Cortical gyral anatomy and gross brain dimensions in monozygotic twins discordant for schizophrenia

SCHIZOPHRENIA RESEARCH ELSEVIER

Schizophrenia Research 22 (1996) 27-40

Cortical gyral anatomy and gross brain dimensions in monozygotic twins discordant for schizophrenia J. Thomas Noga, Alycia J. Bartley, Douglas W. Jones, E. Fuller Torrey, Daniel R. Weinberger * Clinical Brain Disorders Branch, DIRP, NIMH, NIH, Neuroscience Center at St. Elizabeth's, Washington, DC20032, USA Received 17 October 1995; revision 15 April 1996; accepted 29 April 1996

Abstract

Background: This study combines recent advances in three-dimensional neuroimaging technology and the genetic constraints inherent in monozygotic (MZ) twins to examine surface gyral anatomy and gross brain dimensions in monozygotic twin pairs discordant for schizophrenia. Results are presented and evaluated with respect to prior observations of cortical anomalies in schizophrenia and the hypothesis that schizophrenia involves cortical maldevelopment. Design: Three-dimensional renderings from volumetric magnetic resonance imaging (MRI) data of 13 MZ twin pairs discordant for schizophrenia and nine normal MZ pairs were studied. Qualitative assessments of left and right hemisphere surfaces were made by raters blind to diagnosis in an effort to identify developmental gyral abnormalities such as vertical temporal gyri or microgyria. Measurements of brain hemisphere length, area, and volume were also determined. These data were compared within discordant MZ schizophrenia pairs, within normal MZ pairs, and between matched unaffected discordant and normal MZ groups. Results: Raters did not identify qualitatively abnormal gyri in the schizophrenia subjects to enable distinction from their unaffected co-twins or from normal controls. Brain hemisphere volumes in the affected DS were significantly smaller bilaterally by about 3% compared with their unaffected DS co-twins, who did not differ from normals on this measure. Conclusions: We were unable to confirm previous reports of vertical gyri or localized gyral thinning as being characteristic of the cortical anatomy of schizophrenia. If cortical maldevelopment is associated with schizophrenia, it does not appear to disrupt gross gyral pattern formation in these ways. The quantitative results of diminished hemisphere volume and length in the twins with schizophrenia are consistent with previous reports of smaller brain size in schizophrenia. Our results suggest that this is a bilateral phenomenon that may be dependent, at least in part, on environmental factors. Keywords: Cortex; Gyral anatomy; Monozygotic twins; Discordance; Schizophrenia

I. Introduction

Current conceptualizations o f schizophrenia that emphasize cortical maldevelopment are based * Corresponding author. 0920-9964/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved P I I S0920-9964 ( 9 6 ) 00046-1

primarily on indirect morphometric data from in vivo and p o s t m o r t e m studies (Bogerts et al., 1985; Pakkenberg, 1987; Roberts et al., 1987; Shelton and Weinberger, 1987; Altshuler et al., 1990; Suddath et al., 1990; Jaskiw et al., 1994; for a review, see Weinberger, 1994). N o n e of the findings

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J. Thomas Noga et al./Schizophrenia Research 22 (1996) 27 40

