Infratentorial atrophy on magnetic resonance imaging and disability in ...

Brain (1999), 122, 291–301

Infratentorial atrophy on magnetic resonance imaging and disability in multiple sclerosis S. G. M. Edwards,1 Q. Y. Gong,3 C. Liu,1 M. E. Zvartau,1 T. Jaspan2, N. Roberts3 and L. D. Blumhardt1 1Division

of Clinical Neurology, University of Nottingham, of Diagnostic Imaging, University Hospital, Queen’s Medical Centre, Nottingham and 3Magnetic Resonance Research Centre, University of Liverpool, UK

2Department

Correspondence to: Professor Lance D. Blumhardt, Division of Clinical Neurology, Faculty of Medicine, University Hospital, Queen’s Medical Centre, Nottingham NG7 2UH, UK

Summary Loss of tissue volume in the central nervous system may provide an index of fixed neurological dysfunction in multiple sclerosis. Recent magnetic resonance studies have shown a modest relationship between clinical disability rating scores and transverse sectional area of the cervical spinal cord. To explore further the relationship between atrophy and disability in multiple sclerosis, we estimated the volumes of infratentorial structures from MRIs in a cross-sectional study of 41 patients, 21 with relapsing– remitting multiple sclerosis and 20 with secondary progressive multiple sclerosis. We used the Cavalieri method of modern design stereology with point counting to estimate the volume of brainstem, cerebellum and upper cervical spinal cord from three-dimensional MRIs acquired with an MPRAGE (Magnetization-prepared

Rapid Acquisition Gradient Echo) sequence. The volume of the upper (C1–C3) cervical spinal cord was significantly correlated with a composite spinal cord score derived from the appropriate Functional Scale scores of the Expanded Disability Status Scale (r J –0.50, P < 0.01). The cerebellar (r J 0.49, P < 0.01) and brainstem (r J 0.34, P < 0.05) volumes correlated with the Scripp’s Neurological Disability Rating Scale scores. The upper cervical cord volumes (r J –0.39, P < 0.01), but not the brainstem or cerebellar volumes, were significantly associated with disease duration. MRI-estimated structural volumes may provide a simple index of axonal and/or myelin loss, the presumed pathological substrates of irreversible impairment and disability in multiple sclerosis.

Keywords: multiple sclerosis; MRI; MPRAGE; infratentorial; atrophy Abbreviations: EDSS 5 Expanded Disability Status Scale; IFS 5 Infratentorial Functional Score; KFS 5 Kurtzke Functional System Scores; SNRS 5 Scripp’s Neurological Rating Scale; UCC 5 upper cervical spinal cord

Introduction In patients with established multiple sclerosis, the relationship between disability and measurements of the ‘burden of disease’ in the cerebral hemispheres or spinal cord on conventional MRI is at best weak (Isaac et al., 1988; Koopmans et al., 1989; Baumhefner et al., 1990; Thompson et al., 1990; Wiebe et al., 1992; Kidd et al., 1993; Paty et al., 1993; Filippi et al., 1995a). The factors which contribute to this apparent paradox include the insensitivity of conventional T2-weighted spin echo MRI for the pathological heterogeneity of white matter lesions and, in particular, the non-specificity of this type of MRI for axonal loss and demyelination, the presumed pathological substrates of the fixed deficits in patients with multiple sclerosis. It has been proposed that alternative MRI techniques, namely magnetic resonance spectroscopy (Arnold et al., 1994; Davie et al., 1995, 1997), magnetization transfer imaging (Gass et al., 1994; Filippi © Oxford University Press 1999

et al., 1995b) or magnetization decay images (Thorpe et al., 1995; Kidd et al., 1997) may provide better measures of underlying pathology. However, recent studies of T2 relaxation decay data (Thorpe et al., 1995; Kidd et al., 1997) and magnetization transfer ratio imaging (Thorpe et al., 1995) have failed to demonstrate correlations with disability. A simple index of the loss of axons and myelin and perhaps a useful correlate of disability could be provided by three-dimensional MRI estimation of volume loss in critical brain structures. Previous studies of the relationship between cerebral atrophy in multiple sclerosis and various measures of disability, including dementia and cognitive dysfunction, gave inconsistent results (Loizou et al., 1982; Rao et al., 1985; Huber et al., 1987; Clark et al., 1992; Maurelli et al., 1992; Comi et al., 1993; Losseff et al., 1996a). As one might predict from the emphasis on mobility in the major clinical

