Childs Nerv Syst
DOI 10.1007/s00381-013-2351-x

CASE REPORT

Diffusion tensor imaging of pyramidal tract reorganization
after pediatric stroke
Emilie George & Linda Heier & Ilhami Kovanlikaya &
Jeffrey Greenfield

Received: 3 December 2013 / Accepted: 26 December 2013
# Springer-Verlag Berlin Heidelberg 2014

Abstract
Background Plasticity of the developing motor tracts is a
contributor to recovery of motor function after pediatric
stroke. The mechanism of these plastic changes may be functional and/or structural in nature. The corticospinal tract (CST)
represents the major pathway responsible for voluntary movement. Stroke-induced damage to the CST as well as to other
motor tracts leads to motor deficits which may show favorable
functional recovery particularly in the pediatric population.
Methods We report the case of a 3-year-old girl demonstrating
reorganization of the pyramidal tracts after an extensive left
MCA territory stroke secondary to head trauma. Reorganization
is characterized using serial diffusion tensor imaging (DTI) of the
pyramidal tracts which contain the CST.
Results Imaging shows decreased ipsi-lesional fractional anisotropy (FA) suggestive of Wallerian degeneration and increased contralesional FA.
Conclusions These results point to plastic reorganization of
the pyramidal tract post-stroke and the utility of DTI in recognizing these changes.
Keywords Neuroplasticity . DTI . Corticospinal tract .
CNS reorganization . Pediatric stroke

Introduction
Neural plasticity contributes variably to recovery after stroke,
but is thought to be particularly important after insults to the
E. George : J. Greenfield (*)
Department of Neurosurgery, Weill Cornell Medical College,
525 East 68th Street, Box 99, New York, NY 10065, USA
e-mail: jpgreenf@med.cornell.edu
L. Heier : I. Kovanlikaya
Department of Radiology, Weill Cornell Medical College,
525 East 68th Street, Box 141, New York, NY 10065, USA

developing brain. Pediatric patients often exhibit better poststroke functional outcome than adults, especially regarding
motor function and mobility [1, 2]. The majority of this
functional recovery, both in adult and pediatric populations,
occurs within a 2–3 month period after the initial insult with
smaller gains made in the subsequent months [3]. Plasticity of
cerebral connectivity includes functional changes such as
alterations in synaptic efficiency or recruitment of existing
otherwise “silent” pathways as well as structural changes such
as synaptogenesis and enhanced dendritic arborization [4, 5].
The corticospinal tract (CST), as the major facilitator of
voluntary movement, is particularly important for the recovery of motor functioning following stroke. The majority of
CST fibers crosses in the pyramidal decussation at the level of
the cervicomedullary junction and descend to form the lateral
corticospinal tract while a minority remains ipsilateral forming
the anterior corticospinal tract. Limb movement is mediated
by the lateral CST resulting in contralesional motor deficits
when disrupted. The anterior CST, however, innervates axial
and girdle muscles bilaterally, often preserving these movements post-stroke dependent upon the site of the lesion.
Several hypotheses exist regarding the mechanisms underlying motor recovery. The possibilities include the advent of
new connections from the ipsilateral (anterior) CST, recruitment of other pathways such as the reticulospinal tract, enlistment of unaffected cortical areas, or sprouting from intraspinal
CST collaterals which recross to the ipsi-lesional side after
decussation [6, 7]. Mirror movements which can often be
demonstrated in stroke patients [8] suggest novel branching
of pathways originating from the undamaged hemisphere.
The CST can be beautifully visualized utilizing diffusion
tensor imaging (DTI), a specific application of magnetic resonance imaging (MRI) to enhance structural analyses. DTI
takes advantage of the anisotropic movement of water molecules within biological structures such as the axon.
Unrestricted, water molecules undergo isotropic motion; however, when constrained within the linear space within or along