of cortical volume or cell count reductions are pathognomonic of maldevelopment, though tissue loss, lack of gliosis, and failure to demonstrate progression suggest a pathogenesis other than adult-onset chronic neurodegenerative disease. All of these findings, however, are indirect and circumstantial evidence of neurodevelopmental deviance. Potentially direct and compelling evidence for pathological neurodevelopment comes from reports of cytoarchitectural anomalies of prefrontal and temporal cortex (Jakob and Beckmann, 1986; Arnold et al., 1991; Benes et al., 1991; Conrad et al., 1991; Akbarian et al., 1993a,b; Weinberger, 1994). Since lamination and organization of the cortex become fixed for the most part in the second trimester of gestation, cytoarchitectural abnormalities are difficult to attribute to postnatal events. However, these studies involved small samples and have yet to be replicated widely. A neuroimaging approach to test the hypothesis that schizophrenia is associated with or caused by brain maldevelopment is to search for evidence of a brain abnormality that directly implicates neurodevelopmental events. The advent of three-dimensional (3D) cortical surface rendering techniques makes it possible to assess sulcal-gyral patterns in vivo. Since these gyral patterns are formed primarily before birth, anomalous patterns found in association with schizophrenia would be convincing evidence that a pathological event occurred early in development. Cortical gyral and gross dimensional abnormalities in schizophrenia have been reported. Jakob and Beckmann (1989) described 'definite deviations of the sulcal-gyral pattern of the temporal lobe', including that temporal gyri are not differentiable, that they pass in an oblique rather than sagittal direction from the base toward the superior temporal gyrus and are interrupted by vertical passing sulci, and that they lack the fine relief that is normally present; anomalous frontal regions and a tendency toward microencephaly were also mentioned. Bruton et al. (1990) in a blinded examination of normal and schizophrenia postmortem brains, observed the presence of unusual vertical temporal gyri and sulci. They also observed shorter brain length in their schizophrenia sample compared to controls. Other groups

have recently drawn attention to the possibility of thinning of the superior temporal gyrus (STG) (Barta et al., 1990; Shenton et al., 1992). In this study we attempted to confirm these previous observations in a unique patient sample examined during life with a high fidelity method for brain visualization.

2. Methods

2.1. Sample 2. l. 1. Patients Thirteen MZ twin pairs discordant for schizophrenia (DS) were recruited with the help of the National Alliance for the Mentally Ill and the Schizophrenia Society of Canada. Diagnosis was confirmed by SCID (Structured Clinical Interview for DSM-III-R) (Spitzer et al., 1990a) administered by two clinical psychiatrists. This sample has a mean age_+SD of 31.8_+7.7 years; the sex ratio is 8:5 male:female; handedness - male, six righthanded pairs, one left-handed pair, and one pair in which the unaffected twin is right-handed and the affected twin has equivocal handedness; female - two right-handed pairs, two left-handed pairs, one pair equivocal. Each subject and his or her mother completed an extensive questionnaire about prenatal, perinatal, and developmental events and attributes. The subjects were given complete medical histories and structured neurological examinations. Subjects were excluded for history of significant head trauma (i.e., involving either an intracranial lesion, seizures, or more than brief loss of consciousness that affected their subsequent function); none had overt neurological disease; none had concurrent or past medical history which would affect brain structure or function (except case 109 in which the ill twin had suffered a CNS viral illness, possibly meningitis or encephalitis shortly after birth, which did not have obvious effects on subsequent development except that he seemed somewhat clumsy by parental report); a comprehensive substance abuse history was taken and was negative in all but twin pairs 103, 108 and 109 who each had brief circumscribed periods of drug abuse (amphetamine, case 103, well twin

J. Thomas Noga et al./Sehizophrenia Research 22 (1996) 27-40

more involved than sick twin; cannabis, both twins, cases 108, 109) and/or alcohol abuse (well twin of pair 108). An estimate of lifetime neuroleptic exposure was computed by converting antipsychotic medications to a fluphenazine-equivalent dose. One twin pair (case 108, see Fig. 3a) could not be used for hemisphere length or volume measurements because of incomplete MRI volume data acquisition from the frontal lobes. The only well twins with a DSM-III-R diagnoses were pair 112 ('well' twin received a DSM-III-R diagnosis of antisocial personality disorder); and pair 114 (the 'well' twin had a prior major depressive episode and a simple phobia of blood).

2.1.2. Controls Nine pairs of normal monozygotic twins (NC) recruited by newspaper advertisements and word of mouth consented to participate as controls. They were screened with SCID (Structured Clinical Interview for DSM III-R) (Spitzer et al., 1990b) by two experienced clinical psychiatrists to rule out significant psychopathology. They were of normal IQ and without history of head trauma or serious medical/neurological/substance abuse condition that would affect brain size or morphology. This sample has a mean age_+SD of 31.6+10.7 years, range 19-54; male:female ratio 5:4; handedness- male, four right-handed pairs, one pair with one twin right-handed and one left-handed; female, four right-handed pairs. The first-born and secondborn in each normal MZ pair are designated Twinl and Twin2, respectively. Subsets of normal twins were used to generate additional control data sets matched as closely as possible for age, sex, and handedness with similar subgroups of unaffected DS twins in order to address the genetic determinants of any structural deviations that might be present in the unaffected DS twin group. Two normal twin subgroups were created. One has 12 subjects (five female, all right-handed; seven male, six right-handed, one left-handed; mean age_+ SD, 32.3+8.5 years), derived from eight of the nine NC pairs; this group is matched for age and sex with 12 of the unaffected schizophrenia twins (DS); handedness is matched except for three nonright-handed DS females. The second subset was derived to examine the group of seven unaffected

