Gray and white matter alterations in hereditary spastic paraplegia type SPG4 and clinical correlations

Abstract

Hereditary spastic paraplegias (HSP) are a group of clinically and genetically heterogeneous disorders with the hallmark of progressive spastic gait disturbance. We used advanced neuroimaging to identify brain regions involved in SPG4, the most common HSP genotype. Additionally, we analyzed correlations between imaging and clinical findings. We performed 3T MRI scans including isotropic high-resolution 3D T1, T2-FLAIR, and DTI sequences in 15 adult patients with genetically confirmed SPG4 and 15 age- and sex-matched healthy controls. Brain volume loss of gray and white matter was evaluated through voxel-based morphometry (VBM) for supra- and infratentorial regions separately. DTI maps of axial diffusivity (AD), radial diffusivity (RD), mean diffusivity (MD), fractional anisotropy (FA), and measured anisotropy (MA1) were analyzed through tract-based special statistics (TBSS). VBM and TBSS revealed a widespread affection of gray and white matter in SPG4 including the corpus callosum, medio-dorsal thalamus, parieto-occipital regions, upper brainstem, cerebellum, and corticospinal tract. Significant correlations with correlation coefficients r > 0.6 between clinical data and DTI findings could be demonstrated for disease duration and disease severity as assessed by the spastic paraplegia rating scale for the pontine crossing tract (AD) and the corpus callosum (RD and FA). Imaging also provided evidence that SPG4 underlies a primarily axonal rather than demyelinating damage in accordance with post-mortem data. DTI is an attractive tool to assess subclinical affection in SPG4. The correlation of imaging findings with disease duration and severity suggests AD, RD, and FA as potential progression markers in interventional studies.

Full text

ORIGINAL COMMUNICATION
Gray and white matter alterations in hereditary spastic paraplegia
type SPG4 and clinical correlations
Tobias Lindig
1
•Benjamin Bender
1
•Till-Karsten Hauser
1
•Sarah Mang
1
•
Daniel Schweikardt
1
•Uwe Klose
1
•Kathrin N. Karle
2,3
•Rebecca Schu
¨le
2,3,4
•
Ludger Scho
¨ls
2,3
•Tim W. Rattay
2,3
Received: 23 December 2014 / Revised: 20 May 2015 / Accepted: 21 May 2015
ÓSpringer-Verlag Berlin Heidelberg 2015
Abstract Hereditary spastic paraplegias (HSP) are a group
of clinically and genetically heterogeneous disorders with
the hallmark of progressive spastic gait disturbance. We
used advanced neuroimaging to identify brain regions
involved in SPG4, the most common HSP genotype. Addi-
tionally, we analyzed correlations between imaging and
clinical findings. We performed 3T MRI scans including
isotropic high-resolution 3D T1, T2-FLAIR, and DTI
sequences in 15 adult patients with genetically confirmed
SPG4 and 15 age- and sex-matched healthy controls. Brain
volume loss of gray and white matter was evaluated through
voxel-based morphometry (VBM) for supra- and infraten-
torial regions separately. DTI maps of axial diffusivity (AD),
radial diffusivity (RD), mean diffusivity (MD), fractional
anisotropy (FA), and measured anisotropy (MA1) were
analyzed through tract-based special statistics (TBSS). VBM
and TBSS revealed a widespread affection of gray and white
matter in SPG4 including the corpus callosum, medio-dorsal
thalamus, parieto-occipital regions, upper brainstem, cere-
bellum, and corticospinal tract. Significant correlations with
correlation coefficients r[0.6 between clinical data and
DTI findings could be demonstrated for disease duration and
disease severity as assessed by the spastic paraplegia rating
scale for the pontine crossing tract (AD) and the corpus
callosum (RD and FA). Imaging also provided evidence that
SPG4 underlies a primarily axonal rather than demyelinating
damage in accordance with post-mortem data. DTI is an
attractive tool to assess subclinical affection in SPG4. The
correlation of imaging findings with disease duration and
severity suggests AD, RD, and FA as potential progression
markers in interventional studies.
Electronic supplementary material The online version of this
article (doi:10.1007/s00415-015-7791-7) contains supplementary
material, which is available to authorized users.