Childs Nerv Syst

an axon, the molecules move preferentially in the direction of
the axonal process and pathways. Anisotropy may additionally be modulated by the degree of axon myelination which
causes further restriction of perpendicular diffusion due to the
lipid-dense myelin sheath surrounding most axons. By detecting the degree of anisotropy, the orientation of fiber tracts can
be calculated and illustrated. Fractional anisotropy (FA) quantifies the degree of anisotropic motion of the water molecules.
As motion becomes more coherent and aligned, FA values
approach 1.0. Clinical applications of measuring diseaseinduced variations in DTI have primarily focused on the
normal development of white matter and traumatic brain
injury; however, other clinical applications have emerged
including identifying structural derangements in the integrity of white matter due to multiple sclerosis, epilepsy,
and brain tumors.
We describe a pediatric patient who suffered an extensive
middle cerebral artery (MCA) territory infarct which resulted
in profound hemiparesis who subsequently recovered the
majority of her motor function. DTI imaging attained 7 months
and 3.5 years post-stroke indicates significant change in CST
anatomy, presumably the structural basis for her functional
improvement.

Case report
A 3-year and 6-month-old female presented to the emergency
room after being struck by a motor vehicle as a pedestrian.
Fig. 1 T2-weighted axial images
obtained in 2010 (a) and 2013 (b)
reveal chronic infarction with
progressive cystic
encephalomalacia and Wallerian
degeneration seen as asymmetric
atrophy of the left cerebral
peduncle

Initial head computed tomography (HCT) revealed a closed,
displaced left occipital bone fracture, a non-displaced right
temporal bone fracture with underlying subarachnoid hemorrhage, and a left temporal bone fracture through the foramen
magnum and the posterior margin of the left jugular foramen
extending into the base of the left sphenoid bone and pterygoid plate. There was no initial evidence of intraparenchymal,
subdural, or epidural hemorrhage. A magnetic resonance
venogram demonstrated thrombosis of the left transverse and
sigmoid sinuses involving the left jugular vein. Occlusion of
the left internal carotid artery (ICA) resulting from the skull
base fracture was noted on the MRI. Flow within the left
anterior cerebral artery and MCA was reestablished distally.
The patient was placed on antiplatelet therapy with aspirin.
Approximately 40 h after admission, she became combative
and aphasic with an acute right hemiparesis. At this point, HCT
demonstrated extensive cerebrovascular compromise of the left
MCA territory. Angiogram confirmed dissection of the left ICA
and occlusion of the distal M1 segment of the left MCA. At this
point, an emergent hemicraniectomy was performed. After
extubation on postoperative day 11, the patient was successfully following commands on the left; however, she demonstrated
only minimal movement on the right side accompanied by
right-sided spasticity. At 4 weeks postoperatively, the right
lower extremity had progressed to 3/5 strength while the right
upper extremity continued to show only minimal movement.
By 3 months, both the upper and lower right extremities had
improved to 4/5 strength with a stable right-sided facial weakness. At 5 months, the patient was able to stand unassisted. By

Childs Nerv Syst
Fig. 2 Colored FA map(a, b): the
thickness and FA values of the R
medullary pyramid have
increased, but the left medullary
pyramid remains small due to
Wallerian degeneration on the
2013 exam (b, d) as compared to
the 2010 exam (a, c). The
medullary pyramids are indicated
by arrows, c and d are T2 W (B0)
images

1 year post-stroke, she demonstrated a stable right hemiparesis
with symmetric reflexes and decreased strength on the right
side. With the assistance of a right lower extremity orthosis, the
patient was independently ambulatory and had returned to
preschool. At this time, she began experiencing seizures of
varying frequency and intensity with origins in both the right
and left hemispheres.
Serial DTI imaging was performed 7 months (2010) and
42 months (2013) post-stroke. MRI shows chronic infarction
of the left M1 territory (Fig. 1). The initial DTI imaging of the
medullary pyramids (Fig. 2a) revealed the right pyramidal
tract (PT) with a greater FA value (0.9) than the left (0.6)
and greater cross-sectional area (Table 1). Imaging at the same
level 3 years later (Fig. 2b) showed a more pronounced
difference in the PTs. Both sides increased in cross-sectional
area (Table 1); however, the right side remained substantially
larger than the left. Tractography at the two time points
(Figs. 3 and 4) clearly illustrates the disparity between the
Table 1 Measurements of the medullary pyramids
2010
Right
pyramid

2013
Left
pyramid

Right
pyramid

Left
pyramid

FA values

0.9

0.6

1.0

0.3

Anterior/posterior
diameter (mm)
Right-left diameter (mm)

4.2

2.1

4.4

2.6

6.0

3.1

6.5

4.1

tracts. In the right PT, the FA value increased to maximum
(1.0) and decreased substantially on the left (0.3). The
contralesional to ipsi-lesional FA ratio increases from 1.5 to
3.3 between the two DTI images.