29

male DS subjects and seven age- and handednessmatched NC males to check for consistency of results in a better matched, albeit smaller, pair of groups. Monozygosity of the normal and schizophrenia twin samples was determined by two methods. A questionnaire was completed by each subject (Cederlof et al., 1961). All participants then had zygosity confirmed by typing 19 red blood cell antigens according to a previously described method (Lykken, 1978; Wilson, 1980) Only those twins matching on 19 red cell antigens were considered monozygotic for this study. Population studies indicate that this predicts monozygosity at a conservative minimum 97% confidence level (Vogel and Motulsky, 1986). Cerebral dominance was determined by the Edinburgh Handedness Inventory (Oldfield, 1971 ). This is an ordinal scale from - 1 0 0 to + 100; for this study a subject was considered right-handed with a score of 60 or more; 1-60 was considered equivocal handedness; 0 or less was considered left-handed.

2.2. Image rendering The subjects examined in this study are from a cohort of monozygotic twins discordant for schizophrenia (DS) that have been studied and reported previously. The MRI data in the present study were newly acquired, using a more advanced methodology which greatly enhanced the final anatomic resolution of the slice images and 3D renderings. Three-dimensional MRI data sets were acquired on each subject using a 1.5 Tesla GE Signa magnet, with a Tl-weighted gradient-echo spoiled GRASS sequence (repetition time, TR=24 ms; echo time, TE = 5 ms), in a single sagittal volume acquisition (124 contiguous 1.5-mm thick slices with an in-plane field of view of 240 mm across a 256 x 256 pixel matrix; voxel size 1.5 x 0.9375 x 0.9375 mm). The digital data were transferred to an Apple Macintosh Ilci where they were processed as previously described (Bartley et al., 1992; Kulynych et al., 1993). NIH Image 1.5 (Rasband, 1991) and customized macros developed in our laboratory were used to reslice sagittal slices orthogonally to transverse and coronal slices for precision stripping

30

J. Thomas Noga et al./Schizophrenia Research 22 (1996) 27-40

of extracerebral tissues. Systematic errors of measurement due to variation in brain orientation were minimized by alignment of slices in the 3D coordinate space described by Talairach and Tournoux (Talairach and Tournoux, 1988). This was done with the aid of additional macro programs for NIH Image that rotate each sagittal slice stack so that its midsagittal ac-pc (anterior commissure to posterior commissure) line is in alignment with a predefined horizontal axis. The sagittal stack was then resliced parallel to the ac-pc line to obtain a stack of transverse slices, which were rotated such that their midsagittal line was aligned with the ac-pc line; finally, the transverse slice stack was resliced to obtain a coronal stack whose interhemispheric fissure was aligned with vertical (and orthogonal to the ac-pc line), resulting in three orthogonal stacks of slices. These three stacks are then in the Talairach and Tournot reference frame, but have not been deformed from their original shape or size. Extracerebral tissues were segmented from the brain by manual tracing of the edge of the brain in each slice with a mouse-driven cursor using NIH Image 1.5. This segmentation process was performed in each of the three orthogonal planes to maximize accuracy of cortical surface definition. Volume renderings were then created in DIPStation 1.05 (HIPG, 1990) as previously described (Bartley et al., 1992; Kulynych et al., 1993). DIPStation employs a volumetric ray-tracing method to render the 3D surface; the 'integration' option, which computes the image with a slight degree of penetration of the surface, was used in the DIPStation rendering module (Bomans et al., 1990). The fidelity of the method was assessed qualitatively by comparing the 3D MRI rendering of a Rhesus monkey's brain in vivo with photographs of the same brain after 1 month of postmortem fixation in formaldehyde (Fig. 1). When comparing the postmortem brain with the in vivo rendering it is important to note that postmortem tissue is affected by edema, vascular collapse, loss of perfusion, surface vasculature, and autopsy artifacts, such as knife cuts from opening the skull. These can affect the appearance of the postmortem brain in comparison to the in vivo 3D reconstruction. In spite of postmortem changes, the qualitative fidelity can be seen to be