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disability rating scales, recent MRI studies have shown a significant relationship between motor disability and crosssectional areas of the spinal cord (Kidd et al., 1993; Filippi et al., 1996a; Losseff et al., 1996b), but not with volumes of a ‘slab’ of the cerebral hemispheres (Losseff et al., 1996a). Many earlier CT or MRI studies of multiple sclerosis have reported atrophy of the cerebral hemispheres or corpus callosum, or enlargement of the ventricles (Loizou et al., 1982; Noseworthy et al., 1984; Huber et al., 1987; Brainin et al., 1988; Rao et al., 1989a, b; Pozzilli et al., 1991; Swirsky-Sacchetti et al., 1991). In one study, atrophy of the cerebellum and brainstem was judged present on the CT scans of 26% of patients with suspected, probable or definite multiple sclerosis (Loizou et al., 1982). As the pathological lesions of multiple sclerosis are common in the pathway-rich brainstem, cervical spinal cord and cerebellum and, as damage in these areas may be critically important in determining the type of disability measured on clinical rating scales, we aimed both to estimate the volumes of these structures using three-dimensional acquired MRI data and to investigate whether this correlates with functional disability as estimated by the clinical rating scales that are used most widely in treatment trials.

Methods The study was approved by the Nottingham University Hospital and Medical School Ethical Committees, and informed consent was obtained from all patients and control subjects.

Patients and controls Forty-one patients were recruited from a multiple sclerosis research database at University Hospital, Nottingham. All the patients studied had either ‘clinically definite multiple sclerosis’ or ‘laboratory-supported definite multiple sclerosis’ according to established criteria (Poser et al., 1983). Twentyone patients were classified as relapsing–remitting multiple sclerosis and 20 as secondary progressive multiple sclerosis (see Table 1) based on the following criteria: (i) relapsing– remitting multiple sclerosis—a history of relapses and remissions (either complete or partial) with no deterioration between relapses. In addition, all relapsing–remitting multiple sclerosis patients had at least two relapses in the 24 months prior to the study; (ii) secondary progressive multiple sclerosis—an initial relapsing and remitting course followed by documented progressive and continuing deterioration for at least 6 months, with or without superimposed relapses. At the time of the study, all patients had been relapse-free for at least 3 months. No patient had been treated with immunosuppressives or immunomodulators apart from steroids for relapses and none had any other significant medical condition, past or present, which could confound the study. Exclusion criteria included primary progressive multiple sclerosis, treatment with steroids in the preceding

Table 1 Characteristics of patients according to disease course

Age (years) F:M Disease duration (years) EDSS SNRS IFS CFS

RRMS (n 5 21)

SPMS (n 5 20)

Total (n 5 41)

32.6 6 7.7* (21–47) 15 : 6 5.5 6 4.3** (1–17) 1.8 6 1.0*** (0–4) 91.4 6 9.6*** (62–100) 2.4 6 2.2*** (0–8) 1.8 6 1.7*** (0–5)

40.1 6 6.6 (29–50) 13 : 7 12.1 6 6.6 (3–28) 5.6 6 1.0 (3.5–6.5) 68.8 6 7.8 (53–79) 7.9 6 2.2 (3–12) 6.0 6 1.7 (3–10)

36.2 6 8.0 (21–50) 28 : 13 8.7 6 6.4 (1–28) 3.6 6 2.2 (0–6.5) 80.4 6 14.3 (53–100) 5.1 6 3.5 (0–12) 3.8 6 2.7 (0–10)

RRMS 5 relapsing–remitting multiple sclerosis; SPMS 5 secondary progressive multiple sclerosis; EDSS 5 Expanded Disability Status Scale (Kurtzke, 1983); SNRS 5 Scripp’s Neurological Rating Scale (Sipe et al., 1984); IFS 5 Infratentorial Functional Score (see Methods); CFS 5 Cord Functional Score (see Methods). *P , 0.01; **P , 0.001; ***P , 0.0001.