&Ludger Scho
¨ls
ludger.schoels@uni-tuebingen.de
Tobias Lindig
tobias.lindig@med.uni-tuebingen.de
Benjamin Bender
benjamin.bender@med.uni-tuebingen.de
Till-Karsten Hauser
till-karsten.hauser@med.uni-tuebingen.de
Sarah Mang
sarah.mang@gmx.de
Daniel Schweikardt
daniel.schweikardt@student.uni-tuebingen.de
Uwe Klose
uwe.klose@med.uni-tuebingen.de
Kathrin N. Karle
kathrin.karle@medizin.uni-tuebingen.de
Rebecca Schu
¨le
rebecca.schuele-freyer@uni-tuebingen.de
Tim W. Rattay
tim.rattay@uni-tuebingen.de
1
Department of Diagnostic and Interventional Neuroradiology,
University Hospital Tu
¨bingen, Hoppe-Seyler-Str. 3,
72076 Tu
¨bingen, Germany
2
Department of Neurology, Hertie Institute for Clinical Brain
Research, Hoppe-Seyler-Straße 3, 72076 Tu
¨bingen, Germany
3
German Research Center for Neurodegenerative Diseases
(DZNE), 72076 Tu
¨bingen, Germany
4
Dr. John T. Macdonald Foundation Department of Human
Genetics, John P. Hussman Institute for Human Genomics,
University of Miami Miller School of Medicine, Miami,
FL 33136, USA
123
J Neurol
DOI 10.1007/s00415-015-7791-7

Keywords Hereditary spastic paraplegia (HSP) SPG4 
MRI VBM TBSS
Background
Hereditary spastic paraplegias (HSP) are neurodegenera-
tive disorders of the spinal cord with the clinical hallmark
of progressive spasticity and weakness of lower limbs.
Cases presenting with spastic paraparesis, often associated
with mild sensory abnormalities and urinary dysfunctions,
are classified as pure HSP (pHSP). In complicated forms
(cHSP), additional parts of the nervous system are affected
as indicated by symptoms like cerebellar ataxia, parkin-
sonism, epilepsy, cognitive deficits, deafness, cataract, and/
or optic atrophy [1]. Hereditary spastic paraplegias are rare
diseases with a prevalence of 2–10:100.000 [2] but are
genetically highly heterogeneous with to date about 80
genes and loci proven to cause HSP [3–7].
SPG4 is the most common genotype accounting for up
to 50 % of autosomal dominant families [4,8] but also for
up to 20 % of sporadic cases [8–10]. SPG4 is caused by
mutations in the SPAST-gene [11] and is inherited as an
autosomal dominant trait. The typical phenotype of SPG4
is ‘‘pure’’ HSP although few instances have been described
where SPG4 goes along with cognitive decline, cerebellar
ataxia, and/or peripheral neuropathy [4,6–8,12–14].
Systematic studies of MRI in representative HSP
cohorts are rare and mostly include only few individuals
with defined genotypes. MRI abnormalities reported so far
include thinning of the corpus callosum as in SPG11 [15]
and SPG15 [16], as well as in many rare genotypes like
SPG35 [17] and others.
1
Cerebellar atrophy is a rather
frequent finding in SPG7 [18,19] but occurs in many other
genotypes as well. White matter changes have been
described in SPG2 with PLP1 mutations but also in several
cases of SPG5, SPG11, SPG15, SPG20, SPG21, SPG26,
and several more. In SPG4, cerebral standard MRI is
normal in most cases although few SPG4 patients with
cerebellar atrophy, white matter lesions, or thinning of the
corpus callosum have been described. Few MRI studies
focused on SPG4 including structural MRI [20], regional
cerebral blood flow analysis [21], diffusion tensor imaging
(DTI) [22], and O
15
-PET of the motor cortex [23].
Abnormalities belonging to the Dandy-Walker complex
were found in one single family in the posterior fossa in
structural MRI [20].
To identify brain regions involved in SPG4 we applied
advanced imaging techniques to a representative cohort of
SPG4 patients. Using voxel-based morphometry and tract-
based spatial statistics (TBSS), we revealed quantitative
imaging measures for affected regions. Clinical relevance
was determined by the correlation of imaging findings with
deep phenotyping.