quite good, and we have previously reported the quantitative validity of the renderings for measurements of the Sylvian fissures (Bartley et al., 1993) and the planum temporale (Kulynych et al., 1993).

2.3. Anatomical assessments 2.3.1. Qualitative

Since one purpose of this study was to survey the cerebral cortex for gross anatomical evidence of anomalous gyri (e.g., vertical gyri, polymicrogyria, pachygyria, or atrophic gyri) that have been previously described, observation of the high fidelity renderings of the gyral patterns of these brains is the crux of this study. This is analogous to gross observation of postmortem brain specimens reported previously (Jakob and Beckmann, 1986,1989; Bruton et al., 1990). Five neuroanatomically sophisticated investigators (one boardcertified neuropathologist, two board-certified neurologists, two board-certified psychiatrists), blind to diagnosis but not to laterality, examined in pairs the lateral views of brain renderings of 13 MZ discordant schizophrenia pairs (DS) and three normal MZ (NC) pairs. They independently examined the same set of black and white 5 x 7" photographs of the computer-generated MRI renderings. The viewing and light source angles were held constant for all photographs and the photographs preserved the alignment to the Talairach and Tournot coordinate frame which was established uniformly for all renderings. The following instructions were given to the raters: (1) examine each twin pair's standardized lateral rendering photographs with particular attention to cortical thinning, vertical gyri, thin superior temporal gyri, polygyria, pachygyria, and microgyria; (2) diagnose each pair as either normal or discordant for schizophrenia; (3) if diagnosed as schizophrenic, then identify ill vs. well twin within the twin pair; (4) document gross anatomic features supporting the diagnosis on the form provided which contains an inventory check list, including atrophy, vertical gyri, thin superior temporal gyri, polygyria, pachygyria, and microgyria; space was provided for additional comments.

J. Thomas Noga et al./Schizophrenia Research 22 (1996) 27-40

31

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Fig. 1. Comparison of in vivo 3D MRI reconstructionand actual postmortem Rhesus monkey brain. 2.3.2. Quantitative

Renderings of each case were measured on the computer monitor screen (scale: 2 4 0 x 2 4 0 - m m field of view =256 x 256 pixels) using N I H Image 1.5 measurement tools. Maximum brain height, length, width, area and volume for each hemisphere were measured blind to diagnosis. Fig. 2 shows the linear dimensions measured. DS pair 108 could not be used for length, surface, or volume measurements because of truncation of the

frontal lobes on the original sagittal MRI acquisition as can be seen in Fig. 3a. Brain height for each hemisphere was measured in the scaled lateral view of the hemisphere after graphically dissecting away the cerebellum and brainstem using tools in N I H Image 1.5 (Rasband, 1991). The ( x , y ) coordinates of the most superior and inferior points on the lateral view were recorded, then vertical extent was determined as the difference between the y coordinate values. The length and width of the

Fig. 2. External brain dimensions measured from 3D MRI renderings.

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J. Thomas Noga et al./Schizophrenia Research 22 (1996) 27-40

hemispheres were determined similarly from a superior view of the brain by measuring the distance in each hemisphere from the most rostral to the most caudal point. Lateral view projected areas were measured for each hemisphere with a simple thresholding procedure. After dissecting away the cerebellum and brainstem, hemisphere volumes were also computed from the stacks of contiguous 1.5-mm sagittal slice images as follows: the area of each coronal slice was thresholded, outlined, and measured using NIH Image 1.5 tools; the area was multiplied by the 1.5-mm slice thickness to obtain slice volume; then all slice volumes were summed by a custom macro to obtain total hemisphere volume. Interrater reliability was assessed for the measurements of height, length, width, area, and volume of eight cerebral hemispheres made independently by two raters. The ICC (intraclass correlation coefficient) was >0.94 for all measures (Bartko, 1966).