3 months, alcohol abuse, systemic disease, dehydration, malnutrition, renal disease, liver dysfunction or electrolyte disturbance. All patients had full neurological assessments including evaluation of disability scale scores within 24 h of their MRI scans. Disability was scored on the Expanded Disability Status Scale (EDSS) (Kurtzke, 1983) and the Scripp’s Neurological Rating Scale (SNRS) (Sipe et al., 1984). An ‘Infratentorial Functional Score’ (IFS) was derived by adding the Kurtzke Functional System Scores (KFS) (Kurtzke, 1983) for pyramidal, brainstem, cerebellar, sensory, bowel and bladder systems, and a ‘Cord Functional Score’ (CFS) was derived by subtracting the brainstem and cerebellar KFS scores from the IFS. Fifteen healthy consenting volunteers, matched with 15 multiple sclerosis patients (11 relapsing–remitting multiple sclerosis and four secondary progressive multiple sclerosis) for age (mean control data, 29.0 6 5.4 years versus mean patient subgroup, 30.8 6 5.3 years) and sex (M : F ratio 8 : 7), underwent the same MRI sequence.

MRI MRI was performed on a 1.5T whole body imaging system (Magnetom SP, Siemens Medical Systems) within 10 min of an intravenous administration of the contrast agent gadolinium-DPTA (Gd-DPTA). A low flip angle T1-weighted three-dimensional gradient echo sequence (Magnetizationprepared Rapid Acquisition Gradient Echo–MPRAGE) provided 128 sagittal images within a slab of 18 cm, equivalent to a slice thickness for each image of 1.407 mm. The acquisition parameters were a repetition time of 10 ms, an echo time of 4 ms, a delay time of 100 ms, an inversion

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Fig. 1 Boundary definitions of infratentorial structures (see Methods for details).

time of 300 ms and a flip angle of 10°. The acquisition matrix comprised 256 readings of 192 phase encodings with one excitation per phase encoding. The field of view was 25 cm. The acquisition time was 5.25 min.

MRI analysis The images were transferred to a SPARC 10 workstation (Sun Microsystems, Calif., USA) where volumes were measured by a neuroradiologist blinded to the clinical data using stereology menus within ANALYZE (MAYO Foundation, Minn., USA) image analysis software. Our aim was to measure the volumes of cerebellum, brainstem and upper cervical spinal cord (UCC). We used established neuroanatomical landmarks (Daniels et al., 1987; Aylward and Reiss, 1991) for defining the boundaries of the cerebellum, brainstem and UCC (between levels C1 and C3) in the mid-sagittal plane (Fig. 1). The upper boundary of the brainstem was defined by a line from the superior border of the superior colliculus to the lower margin of the root of the mammillary body. The lower limit of the brainstem and the upper limit of the spinal cord were defined by a line from the lower border of the anterior

arch of the atlas to the upper border of the posterior arch of the atlas. A line which extrapolated the outer anterior limits of the cerebellar folia above, to a similar point below the cerebellar peduncles, defined the portion of the peduncles that was included with the cerebellar volumes. The portion of the cerebellar peduncles outside this line were included with the brainstem volumes. The lower margin of the UCC was identified by extrapolating a line joining the anteroinferior and postero-inferior corners of the third cervical vertebral body. These three major anatomical boundaries are illustrated in Fig. 1. The UCC volumes could not be obtained in two patients due to technical errors that occurred during imaging which meant that they were not included in the field of view.

The Cavalieri method According to the Cavalieri method (Gundersen and Jensen, 1987; Mayhew and Olsen, 1991; Roberts et al., 1993), an unbiased estimate of the volume V (est1V) of an object can be obtained by exhaustively sectioning the object with a series of parallel planes a distance T apart. To avoid bias,

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the first section must be placed at a uniform random position within the sectioning interval. If the area of the object transects on the images can be measured exactly (e.g. by automatic image analysis techniques), then, est1V 5 T (A1 1 A2 1 . . . An) [cm3]