Materials and methods
MRI was performed on a 3T whole-body MR system
(Skyra, Siemens, Erlangen, Germany) at the University
Hospital Tu
¨bingen using a 32-channel head coil. 15 adult
patients (online resource 1) with confirmed mutations in
SPG4 (7 females, 8 males; mean age 52.7 ±10.5; all right
handed) and 15 age- and sex-matched healthy volunteers (7
females, 8 males; mean age 51.1 ±9.2; all right handed)
were recruited from the HSP outpatient clinic in Tu
¨bingen
and underwent a standardized clinical examination
including the spastic paraplegia rating scale (SPRS) as a
valid measure of disease severity [24]. Diagnostic MRI was
carried out to exclude pathological conditions not related to
HSP like tumor, infarction, or other focal lesions. Struc-
tural 3D volume data were collected with T1 and T2-
FLAIR contrast; MPRAGE (TR/TE/TI, 2300/2.32/900 ms;
voxel size, 0.9 mm isotropic; acquisition time, 5.19 min),
3D T2-FLAIR (IR-SPACE) (TR/TE/TI, 5000/3872/
1800 ms; voxel size, 0.9 mm isotropic; acquisition time,
6.55 min). Diffusion-weighted images were acquired by
using an EPI Sequence (TR/TE, 13100/91 ms; bandwidth,
1685 kHz; voxel size, 2.0 mm isotropic; bvalue =0 and
1000 s/mm; 64 gradient directions; 80 slices; acquisition
time, 14.52 min).
VBM of supratentorial structures
Voxel-based morphometry (VBM) of supratentorial struc-
tures was carried out on T1-weighted and T2 FLAIR data
sets using the standard routines of SPM12 for multi-
channel segmentation [25–27].
After a nonlinear normalization step, images were seg-
mented into tissue types using the multi-spectral segmen-
tation algorithm, isolated from the surrounding non-brain
structures.
The resulting whole brain segmentation maps for gray
and white matter of normal controls and SGP4 patients
were then checked for group homogeneity using the VBM
toolbox VBM8 to calculate the standard deviation across
the sample (Gaser, http://dbm.neuro.uni-jena.de/vbm). One
patient with poor normalization to the MNI152 template
was identified. Even with improved prealignment, no suf-
ficient normalization to the standardized MNI template
could be achieved, so that this patient and the matched
control were excluded from further analysis.
1
SPG21, SPG46–SPG50, SPG54, SPG56, SPG63, SPG65–67 and
SPG71.
J Neurol
123

The segmented normalized and modulated images were
then smoothed by an 8 9898 mm full width at half
maximum (FWHM) filter. Differences between the 2
groups were analyzed by a paired two-sample ttest in
SPM12. Age, sex, and total intracranial volume were used
as covariates. An uncorrected pB0.001 was considered
the level of significance if not stated otherwise. Finally, we
performed an identical VBM analysis for gray and white
matter but without patient 17 to evaluate the potential
influence of a mild periventricular microangiopathy on our
VBM results.
VBM of infratentorial structures
Voxel-based morphometry of infratentorial structures was
carried out on T1-weighted images using the SUIT toolbox
(SUIT, Version 2.7) [28,29], not yet compatible with
multi-channel segmentation.
After a linear normalization step, the cerebellum and
brainstem were segmented into tissue types using the
SPM8 segmentation algorithm [25,30], isolated from the
surrounding non-brain and supratentorial brain structures,
and cropped to an infratentorial volume of interest. All
isolation maps were hand-corrected to exclude tissue of the
occipital lobe and venous sinuses using a 3D image viewer
(MRICroN) [31]. The cropped images were then normal-
ized to a spatially unbiased atlas template of the cerebellum
and brainstem. The segmented gray and white matter
images were re-sliced into SUIT space using the defor-
mation map generated in the normalization step.
The resulting infratentorial segmentation maps of nor-
mal controls and SGP4 patients were then checked for
group homogeneity using the VBM toolbox VBM8. One
patient with poor normalization to the cerebellar template
was identified as the same patient for the supratentorial
analysis. Even with improved prealignment and exten-
sively hand-corrected cerebellar isolation maps, no suffi-
cient normalization to the standardized cerebellar template
could be achieved, so that this patient and the matched
control were excluded from further analysis.