3. Data analysis

3.1. Qualitative examination of cortical surface Results from the five raters were analyzed to assess the number of correct diagnoses by twin type and diagnosis within twin pair for the DS twins. Documentation of the anatomic features supporting their diagnoses was reviewed to see if consistent sulcal-gyral anomalies were present.

3.2. Quantitative measurements Statistical analysis was performed on each of the brain measures (length, height, width, lateral surface projected area, and volume) using a computerized ANOVA (analysis of variance) (Statistica for Windows, 1994). We compared affected and unaffected DS twins using diagnosis and side as within-group (repeated measures) factors to take into consideration both the high degree of genetic similarity inherent in monozygotic twins and the expected presence of gross structural hemispheric symmetry. ANOVA using only side as a withingroup repeated measure was used to compare

unaffected DS twins with normal MZ twins. The level of statistical significance was set to p <0.05. Tukey's Honest Significant Difference, a standard, rigorous post hoc test for significance, was performed following significant main effects found with ANOVA.

4. Results

4.1. Qualitative assessment of cortical anatomy Figs. 3a, 3b, 3c and Fig. 4 display the renderings which were examined for anomalies and psychiatric diagnosis. Raters averaged 61% (range 19-81%) correct in their assessments of whether each of the 16 sets of 3D renderings was a normal (NC) or discordant-for-schizophrenia (DS) twin pair. Within the correctly categorized twin pairs, raters correctly identified the ill twin an average of 48% (range 40-60%) of the time. These percentages do not differ from chance. The raters were thus consistent in being unable to use qualitative features to correctly identify the affected twin. In making their subjective judgments, the raters varyingly cited generalized sulcal-gyral thinning (e.g., case 108, affected DS), specific gyral size reduction (e.g., STG in case 124, affected DS), twin-cotwin dissimilarity of size and sulcal-gyral patterns (e.g., case 118 affected DS), and gyral morphological abnormality (e.g., vertical gyrus in case 101 unaffected DS; pachygyria in STG in case 126 unaffected DS; polymicrogyria of STG in case 412 NC; macrogyria (e.g., NC case 412, 2nd born; this case was diagnosed unaffected DS by the rater who noted it), though their diagnostic conclusions based on these observations were largely incorrect. The STG was observed to be thin or abnormally configured by one or more investigators in 13 of the 64 hemisphere views; in only two twin pairs did two or more investigators agree that one of the twins had an abnormal STG; and in only one of these was the affected co-twin correctly identified. In seven of the 64 hemisphere renderings two or more investigators cited widespread cortical atrophy as a diagnostic feature; however, their diagnosis proved correct in only one of these instances.

J. Thomas Noga et al./Schizophrenia Research 22 (1996) 2 7 - 4 0

33

4.2. Gross brain dimensions w ; t . . : , ~ c5 ~

Measurements of brain hemisphere height, length, width, projected areas, and volumes are shown in Tables 1-. The affected schizophrenia twins have brains that are significantly shorter bilaterally in length (R, shorter in seven of 12; L, nine of 12 pairs) and in height (R, nine of 13; L, nine of 13 pairs), smaller in lateral view projected area bilaterally (R, nine of 12; L, nine of 12 pairs); and smaller in hemispheric volume bilaterally (R, nine of 12; L, nine of 12 pairs) compared as a group with their unaffected cotwins. In the DS (discordant schizophrenia) twin pairs, a main effect of laterality was found for hemisphere area and volume ( L > R ) , with no interaction effect with illness status. The normal control (NC) MZ twins showed no significant within-pair dissimilarity in any measures performed (Table 2). There was a significant main laterality effect of volume (L > R) and trend-level significant main effect of lateral surface area (L > R) in the NC twin group as well. Lateral surface area and hemisphere volumes were highly co rrelated (Pearson r -- 0.8 7, p < 0.0 5 ). The twelve unaffected DS twins in which complete volumes were measured were compared with the age- and sex-matched subsample of 12 normal control MZ subjects (Table 2). The unaffected DS twins had slightly smaller hemisphere volumes bilaterally, but this result was not statistically significant by ANOVA (p=0.15). There was a significant main laterality effect ( L > R) of hemisphere volumes in both groups ( p = 0 . 1 6 , ANOVA). In a further comparison, seven male unaffected DS (six right-handed) did not differ statistically from age- and handedness-matched group of seven NC males, though a trend-level laterality effect (L > R ) for both groups was seen (Table 3).