(1)

is an unbiased estimator of V, where n is the number of sections and A1, A2, . . . An are the corresponding transect areas on the n sections. If the section areas cannot be properly segmented and measured automatically, then the semiautomatic approach based on manual contour tracing of the digital section images should not be adopted. Instead the method of point counting should be employed. Each image is overlaid with a regular grid of test points and an operator counts the number of points hitting the object transects on the corresponding sections. The unbiased volume estimator becomes est2V 5 Ta/p (P1 1 P2 1 . . . Pn) [cm3]

Table 2 Mean 6 SD and (range) of volume estimates (cm3) for brainstem, cerebellum and UCC

Brainstem n Cerebellum n UCC n

RRMS

SPMS

All patients

21 22.87 6 3.41 (15.59–31.01)

20 41 21.50 6 3.38 22.20 6 3.43 (16.76–26.65) (15.59–31.01)

21 20 41 122.90 6 12.89* 114.30 6 10.35 118.70 6 12.38 (95.67–145.70) (97.45–143.90) (95.67–145.70) 20 2.63 6 0.67* (1.01–3.73)

19 39 2.14 6 0.67 2.39 6 0.70 (1.13–3.73) (1.01–3.73)

RRMS 5 relapsing–remitting multiple sclerosis; SPMS 5 secondary progressive multiple sclerosis; UCC 5 upper cervical spinal cord. *P , 0.02.

(2)

where P1, P2, . . . Pn denote the point counts and a/p represents the area associated with each test point. If a square grid is used with a distance d between test points, then a/p 5 d2. Note that each section area A1 is now estimated by (a/p)/P1. The subscript 2 in est2V indicates that the volume is estimated by a two-stage sampling, namely sectioning and point counting. In the present study, point counting has been facilitated via a stereology interface within the ANALYZE software (MAYO Foundation, Minn., USA). The precision of the estimator est2V may be measured by its coefficient of error or ‘relative standard error’, using techniques previously described (Matheron, 1971; Gundersen and Jensen, 1987; Cruz-Orive and Roberts, 1993; Pache et al., 1993).

Repeatability and reproducibility Intra-operator variability was assessed by a single observer who measured the volume of the cerebellum, brainstem and UCC in 10 patients randomly selected from the total cohort, on two occasions 2 weeks apart. Inter-operator variability was assessed as the difference in the volume measurements obtained by two trained observers in the same 10 patients.

Biological variation The variation in the measured volumes among individuals (CV) has contributions from both the inherent biological variance among the measured volumes (CVb) and the precision of the individual volume estimates (CE) (Gundersen and Osterby, 1981). The mathematical relationship between these quantities is CV2 5 CVb2 1 1/n CE2. Since CV and the CEs on the individual estimates could be calculated, the magnitude of the biological variation could be derived.

Statistical analysis Clinical and demographic differences between controls, relapsing–remitting multiple sclerosis and secondary

progressive multiple sclerosis patients were tested with the Mann–Whitney U-test. The relationship between MRI volume estimates, and between volumes and disability scores, was evaluated using Spearman’s correlation coefficient (rho). All statistical analyses were two-tailed and performed with the Arcus statistical package (version Pro-11 2.15a). The assessment of inter- and intra-observer measurement agreement was based on the statistical methods of Bland and Altman (1986).

Results The mean volumes for all three infratentorial structures were significantly reduced in the 15 multiple sclerosis patients compared with their matched controls (brainstem, 22.68 6 2.47 cm3 versus 27.13 6 3.24 cm3; cerebellum, 126.0 6 11.1 versus 140.2 6 15.1 cm3; UCC, 2.75 6 0.58 versus 4.09 6 0.64 cm3, P , 0.0001 for all three comparisons). There were significant differences between our two patient groups for mean age, duration of disease and disability scores (Table 1). The mean volumes of the brainstem, cerebellum and UCC volumes were larger for relapsing–remitting than for secondary progressive multiple sclerosis patients. There was a wide variance, with overlapping ranges of values in the two patient groups, but the group mean differences were significant for cerebellum (P 5 0.013) and UCC (P 5 0.015) (Table 2). The volumes of the brainstem, cerebellum and UCC in the total patient cohort were significantly correlated with each other: brainstem and UCC (r 5 0.66, P , 0.0001); brainstem and cerebellum (r 5 0.56, P 5 0.0002); and cerebellum and UCC (r 5 0.61, P , 0.0001). In the total patient cohort, the UCC and cerebellar volumes were both correlated with disease duration and disability rating scores (Fig. 2 and Table 3). These volumes were also correlated with scores on the standard disability scales EDSS and SNRS and the derived disability scores of CFS (for UCC volume) and IFS (for cerebellar volume). Brainstem volume