The segmented normalized and modulated infratentorial
images were then smoothed by a 6 9696 mm full width
at half maximum (FWHM) filter. Differences between the
2 groups were analyzed by a paired two-sample ttest in
SPM8. An absolute threshold masking of 0.1 was used to
restrict the analysis to gray and white matter, respectively.
An uncorrected pB0.001 was considered the level of
significance if not stated otherwise.
TBSS
Tract-based spatial statistics (TBSS), part of fsl version
4.1.1 (http://fsl.fmrib.ox.ac.uk/fsl)[32], were used to
investigate differences between 14 patients and matched
controls along the major white matter tracts without bias
[33], the same cohort as with VBM before.
Prior to the TBSS evaluation, the diffusion data were
corrected for motion during data acquisition and corre-
sponding gradient directions were corrected accordingly.
The two basic diffusion measures axial diffusivity (AD)
and radial diffusivity (RD) were created by fitting a tensor
model to the raw diffusion data. The thereof derived dif-
fusion measures such as fractional anisotropy (FA), mean
diffusivity (MD), and measured anisotropy
(MA1 =AD -MD) were calculated afterwards. All
subjects’ data were then aligned into a common space
using the nonlinear registration tool within fsl. The rec-
ommended FMRIB58-FA standard-space image, already
aligned to the MNI152 template, was used as the target in
TBSS.
The resulting spatially normalized maps of normal
controls and SGP4 patients were then checked for group
homogeneity using the VBM toolbox VBM8. Three further
patients with poor normalization to the FMRIB58-FA
standard-space template were identified and excluded from
further analysis, together with their matched controls.
The mean FA image of all subjects was created and
thinned (threshold FA value of 0.25) to create a mean FA
skeleton which represents the centers of all tracts common
to the group. Finally, each subjects’ spatially normalized
AD, RD, MD, FA, and MA1 data were projected onto the
mean skeleton and the resulting data were fed into voxel-
wise cross-subject statistics. Differences between the 2
groups were analyzed by a paired two-sample t-test using
the randomized tool from fsl. After a cluster-based cor-
rection for multiple comparisons, p\0.05 was considered
the level of significance if not stated otherwise.
Correlation of DTI parameters with clinical data
Based on the VBM and TBSS results, the pontine crossing
tract with fibers from the superior cerebellar peduncles, the
CC (genu, body, and splenium), the posterior corona
radiata for the parietal WM, the posterior thalamic radia-
tion for the occipital WM, and the corticospinal tract as the
clinical focus of the disease were preselected for correla-
tion analyses. Atlas-based ROI-identification of those
structures was carried out using the ICBM-DTI-81 labels
atlas (white matter tract labels created by hand segmenta-
tion of a standard-space average of diffusion MRI tensor
maps from 81 subjects); provided by Dr. Susumu Mori,
Laboratory of Brain Anatomical MRI, Johns Hopkins
University. The atlas labels were used to cut out the
selected structures from the previously created (TBSS)
normalized AD, RD, MD, FA, and MA1 skeletons, and the
mean values were calculated for each subject and label.
J Neurol
123

Statistical analysis were performed for all 5 white matter
tract labels using the Statistics Toolbox within MATLAB,
version 2013b. The calculated mean DTI values of all
subjects were correlated with age, disease duration, SPRS
total score, and SPRS spasticity subscore (items 7, 8, 9, and
10) through Pearson correlation coefficients and were
corrected for multiple comparisons (Bonferoni-Holms).
Results
VBM of supratentorial structures
Voxel-based analysis of the supratentorial gray matter
(Fig. 1) showed symmetric volume reduction in the medio-
dorsal thalamus (medial pulvinar, latero-dorsal nuclei, and
medio-dorsal nuclei) and in the cingulum (mid cingulate
gyrus).
Voxel-based analysis of the supratentorial white matter
(FDR corrected p\0.001; Fig. 2) revealed symmetric
volume reduction in the deep white matter predominantly
parieto-occipital and to a lesser extent frontal, and in the
corpus and splenium of the corpus callosum.
The exclusion of patient 17 with a mild pattern of
periventricular microangiopathy showed no focal atrophy
in the right supramarginal gyrus and in the subcortical
white matter of the supramarginal gyrus, whereas an
analysis including patient 17 revealed some regional vol-
ume reduction in these regions.
No significant volume loss was found for the precentral
gyrus.