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34

J. Thomas Noga et al./Schizophrenia Research 22 (1996) 2 ~ 4 0

Fig. 3. Three-dimensional MRI reconstructions of brains of monozygotic twins discordant for schizophrenia. (b) Discordant schizophrenia brains (contd.). (c) Discordant schizophrenia brains (contd.).

J. Thomas Noga et al./Schizophrenia Research 22 (1996) 27-40

35

Fig. 3 (continued) Table 2 Gross brain measurements of age- and sex-matched groups of normal twins and those discordant for schizophrenia A. MZ (n = 12)

B. MZ (n = 12)

Unaffected discordant (n = 12) Normal

Affected discordant (n = 12) Normal

L Height (mm) 114.5+3.5 Length (mrn) 172.4+6.5 Width (mm) 64.8+3.5 Area(cm2) a 136.1+7.6 Volume (crn 3) 584.7+50.9

R

L

R

L

R

L

R

114.7+3.0 171.6+6.4 63.8+2.1 135.4+8.9 580.1___52.5

114.2+6.2 172.6___8.3 63.1___1.8 137.6+11.3 632.7+99.8

114.7+5.4 171.9___7.3 64.1+2.6 135.5+10.0 624.9___95.2

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Values are mean SD (n). 2-way ANOVA results (side as repeated measure): amain effect of side; F = 11.6, p =0.003. bmain effect of diagnosis; F = 5.5, p = 0.028 before birth, anomalies of these patterns would represent direct evidence of a neurodevelopmental a b e r r a t i o n in this d i s o r d e r . M o r e o v e r , t h e use o f a d i s c o r d a n t M Z t w i n p o p u l a t i o n m a k e s it p o s s i b l e to address not only cortical maldevelopment assoc i a t e d w i t h illness p e r se b u t also g e n e t i c risk as a

represented by the unaffected co-twins. Traditional 2D morphometry methods that have been the m a i n s t a y o f n e u r o i m a g i n g studies o f s c h i z o p h r e n i a to d a t e a r e p o o r l y s u i t e d to t h e s t u d y o f c o r t i c a l gyral patterns. However, even using recently develo p e d h i g h fidelity 3 D r e n d e r i n g m e t h o d s , n o o b v i -

J. Thomas Noga et al./Schizophrenia Research 22 (1996) 27-40

36

Fig. 4. Three-dimensional M R I reconstructions of brain of normal monozygotic twins. Table 3 Gross brain measurements in unaffected discordant vs. normal monozygotic twin groups matched for age, sex, and handedness M Z (7 male pairs)

Height (cm) Length (cm) Width (cm) Area (cm2) a Volume (cm 3)~

Unaffected discordant (7 male pairs)

Normal

L

R

L

R

116.2+2.9 174.84 ± 6.6 65.4 + 3.8 140.3 ± 7.0 614.6 ± 46.1

116.1 +2.7 174.4 + 6.9 64.4 + 2.0 140.5 ___8.0 606.9 ± 51.9

116.5+7.2 176.9 + 7.5 63.6 + 1.6 141.3 ± 12.8 671.6 ± 91.7

117.0+6.1 176.1 + 5.6 64.5 ± 2.6 138.9 + 11.4 663.0 ± 88.4

Values are m e a n SD. Results from ANOVA: adiagnosis x side interaction, F = 4.93, p = 0.046; L > R in Normal MZ, p = 0.05, post-hoc Tukey HSD. bmain effect of side; F = 4 . 0 , p =0.07.

ous qualitative cortical anomalies in our discordant M Z pairs are observed. In particular, no evidence of qualitatively thinned STG, vertical temporal gyri, or anomalous frontal gyri as previously reported are seen (Jakob and Beckmann, 1986,Jakob and Beckmann, 1989; Bruton et al., 1990). Infrequent instances of such findings were noted in the subjective 'readings' in our DS pairs, but they were also noted in the N C cases.