Infratentorial atrophy in multiple sclerosis correlated weakly with the disability rating scales, but not with disease duration. The volume of the UCC was correlated with disease duration in the relapsing–remitting multiple sclerosis subgroup (–0.50, P 5 0.026). There was no significant association between age and volume for any structure. The mean predicted coefficients of error in measuring the volumes of the brainstem, cerebellum and UCC were 4.3% (range 2.28–5.59%, SD 0.83%), 4.7% (range 2.93–5.83%, SD 0.79%) and 3.4% (range 1.47–5.47%, SD 0.97%), respectively. The biological CV in the volume estimates of the brainstem, cerebellum and UCC overall patient groups were 13, 8 and 27%, respectively. The intra-operator repeatability of the measurements for all three structures was between 0.02 and 0.03. The inter-operator reproducibility was 0.05–0.06. None of the repeated observations fell outside 95% confidence limits.

Discussion This study adds to previous reports of decreased cerebellar volume (Davie et al., 1995) and cross-sectional cervical spinal cord area (Filippi et al., 1996a; Losseff et al., 1996b) in multiple sclerosis, by demonstrating that all infratentorial structural volumes, including the brainstem, are significantly atrophied when compared with matched control subjects. We have found significant correlations between disability and the volumes of brainstem, cerebellum and UCC on MRI, despite a considerable variance in volume estimates at each disability level and the well-recognized limitations of the disability rating scales (Francis et al., 1991; Noseworthy, 1994). As the EDSS and SNRS scales include scores from functional systems which are not relevant to infratentorial structures, we derived two composite KFS scores which would relate more closely to brainstem, cerebellar or spinal cord function. This approach made little difference to the strength of the correlations. Confounding factors include the non-linearity and poor reproducibility of the commonly used disability rating scales (Willoughby and Paty, 1988; Noseworthy et al., 1990; Francis et al., 1991; Noseworthy, 1994), the large biological variance and the errors inherent in the stereological technique. A further error is inevitable due to the variable anatomical relationship between central nervous system structures and skeletal landmarks. In addition, atrophy is non-specific for the axonal loss or dysfunction which is thought to account for fixed neurological deficits and, at low levels of disability, other pathophysiological mechanisms, such as conduction block, may have a significant affect on function without contributing to volume loss. The relationship between structural volume and disability requires further exploration in a larger study. The association between brainstem volume and disability was weak, perhaps indicating that axonal or myelin tissue loss in this structure is either less severe than that in the cerebellum or spinal cord, or relatively ‘compensated’ for by gliosis, remyelination or expanded extracellular space.