VBM of infratentorial structures
Voxel-based analysis of the infratentorial gray matter
showed symmetric volume reduction in the inferior
Fig. 1 Supratentorial VBM results for gray matter. Significantly
(p\0.001) decreased supratentorial gray matter volumes in SPG4
patients compared to controls are displayed on top of MNI152_T1
template in axial view; medio-dorsal thalamus (asterisks) and
cingulum (hash). With correction for multiple comparisons (FDR
p\0.1) no significant volume reduction was left
J Neurol
123

semilunar lobe of the posterior lobe and in the left lobus
culminis of the anterior vermis.
Voxel-based analysis of the infratentorial white matter
(FDR corrected p\0.05) revealed volume reduction in the
upper brainstem (pons and midbrain) and symmetrically in
the deep cerebellar white matter including the superior,
middle, and inferior cerebellar peduncles (Fig. 3).
TBSS
TBSS revealed significant differences of AD, RD, FA, and
MA1 values (TFCE corrected p\0.05) between patients
and controls (Fig. 4,5). In general, TBSS disclosed a
widespread reduction of AD and some increase of RD as
the two major effects, both result in no significant change
regarding MD. In MA1 both effects, the reduction of AD
and the increase of RD, are added.
Specifically, SPG4 patients showed lower MA1 in almost
all major subcortical and deep white matter fiber bundles of
the supratentorial and infratentorial brain, including the
genu, body, and splenium of the corpus callosum, the corti-
cospinal tract, the cingulum, the optic radiation, the superior,
middle and inferior cerebellar peduncle. Significance level
up to p\0.003 (TFCE corrected) are found for almost all
major subcortical and deep white matter tracts, predomi-
nantly in the parieto-occipital white matter and to a lesser
extent frontal (lower row in Fig. 4).
The metaparameter FA was reduced to a lesser extent
than MA1 but was also reduced in all major supratentorial
white matter tracts, including the fornix and the chiasma
Fig. 2 Supratentorial VBM results for white matter. Significantly
(FDR corrected p\0.001) decreased supratentorial white matter
volumes in SPG4 patients compared to controls are shown in the deep
white matter predominantly parieto-occipital and to a lesser extent
frontal, and in the corpus callosum. Results are displayed on top of
MNI152_T1 template in axial view
J Neurol
123

opticum and some additional subcortical frontal white
matter tracts. FA did not reach the statistical significance
level for infratentorial structures.
AD was altered to a lesser extent than MA1 but was also
reduced in all major white matter tracts including the
corticospinal tract. The splenium and the dorsal body of the
corpus callosum was spared.
RD showed an increase in the middle and dorsal parts of
the corpus callosum (splenium and body) and in the pari-
eto-occipital deep white matter. No significant increase for
the corticospinal tract was found.
MD did not reach a level of statistical significance.
Correlation of DTI parameters with clinical data
DTI results correlated closely (correlation coefficients
r[0.6, p\0.0005 corrected for multiple comparisons)
with disease duration for the pontine crossing tract (AD)
and with disease severity (SPRS total score and SPRS
spasticity subscore) for the corpus callosum (RD and FA).
No significant correlation of DTI values with clinical data
could be found for the corticospinal tract. No significant
correlation with age was found. (see online resource 2)
Discussion
In contrast to standard MR imaging, VBM and TBSS
revealed widespread affection of gray and white matter
in SPG4, including the corpus callosum, medio-dorsal
thalamus, parieto-occipital regions, upper brainstem,
cerebellum, and corticospinal tract. Given the pure
spastic phenotype in most patients of our SPG4 cohort,
the finding of such extensive abnormalities on MRI was
quite unexpected. Alterations in the same direction have
been reported in a voxel-based FA-analysis, which
revealed changes exceeding the corticospinal tracts to
the temporal lobes, but this study included only six
SPG4 patients and diffusion data was acquired with a
highly anisotropic voxel size [34]. Similarly, a recent
DTI study with limited SNR (bvalue 800 s/mm
2
,15
directions, no average, highly anisotropic voxel size) and
only three SPG4 patients found increased MD and
decreased FA of the semioval centers, peritrigonal white
matter, genu, and trunk of the corpus callosum as well as
the posterior limb of the internal capsule and cerebral
pedicle [22]. No DTI alterations were observed in the
cerebellar white matter. The more widespread affection
Fig. 3 Infratentorial VBM results. Significantly (p\0.05 blue;
p\0.001 pink) decreased infratentorial gray matter volumes in
SPG4 patients compared to controls are presented on top of patients
mean_T1 template (upper row), inferior semilunar lobe (hash) of the
posterior lobe left and right, and left lobus culminis (asterisks) of the
anterior vermis. Significantly (p\0.001 blue; FDR corrected
p\0.01 pink) decreased infratentorial white matter volumes in
SPG4 patients are shown in the upper brainstem and symmetrically in
the deep cerebellar white matter including the superior, middle, and
inferior cerebellar peduncles (lower row). Sagittal, coronal, and axial
views form left to right
J Neurol
123

in our study is most likely reflecting the advanced
approach with the focus on image quality and SNR (3 T,
32 channel head coil, advanced B0 field shim and patient
specific B1 field shim, isotropic high-resolution proto-
cols, DTI with an acquisition time of 15 min for 64
directions and 80 slices). Our findings are based on a
representative cohort of 15 SPG4 patients including two
patients with cerebellar signs and a broad spectrum of
age ranging from 39 to 73 years.