Furthermore, there was no increased association with the presence of schizophrenia. These results indicate that in this sample, if schizophrenia is associated with cortical maldevelopment, it appears to be at a more subtle level that the formation of gross gyral patterning. It should be noted that we have not performed an exhaustive survey of the cortex in these cases. In fact, only the exposed lateral cortex was assessed

J. Thomas Noga et al./Schizophrenia Research 22 (1996) 2 ~ 4 0

in these renderings, precluding examination of deep sulci and of mesial surfaces. Moreover, the quality of the rendering of cortical detail is probably not sufficiently resolute to identify small anomalies, which may thus go unnoticed. But it seems that the fidelity of these renderings is adequate to detect the presence or absence of previously described gross gyral deviations. Their fidelity is established in prior quantitative analyses (e.g., Bartley et al., 1993) and in the comparison with the rhesus monkey brain (Fig. 1). The infrequent and sporadic citings of gyral anomalies in the subjective 'readings' illustrates most probably that they are not there and that the raters were to some degree overreading them. The ultimate proof of the absence of these anomalies lies in the renderings themselves and not in the raters' interpretations of them. We also recognize that a quantitative analysis of the gyral structure of the brains might reveal differences not detected by raters in this study. However, there are currently few, if any, methodologies available to perform such a comparison in a meaningful way. In a recent study (Kikinis et al., 1994) an algorithm was developed to compute the verticality vs. horizontality of sulci from lateral view renderings. While the results suggested that their schizophrenia group had a more vertical orientation of sulci in the left temporal lobe than their normal group, the algorithm has not been validated in terms of known gross pathological entities such as microgyria, pachygyria, or sulcalgyral atrophy or gyral developmental patterns as addressed here. Moreover, the anatomic fidelity of the renderings in that study was not lifelike, raising further question about the results, since the validity of any procedure to examine the cortical surface depends critically on the fidelity of the renderings. It is hoped that the renderings presented here will themselves serve as a visual archive of the brains of this unique twin sample in order that further inspection and/or quantification by other neuroanatomists, with better methodologies, may reveal anomalies not yet recognized. While we did not find obvious qualitative differences, we did find some important quantitative differences within the schizophrenia twin sample; namely, the hemispheric volumes were smaller in the affected DS twins than in the unaffected DS

37

twins bilaterally by 2.9% and 2.8% (left and right hemispheres, respectively). As one might then expect, hemisphere length and lateral view projected area measures were also smaller in affected compared with unaffected twins (Table 1). These results are consistent with several earlier reports suggesting widespread reduction in cortical volume (Weinberger et al., 1979; Pearlson et al., 1989; Gur et al., 1991; Zipursky et al., 1992; Harvey et al., 1993) as well as hemispheric length (Bruton et al., 1990). While we did not measure ventricular volumes on these scans, ventricular enlargement per se would not explain the findings, since even if ventricular enlargement were to increase overall brain size it would tend to diminish rather than accentuate the differences in cerebral hemisphere volumes between sick and well twins. With regard to the laterality results, i.e., L > R hemisphere volumes for affected, unaffected, and normal twins, it is unknown whether this is representative of twin populations in general, or if it is representative of normal, singleton populations since the current literature is not conclusive on this point. There is, however, a suggestion of altered laterality in the schizophrenia twins (i.e., L > R volume in NC twins but not in DS twins; see Table 1). The comparison of discordant twins permits a unique assessment of the potential roles of genes and environment in the origin of findings. The results indicate a process that is partially nongenetic. To test for genetic determinants of risk for structural change, we compared the unaffected DS twins with an age- and sex-matched group of normal MZ twin subjects. There were no differences found. Because of the small sample size and incomplete control for handedness in this comparison, we cannot rule out the possibility that a small difference might be missed; thus, although our data do not support it, a role for preconception genetic factors affecting brain structure in the unaffected DS twin group is not excluded. The use of diagnosis as a repeated measure in the ANOVA accounted statistically for the monozygosity of the twins. That is, statistically we are assuming MZ twins should have complete homology of gross structural size and distribution of brain tissue except for the influences of environmental shaping forces. If the interaction of genes and environment is the likely mechanism of normal