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Alternatively, the percentage of destructive lesions relative to the volume of the brainstem may be less than that for the spinal cord. In the cerebellum, we found a weak correlation between volume and the SRNS, but not with the EDSS, perhaps in accord with claims of increased sensitivity of the SNRS to disability (Sipe et al., 1984). Our finding of a modest relationship between UCC volume and disability is consistent with studies of the cross-sectional area of the cervical spinal cord (Kidd et al., 1993; Filippi et al., 1996a; Losseff et al., 1996b) and with the concept that the frequent and extensive demyelination and axonal damage and loss seen pathologically in the cervical spinal cord (Oppenheimer, 1978) is more likely to cause disability than brainstem lesions. Many radiological studies which have attempted to relate atrophy to dysfunction in multiple sclerosis have been concerned primarily with correlating cognitive performance with measurements of supratentorial structures, particularly the ventricles or corpus callosum (Zimmerman and Netsky, 1950; Simon et al., 1987; Dietemann et al., 1988; Laissy et al., 1993; Pelletier et al., 1993). In some early CT studies, ventricular enlargement and prominent sulci, basal cisterns and sylvian and interhemispheric fissures were observed to be common in patients with established clinically definite multiple sclerosis. Atrophy was noted to be the only CT abnormality in many young patients ‘with or without cerebral symptoms of brief duration’ (Noseworthy et al., 1984). Attempts to relate cognitive dysfunction to MRI measures of atrophy have produced inconsistent results. Some MRI studies have been able to demonstrate significant correlations between various cognitive tests and either corpus callosum atrophy (Swirsky-Sacchetti, 1991) or ventricular dilatation (Huber et al., 1987; Pozzilli et al., 1991), whereas others have not (Brainin et al., 1988; Franklin et al., 1988; Rao et al., 1989b). We have reported recently that supratentorial volumes of white matter, as estimated from threedimensional-acquired MRI, are significantly correlated with cognitive performance, but have little if any relationship with scores on the commonly used clinical disability rating scales (Edwards et al., 1997). This is not unexpected, considering the relatively crude cognitive domains in these scales. There are few reports of quantitative MRI studies of atrophy of infratentorial structures or spinal cord in multiple sclerosis with which to compare our data. One recent study used contouring methods and extrapolation from twodimensional sagittal slices to show that multiple sclerosis patients with ataxia had significantly smaller cerebellar volumes than either patients without ataxia or controls, but there was no correlation between the estimated volumes and either EDSS or the KFS scores (Davie et al., 1995). A series of MRI studies have correlated cross-sectional area of the spinal cord with disability (Filippi et al., 1996a; Kidd et al., 1996; Losseff et al., 1996b). In one report, the estimated cross-sectional area of the cord at C5 was found to be significantly less in patients with secondary progressive disease and more disability, than that found in patients with benign disease and less disability (Filippi et al., 1996a). In

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this same study, the degree of atrophy, as measured by both cross-sectional area and transverse diameter, was correlated with EDSS scores. However, change in cross-sectional area over a 12-month period did not correlate with change in disability (Kidd et al., 1996). The use of improved MRI techniques, including phased array coils, has revealed strong graded correlations between EDSS scores and cross-sectional spinal cord area (Losseff et al., 1996b). Why are the correlations between UCC volume and disability scale scores in the present study less strong than those seen for cross-sectional area of the spinal cord at a single level in other reports (Filippi et al., 1996a; Losseff et al., 1996b)? Our observation that the largest volumes were not necessarily associated with the least disabled patients suggests that at least some disability may be present without any detectable changes in the cervical cord volume. Nevertheless,

the volumes of the UCC and cerebellum were significantly smaller in secondary progressive multiple sclerosis than in relapsing–remitting multiple sclerosis (P , 0.01), a result suggesting that there is a greater mean loss of axons or myelin per unit tissue volume in secondary progressive multiple sclerosis and/or a larger proportion of lesions in relapsing– remitting multiple sclerosis with little or no axon or myelin loss, perhaps associated with more remyelination. In this study, we used the MPRAGE technique of T1weighted three-dimensional gradient echo imaging because it offers important benefits over conventional spin echo sequences, particularly when measuring structural volumes (Brant-Zawadzki et al., 1992). It has fewer susceptibility, vascular and motion artefacts due to its relatively short acquisition time and high spatial resolution with decreased partial volume effects (Shah et al., 1992). It provides thin

Infratentorial atrophy in multiple sclerosis contiguous slices without the ‘cross-talk’ between twodimensional-acquired slices. There is no need for a localizer image, and the image set can be processed and reformatted in any plane with minimal effect on image quality. Lastly, the method allows rapid and simultaneous acquisition of both brain and UCC images. Recent studies have shown that it is superior to conventional spin-echo sequences in the evaluation of both T1-weighted lesions (Gong et al., 1996) and gadolinium-enhancing (Filippi et al., 1996b) lesions. We analysed our MRI data using the Cavalieri method. This technique has been used previously to estimate the volume of cortex, white matter, central grey structures and

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ventricles of the human brain at autopsy (Regeur and Pakkenberg, 1989), the volume of the ventricles in hydrocephalic patients (Pakkenberg et al., 1989), the effects of age and sex on cerebellar volume (Escalona et al., 1991) and the lesion load on T2-weighted MRI in multiple sclerosis (Roberts et al., 1994). The lack of correlation between age and atrophy in our patients is consistent with other reports (Chida et al., 1989; Raininko et al., 1994). Our method is both repeatable and reproducible. There were no significant differences between two independent measurements of infratentorial stuctures by either a single observer, or by two observers (P . 0.05). Volume estimates

