of the immediately preceding head position, we studied a subsamplc oj- the infants by placing their heads in the midline and observing the initial direction of turn and the position 15 min later. Examination of the direction of initial turn faded to indicate a clear-cut relationship between conceptional age and direction of 1st turn. In addition, we found no significant differences between the number of infants initially turning right and the number initially turning left at any age (sign tests) .

In marked distinction to all of the other data, examination of the infants’ head positions 15 min after release from the midline suggested an age-related

I I I I 33-35 36 37 38 39

CONCEPTIONAL AGE ( w e e k s )

( n ) 141) 1331 132) 137) (17)

Fig. 3. Percent of infants having their heads to the right in 80% or more of the observations.

POSTURAL ASYMMETRY IN PREMATURE INFANTS 477

20 40L 36 37 3a 39

(n l ( 9 ) (11) (8) (7)

CONCEPTIONAL A G E ( w e e k s )

Fig. 4. Percent of infants with their heads to the right 15 min after release from a midline position.

decline, rather than an increase, in the presence of a head-right posture (Fig. 4). Sign tests of the difference between the number of infants with their heads to the right as opposed to the left indicated a significant difference at 36 weeks of age but not at any of the older ages. This apparent discrepancy could have resulted from either a difference in head posture following release from a mid- line position and that following routine hospital care, or it could have stemmed from a difference in the nature of the babies examined with the 2 procedures. To determine whether a sampling bias was responsible for the discrepant findings we compared the head position following routine hospital care of those infants subjected to the midline procedure with that of the entire sample. No differences between the subsample and the entire sample were obtained at any conceptional age ( t tests).

Discussion

The results of the present study indicate that development of the head-right position bias of the human infant is positively associated with conceptional age. This relationship is not dependent on whether or not the last 2 inonths of the normal period of gestation occur in utero. The position bias appeared in pre- maturely born infants at 35 weeks of age and increased thereafter, until by 39 weeks the infants were Iying with their heads to the right 96% of the time. By this age 88% of the infants were spending 80% or more of their time with their heads to the right and none of the infants had their heads to the left more than to the right. The regularity of the association between head position and con- ceptional age argues for the relative unimportance of both intrauterine and extra- uterine experience for the establishment of the head position bias, in that the amounts of such experience differed for babies observed at the same conceptional

478 GARDNER, LEWKOWICZ, AND TURKEWITZ

age. In fact, prematurely born infants at 39 weeks of age are very similar t o infants born at full term (Turkewitz et aZ., 1965) with regard to the percent of time spefit with the head to the right. However, when the results obtained follow- ing release of the head from a midline position were considered, prematurely born infants, even when 39 weeks of age, were markedly different from newborn full-term infants. When the heads of full-term infants are released from a mid- line position they make an initial right turn. In contrast, premature infants were as likely to turn left as right following release. A comparable absence of a turning bias has been seen only in those full-term infants who are under 12 h r old and who have had their heads in a midline position for a relatively long time (15 min; Turkewitz & Creighton, 1974). Older full-term infants (12-72 hr) continue l o show a right turning bias even following the maintenance of the midline position for this period of time. These data therefore suggest a developmental continuum for the establishment of a right position and turning bias with the position bias appearing prior to the normal age at birth. However, this bias is easily disrupted. Merely placing the head in a midline position momentarily is sufficient to eliminate the bias for a relatively prolonged period of time. This effect continued for at least 15 min with the infants still failing to show a systematic position bias at the end of this time. Therefore, certain aspects of the intrauterine environment may be involved in strengthening an endogenously based postural bias,. However, as is always the case when a difference between premature and full-term infants is observed, the difference may stem not from the absence of intrauterine experience but rather from the same cituses and physiological consequences as contributed to the prematurity itself. This possibility receives some support from the previous finding that babies born in poor condition as evidenced by low Apgar scores exhibit a weaker lateral difference in response to somesthetic stimulation than do infants born in good condition (Turkewitz, Moreau, & Birch, 1968).

Absence of a bias for at least 15 min following maintenance of a midline position raises the possibility that the head position of the infants reflected the manner in which they were placed rather than any characteristic of their behavior. In view of the observed age-related changes in the position bias such a hypothesis is unlikely in that an increase in placement of the baby with its head to the right is not apt to occur with increases in age. Although older babies are more likely to be in bassinettes than younger babies, this could not, in all likelihood, affect head positioning. If the infants were responsive to the direction of environmental events this could result in a systematic position bias if all or most infants were placed with a common orientation to such events. However this was not the case, as no common positioning of the infants occurred with regard to the direction of light, sound, or temperature gradients.

The finding of a weakened turning bias in premature infants is consistent with, and indeed may be contributory to, the finding that during childhood prematurely born individuals are likely to be less stable and consistent in their preferred lateral usage than are individuals born at term (Eames, 1957). The head position bias of the infant contributes to lateral differences in his response to stimulation (Turkewitz et aZ., 1969). Such lateral differences may contribute to the development of functional asymmetry as well as to the hemispheric differentiation which characterizes normal adult functioning. The c:urrent data

POSTURAL ASYMMETRY IN PREMATURE INFANTS 479

suggest that the determinants of the basic postural asymmetry are complex including both factors intrinsic to the developing fetus as well as components contributed by the uterine environment. Interference with either of these, as is the case with prematurely born infants, may result in a failure to develop the marked asymmetry of turning as well as posture which characterizes the full-term infant and may result in inconsistent lateralization and its associated problems later in life (Hacean & de Ajuriaguerra, 1964). On the other hand, the fragile position bias of the pre- mature infant may reflect a defective nervous system which might be concomitant with, but not causative of, these associated problems. In either case, further investigation of the premature infant’s head position and turning bias, and the relationship between these early appearing aspects of behavior and subsequent functioning, might provide the basis for the development of a useful diagnostic tool.

Notes

‘Although a number of the infants remained on the unit after they were 39 weeks of conceptional age, most went home by that time. Therefore, in order to avoid a sample heavily biased towards infants having particularly long lasting medical problems, we restricted this paper to consideration of infants at or below 39 weeks of conceptional age.

*A!though 14 infants in the sample were born between 30 and 32 weeks of gestational age, the high incidence of respiratory distress in such immature infants and the accompany- ing need for mechanical assistance in respiration resulted in the collection of usable data from only 2 infants at 31 weeks and 3 infants at 32 weeks. In view of the extremely limited number of observations at these ages they were omitted from consideration. In addition, because the number of infants observed at each age from 3 3 to 35 weeks was relatively small the data from observations at these ages were combined.

3Because subjects entered and left the study at different conceptional ages and because restrictions on usable data resulted in discontinuities in the data record of individual infants some infants were seen at both 33-35 weeks and 39 weeks and others were not. In order to compare the data from independent subjects we omitted the data collected at 33-35 weeks (the age with the larger n) for the subjects whom we saw at both ages.

4As was the case for the analysis of conceptional age some infants were seen during both the 1st and subsequent postnatal weeks and others were not. To maintain the inde- pendent character of the data for statistical analysis we eliminated the data from the age with the larger n. This resulted in eliminating the data from 17, 8, and 4 babies seen during the 2nd, 3rd, and 4th weeks, respectively. For the comparisons with the 5th, 6th, and 7th weeks we eliminated the data collected during the 1st week of life from 5, 4, and 3 babies, respectively.

This research was in part supported by the National Institute of Child Health and Human Development, National Institutes of Health (H001799).

We thank Ms. Emerita Caputo and the nurses of the Intensive Care Unit of the Bronx Municipal Hospital Center for their cooperation.

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480 GARDNER, LEWKOWICZ, AND TURKEWITZ

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