Extensive white matter changes have also been shown
with DTI in amyotrophic lateral sclerosis (ALS), a com-
parable neurodegenerative motor neuron disease, including
abnormal FA values in the internal capsule, frontal white
matter, genu and splenium of the corpus callosum, and less
significant in the parietal and temporal lobe, and posterior
cingulum [35]. In ALS, frontal white matter was more
affected than parietal white matter, in contrast to our
findings in SPG4 with predominant involvement of the
parieto-occipital white matter. Comparison to other motor
neuron diseases would be interesting, but only very limited
imaging data is yet available due to rarity of these diseases.
Our VBM analyses revealed much less extensive
structural changes than functional DTI in SPG4 but con-
firmed some atrophy of the cerebellum. Alterations of the
posterior fossa have been reported earlier in SPG4 [20] but
were described as being part of a Dandy-Walker complex
what is not the case in our series. In contrast, our findings
do not represent developmental abnormalities but degen-
erative changes with mild but significant cerebellar atro-
phy. Excluding the two patients with clinical cerebellar
involvement from the VBM analysis had no influence on
the presented cerebellar effects. In accordance with our
findings, a detailed pathological study of a genetically
undefined case of late onset autosomal dominant HSP
Fig. 4 TBSS results for measured anisotropy. MA1 skeleton pre-
sented on top of TBSS mean_FA template in sagittal, coronal, and
axial view. Compared to controls, SPG4 patients show decreased
MA1 in almost all major subcortical and deep white matter tracts of
the supratentorial and infratentorial brain (tfce corrected p\0.05
blue and p\0.003 pink) pointing to a subclinical affection far
beyond the corticospinal tract. MA1 together with FA (shown in
Fig. 5) provide the best contrast and most sensitive representation of
the extent of the underlying neurodegeneration in SPG4 patients but
are fairly unspecific marker of microstructural architecture and
neuropathology
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123

found loss of Purkinje cells and neuronal loss in the deep
cerebellar nuclei [36]. Additionally, Seidel and colleagues
found severe neuronal loss in several thalamic nuclei which
has not been reported before in HSP. Interestingly, this
finding nicely matches with our VBM results, which
demonstrate extensive thalamic abnormalities and may
reflect transsynaptic degeneration from the fasciculus gra-
cilis which is known to be clearly affected in HSP (e.g.,
[37]). An influence of mild periventricular microangiopa-
thy present in patient 17 could be ruled out by excluding
this patient from our VBM analysis. The patient included
showed volume reduction in the right supramarginal gyrus
Fig. 5 TBSS results for
different DTI parameters. TBSS
skeletons (tfce corrected
p\0.05) of different DTI
parameters are shown on top of
the mean_FA template in
sagittal, coronal, and axial view.
SPG4 patients compared to
controls show a widespread
reduction (blue) of AD and
some increase (red)ofRDas
the two major effects, and no
significant change in MD. This
results in a reduction of FA and
MA1 where both effects of AD
and RD are added. Thus TBSS,
including the corticospinal tract
(asterisks), indicates most likely
a primarily axonal degeneration
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123

and in the corresponding subcortical white matter, pre-
sumably a local effect of the underlying microangiopathy.