Fig. 2 Scatter plots of association between structural volumes and clinical parameters: (A) upper cervical cord volume versus disease duration (n 5 39); (B) upper cervical cord volume versus Cord Functional Scale (CFS); (C) cerebellar volume versus Scripps Neurological Rating Scale (SNRS); and (D) upper cervical cord volumes versus SNRS. r 5 Spearman’s rho; open circles 5 relapsing–remitting multiple sclerosis; closed circles 5 secondary progressive multiple sclerosis.

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Table 3 Coefficients (Spearman’s rho) and significance (P) for correlations between infratentorial volumes and age, disease duration and disability scores

Age Disease duration EDSS SNRS IFS CFS

Cerebellum

Brainstem

UCC

ns –0.34* –0.34* 0.49** –0.44** –0.39*

ns ns –0.13 ns 0.34* –0.32* –0.30 ns

ns –0.39* –0.34* 0.46** –0.47** –0.50**

EDSS 5 Expanded Disability Status Scale (Kurtzke, 1983); SNRS 5 Scripp’s Neurological Rating Scale (Sipe et al., 1984); IFS 5 Infratentorial Functional Score (see Methods); CFS 5 Cord Functional Score (see Methods). ns 5 not significant, *trend (0.01 , P , 0.05), **significant (P , 0.01).

on a mean difference plot were scattered equally about a mean difference line, with no significant variance with respect to inter- or intra-observer variability. Although measurements of the cervical spinal cord volumes yielded larger coefficients of variation than for the brainstem and cerebellum, the CV for intra- and inter-operator measurements for this structure was ,3% and 6%, respectively. What are the pathological correlates of the structural volume changes found in multiple sclerosis? Although it is commonly considered that axons are relatively spared in the essentially demyelinating lesions of multiple sclerosis, it is well recognized that a significant percentage of axons may be lost during the earliest stages of plaque formation, and with time and associated fibrosis the axonal density decreases further (Greenfield and King, 1936; Adams and Kubik, 1952; Madrid and Wisniewski, 1977; Ozawa et al., 1994; Raine, 1997a, b). Recent studies have provided both histological and biochemical evidence of axonal damage occurring in acute lesions early in the disease course (Ferguson et al., 1997; Trapp et al., 1998). Secondary or Wallerian degeneration of axons is generally considered a feature of patients with extensive or late disease (Prineas, 1996). Individual cases have been described in which the loss of an estimated onethird or more of the axis cylinders in the pyramidal tracts of the lumbar spinal cord was attributed to involvement of the pyramidal tracts at the cerebral level (Lumsden, 1972). Chronically demyelinated axons have a significantly smaller diameter (Prineas and Connell, 1978; Raine 1985). Loss of myelin must also be considered as a factor in volume loss, but the relative contributions of myelin loss, axonal loss or shrinkage and secondary gliotic scarring to structural volume reduction in the white matter of the central nervous system remain uncertain. Serial studies are required to determine the sensitivity of UCC, brainstem and cerebellar volumes to the accumulating disability scale scores in patients with multiple sclerosis with different disease courses and in different stages of the disease. A recent serial MRI study of a ‘slab’ of cerebral hemispheres suggests that significant volume changes can be detected within the time frame of therapeutic trials (Lossef et al.,

1996a). We currently are evaluating the role of serial volume estimates of both supratentorial and infratentorial white matter on three-dimensional MRI in the investigation of the efficacy of putative treatments aimed at preventing disease progression in multiple sclerosis.

Acknowledgements We wish to thank Steve Evans, Charge Radiographer in the Diagnostic Neuroimaging Unit at University Hospital, Nottingham, for his assistance with MRI data collection, and Vicky Orpe, Multiple Sclerosis Trial Coordinator in the Division of Clinical Neurology, for essential work with patients and collation of clinical data.

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Received August 21, 1998. Accepted September 30, 1998