Discussion
Reorganization of the CST is an important substrate for the
recovery of motor function after stroke, especially in the pediatric population. Visualizing white matter tracts like the CST
and the plastic changes that occur within the brain during poststroke recovery is accomplished particularly well with DTI. The
response to injury like stroke in both the central and peripheral
nervous system involves Wallerian degeneration (WD).
Occurring distal to the site of injury, WD is the disintegration
of axonal structures followed by eventual degradation of myelin
and atrophy of affected fiber tracts. In the case presented here,
the reduction in FA values of the left medullary pyramid between the two DTI images is most likely due to WD of the leftsided PT [9, 10]. As myelin and axonal microfilaments disintegrate, water molecules are less constrained to move anisotropically. WD measured by FA and the FA ratio has been shown to
correlate with concurrent motor deficits and to be predictive of
motor deficits 1 month post-injury [11–13]. While the ipsilesional tract demonstrated a decrease in FA values, the
contralesional tracts show a compensatory increase which may
suggest a more anatomically robust tract including greater axonal diameter and increased myelination of fibers [14].

Childs Nerv Syst
Fig. 3 FA colored maps of DTI
(a 2010, b 2013) show a
progressive asymmetry between
the R and L corticospinal tracts
indicated with arrows (L smaller
than R side) which is more
prominent on the 2013 exam
(b, d). c and d are axial
T2-weighted images

This imaging clearly indicates changes in the PTs as a
whole; however, it is not apparent which component(s) of
the PT may be responsible for return of motor function. The
PT contains not only the CST which is itself comprised of the
lateral and anterior CSTs but also the reticulospinal tract.
Thus, there are three distinct possibilities to explain the growth
and increased FA of the contralesional pyramid. Evidence
drawn from studies of stroke recovery as well as motor reorganization in cerebral palsy suggests that all three of these
pathways may demonstrate compensatory reorganization following CST compromise.
First, there is evidence suggesting that the reticulospinal
tract is an important route of ipsilateral motor control after
CST damage [15, 16]. Although in healthy subjects the
crossed lateral CST facilitates the vast majority of limb movement, there are indications that ipsilateral innervation of the
distal limbs (at least the upper extremities) does exist and is
Fig. 4 Comparison of the CST
fiber tractography from 2010 (a)
and 2013 (b) created by using the
same imaging parameters and FA
threshold values: reduced
corticospinal tracts in the left
cerebral peduncle and brainstem
while the thickness on the right
side is increased

potentially mediated through corticoreticulospinal projections
[17, 18]. A second explanation involves persistent ipsilateral
CST projections. Ipsilateral CST projections are more abundant in childhood after which they are pruned in favor of
contralateral ones [19, 20]. Early childhood insult, therefore,
may result in improved recovery due to the presence and
preservation of these fibers which have yet to be eliminated.
A third possibility involves the abnormal branching of
corticospinal axons descending from the contralesional cortex. In this case, homologous left and right motor neuron pools
share a common descending axon with presynaptic branching
at the segmental level not seen in healthy anatomy. This
scenario can be accompanied by mirror movements as a result
of the shared input [18, 21].
Although this imaging study is focused only at the medullary pyramids, any of these three proposed explanations of
reorganization could be accompanied by atypical connections

Childs Nerv Syst

between the hemispheres as a further attempt to compensate
[22]. This potential increase in connectivity could theoretically lead to aberrant propagation of electrical activity and manifest as the focally originating seizures displayed by the patient. Furthermore, these explanations do not take into account
afferent sensory pathways that can also be contributing to
functional deficits if they continue to project to the lesioned
hemisphere.
DTI can be utilized post-stroke as a useful detector of early
Wallerian degeneration, a predictor of future motor deficits
and an indicator of patterns of reorganization. Coupled with
clinical observations, motor mapping, and other forms of
imaging, DTI can help to elucidate neural organization and
contribute to the planning of rehabilitation and, when indicated, surgical intervention.
Acknowledgments Thanks to Barry Kosofsky for his enthusiastic help
in editing this case report.
Conflict of interest The authors declare that they have no conflicts of
interest.

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