38

J. Thomas Noga et al./Schizophrenia Research 22 (1996) 27-40

neurodevelopment, then it is assumed that such interaction may lead to abnormal brain development as well. A recent study has concluded that the popular familial/nonfamilial distinction in schizophrenia is not supported by the existing data, and that schizophrenia is a result of joint effects of genes and environment (Roy and Crowe, 1994). The existence of environmental influences on normal neural development and plasticity is well known, and current knowledge implies that, to the extent that MZ twins differ, the environment can play a significant role (Phillips, 1993). We interpret our result that the affected DS twins have significantly smaller brains bilaterally as evidence that an environmental insult or a mishap of a neurobiological/developmental interaction with the environment has adversely affected one MZ twin but not its genetically identical co-twin. One may ask how an environmental process could affect one MZ twin but not the other in spite of their closely shared environment. What forms could these processes take? They may include relative intrauterine and intraabdominal position, mechanical forces, blood supply, infection, trauma or other factors. Further careful study of differences in processes affecting the intrauterine environment may yield useful hypotheses. For example, though genetically identical at conception, monozygotic twins have differences in their intrauterine environment (Phillips, 1993). Specifically, two-thirds of all MZ twin pairs are monochorionic, sharing a common chorionic membrane but developing within separate amniotic sacs. The other one-third are dichorionic, in which the twins have separate chorionic and amniotic sacs, and in this sense resemble dizygotic twins. Monochorionic twins tend to form after the blastocyst stage, while dichorionic twins separate earlier between zygote formation and formation of the embryonic disk. Rarely, twins will survive that share common chorionic and amniotic membranes when twin formation occurs at a delayed stage of development after the embryonic disk has separated from the cavities that eventually form the fetal membranes. These variations in the twinning process have important effects on subsequent development of the placenta. Monochorionic twins will share a common placenta, resulting in compe-

tition for a limited nutrient supply; vascular anastomoses may form in these cases resulting in a twin-twin transfusion syndrome in which one twin reaches term larger and plethoric while the other is smaller and anemic. Evidence shows that monochorionic MZ twins are lighter at birth than dichorionic MZ twins and dizygotic twins. Monochorionic MZ twins' weights also seem to have greater within-pair variability than dichorionic twins, by as much as 1000 g. Monochorionic MZ twins also reportedly have a higher perinatal mortality rate than dichorionic MZ twins (Phillips, 1993). The consequences of such differences on brain development in utero and postnatally are currently unknown, but it is likely that these confound the extent to which twin studies can isolate the genetic components of disease. Unfortunately we are not aware of the chorionic status of our twins in order to examine these considerations in our cohort of subjects. A further consideration is that postconception genetic mutations occur that could both alter neurodevelopment and have an impact on the effect of environmental factors. In conclusion, these data suggest a possible role for environmental/ecological forces that act to result in slightly smaller brain volume by about 3% in association with clinically documented schizophrenia and do not strongly support a shared genetic aspect to the development of these neuropathological changes associated with schizophrenia. If schizophrenia is associated with brain maldevelopment, the underlying process does not seem to affect the formation of gyral patterns on the lateral cortical surface of the brain at the gross anatomic level as observed in vivo with current MRI techniques.

Acknowledgment We are very grateful to our colleagues T.M. Hyde, MD, PhD, M. Herman, MD, and J.E. Kleinman, MD, PhD, for their diagnostic evaluations of the renderings; S.I. Han, MD, for his assistance in the determination of interrater reliability; R. Saunders, PhD, for the postmortem photographs of the Rhesus monkey brain; L.B. Bigelow, MD for assistance in the clinical diagnosis

J. Thomas Noga et al./Schizophrenia Research 22 (1996) 27-40

of the subjects; and J.J. Bartko, PhD, for statistical consultation.

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