Excluding patient 17 from our TBSS analysis reduced the
statistical power (n=10) considerably but did not change
the analysis qualitatively.
Only a small number of four autopsy studies of geneti-
cally confirmed SPG4 cases [37,38] have been reported so
far. Wharton and colleagues studied the expression of
spastin, the SPG4 protein, in human brains and related
pathology in three SPG4 cases. Using immunohistochem-
istry, they found spastin to be widely expressed in the CNS.
In addition, they demonstrated tau pathology outside the
motor system in their SPG4 cases suggesting a neu-
ropathological affection far beyond the motor system. This
may well represent the morphological background of DTI
abnormalities observed in our study.
The core neuropathology of HSP is a distal degeneration
of the lateral corticospinal tract and the fasciculus gracilis. In
three autopsy cases with genetically confirmed SPG4, distal
degeneration of long tracts in the spinal cord was consistent
with dying back axonopathy [37]. Until recently, no evi-
dence from imaging studies was pointing into the direction of
an axonal process. Here, we show (Fig. 5) that TBSS indi-
cates most likely a primarily axonal degeneration with a
widespread reduction of AD and some increase of RD. AD
reflecting the first eigenvalue of the diffusion tensor model
decreases in axonal degeneration due to the reduced fiber
density [39–43]. A recent study supports our findings and the
concept of SPG4-HSP as a distal motor axonopathy [44]. In
the areas with significant VBM white matter reduction, like
in the body and splenium of the corpus callosum, we find an
increase in RD together with no or only minimal decrease of
AD in accordance with reduced microstructural density, e.g.
sponge-like atrophic tissue with more free intercellular
water.
TBSS changes in our SPG4 cohort were largely sub-
clinical as only 2 of 15 patients presented with cerebellar
signs and none was obviously cognitively impaired (see
online resource 1). In general, in SPG4, pure HSP is the
most common phenotype with only few cases reported in
the literature to suffer from ataxia [8,14] or cognitive
impairment [45–48]. However, detailed neuropsychologi-
cal testing has not been performed in our cohort and may
well disclose minor neuropsychological deficits as clinical
equivalent of the extended DTI alterations found consis-
tently in our SPG4 patients. Our study recommends DTI as
an attractive tool to assess subclinical affection in SPG4.
Whether DTI changes are suitable as a quantitative mea-
sure of the degenerative process requires prospective lon-
gitudinal MRI studies. Ideally, these should be combined
with repetitive neuropsychological assessments. In addi-
tion, it will be interesting to learn when DTI abnormalities
start in SPG4. As the earliest stages of the disease were not
included in our study (minimal disease duration 11 years
and minimal SPRS 12 points), our study must leave this
question open.
Conclusion
In this VBM and DTI study, we were able to demonstrate a
widespread affection of gray and white matter in SPG4 in
contrast to normal routine MRI. TBSS including the cor-
ticospinal tract presented characteristics of primarily axo-
nal damage with decreased AD and at most mildly
increased RD in accordance with histopathological findings
[37] and electrophysiological assessment [49,50]. Studies
of early or even preclinical stages of SPG4 can help to
unravel the onset of DTI abnormalities. The correlation of
imaging findings with disease duration and severity sug-
gests AD, RD, and FA as potential progression markers in
interventional studies.
Acknowledgments The research leading to these results has
received funding from the European Community’s Seventh Frame-
work Programme (FP7/2007–2013) under grant agreement n°
2012–305121 ‘‘Integrated European—omics research project for
diagnosis and therapy in rare neuromuscular and neurodegenerative
diseases (NEUROMICS)’’ (to TR und LS) and was supported by the
Interdisciplinary Center for Clinical Research IZKF Tu
¨bingen (grant
1970-0-0 to RS), the European Union ((PIOF-GA-2012-326681)
HSP/CMT genetics) to RS, and the German HSP-Selbsthilfegruppe
e.V. (grant to RS and LS).
Conflicts of interest The authors declare that they have no conflicts
of interest.
Ethical standard The study protocol was approved by the local
ethics review board. Informed written consent was obtained from all
subjects prior to examinations. The study has been performed in
accordance with the ethical standards laid down in the 1964 Decla-
ration of Helsinki and its later amendments.
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