Patients
From June 2002 to February 2003, in Changhua county, Taiwan, 25 patients with essential tremor (ET) (mean age, 43 years;
range, from 17 to 62 years) were enrolled in this study. All these patients met the published diagnostic criteria for ET (22). None had a history of stroke or signs of dystonia, parkinsonism, or spinocerebellar ataxia on neurological examination. Required criteria for enrolment were (1) postural tremor of moderate
amplitude in one or both arms; (2) tremor of moderate amplitude in one or both arms during 4 or more tasks such as: pouring water, using a spoon to eat soup, drinking water, the
finger-to-nose maneuver, and drawing a spiral; (3) tremor interferes with one more activities of daily living; (4)
medications (lithium, prednisone, levothyroxine, beta-adrenergic bronchodilators, valproate, selective serotonin reuptake
inhibitors, and others), caffeine, nicotine, and alcohol are not the cause of the tremor; (5) no other neurologic conditions are the cause of the tremor (23).
The tremor was rated from 0 to 3 based on the tremor scale (Table 6). For the current analysis, we used the tremor score in
the dominant arm (Table 7). All of the 25 patients with ET had normal findings on the brain MRI, and normal thyroid function.
Patients with evidence of cerebrovascular disease, other
structural brain disease, dementia, head injury, or encephalitis, were excluded. To prevent any potential interaction, the intake of central nervous system stimulants such as amphetamines, anti-anorexia and obesity treatments, sympathomimetics including some nasal decongestants, and antidepressants
(amoxapine, buspirone, and benztropine ) were disallowed for 4 weeks prior to entry. Those with standard co-morbid renal,
hepatic, and cardiac conditions, and patients with drug or alcohol abuse were excluded.
Table 6 . Tremor Scale Score Symptom
0 Absent (no tremor or writing impairment
1
Slight and infrequently present (mild tramor, writing and drawing of spiral is minimally impaired)
2 Moderate; bothersome to most patients (writing and drawing of spiral moderately
impaired)
3 Severe tremor(writing and drawing severely impaired)
4 Marked tremor (interferes with most activities)
99m Tc ECD SPECT of the brain
The 99m Tc ECD was prepared following the instructions on a commercial vial (Neurolite, Dupont Company, USA). The radiochemical purity of the final 99m Tc ECD complex was measured by thin-layer chromatography on Whatman MKC 18 plates developed with acetone and 0.5 M ammonium acetate (60:40). The radiochemical purity was calculated by comparing the peak for the 99m Tc ECD complex with the sum of all other peaks on the plate (24). The radiochemical purities of batches of the 99m Tc ECD were higher than 97%. Brain SPECT with 99m Tc ECD was performed with the patients in a dark and quiet room ( Figure 3). The position of the patient's head was fixed and maintained during SPECT imaging using a hemicylindrical plastic headholder with a radiolucent plastic neck contoured head rest. Fifteen to 45 minutes after intravenous 99m Tc ECD
injection (740 MBq), SPECT data were obtained using a dual headed gamma camera (ADAC, Vertex plus) equipped with
fanbeam collimators. Data were collected from 64 projections in the 140 keV photopeak over 360 deg (1800 for each head) in 128x128 matrices, with an acquisition time of 30 s/view. A zoom factor of 1.46 was used. After data acquisition, the data were normalized for the correction of the rotating camera head speed in different directions (upward and downward) and decay of 99m Tc from the first to last frame, so that the number of counts within each frame of the SPECT was the same.
Transaxial, coronal, and sagittal slices were reconstructed.
Reconstruction of images was performed with attenuation
correction using a Butterworth filter at the optimum cut off, and order levels were determined by acquisition counts. For SPECT images, the transaxial sections were reoriented parallel to the base of the brain. This enabled us to obtain coronal and sagittal reconstructions for the determination of the correct anatomical regions of the brain. After image reconstruction, all slices of the SPECT images were normalized to produce the final SPECT
images, the contrast of which was set within the same range of 0-255 gray scales based on the computer screen.
Data Analysis Visual Assessment
To identify areas of abnormal perfusion, visual interpretation of the SPECT images from each patient was carried out twice in random order with agreement of at least two of three
independent experienced observers blind to the clinical
information. Normal 99"Tc ECD SPECT findings consisted of homogeneous rCBF in the gray matter of the cortex and basal
ganglion without regions of hypoperfusion or visible asymmetry.
Abnormal findings included heterogeneous rCBF with regions of hypoperfusion or visible asymmetry on at least two
consecutive slices noted twice by at least two of three observers.
This meant that at least four of a total of six individual interpretations had to be the same for an agreement.
Semiquantitative Assessments
For semiquantitative analysis, the cortical and subcortical
regions of interest (ROIs) in the acquired SPECT data were defined. The relative blood flow ratio in each region was
calculated as the average tissue activity in the region divided by the activity in the ipsilateral occipital cortex. Using elliptical templates, ROIs were placed in the frontal, parietal, temporal, occipital, cerebellum, striatum, thalamus, and subcortical white matter regions in each hemisphere (Fig 4). The placement of ROI depended on the visual identification of anatomical regions aided by a stereotaxic atlas. The occipital lobe was used as a reference region (Fig 5, 6).
In our preliminary study, data was collected and analyzed in one center, on a standardized form. A correction was made from the occipital cortex readings, because this area receives an equal supply of tracer from the vascular compartment.
To determine the activity ratio (brain region/occipital), the counts per ROI of each cortical area were divided by the average counts per ROI found in each occipital hemisphere.
This ratio was used as a measure of regional cerebral blood flow (rCBF). All SPECT measurements were performed by a nuclear
medicine doctor who was blind to the clinical data.
Statistical Analysis
The ratios of the rCBF in the ROI in each region of the cortex to that in the occipital cortex are presented as the mean (SD). The Mann-Whitney test was used to compare mean values in each group. The Wilcoxon signed rank sum test was used for
comparisons between the right and left rCBF. The Spearman rank correlation coefficient was employed to evaluate
correlations between tremor score and age, sex and duration of illness. The significance level was set at P = 0.05. Data were analyzed using SPSS 9.0 J for Windows (SPSS, Chicago, IL, USA).
n Results
The duration of the tremor history of the 25 patients with essential tremor was from 3 to 20 years (mean: 8.4 years).
Fifteen patients had a positive family history of essential tremor.
The tremor score of all the patients ranged from grade 2 to 3.
Tables 8 shows the detailed data for the patients. (Table 8) The 99m Tc ECD brain SPECT of these 25 patients showed regions of hypoperfusion in many regions of the brain, including the basal ganglia, thalamus, frontal lobe, and temporal lobe (Figure 4). The calculated rCBF values of the thalamus, basal
ganglia, and cerebellum relative to the occipital lobe are shown in Table 9. (Table 9) The thalamus region was the area with the most hypoperfusion in 20 patients, and had a positive statistical significance relative to the contralateral tremor side (P =
0.002797 for the left thalamus; P = 0.0297 for the right thalamus) (Table 10), but no such correlation was found in the basal
ganglia and cerebellum(Table 11,12). Significant hypoperfusion of the rCBF was revealed in the thalamus region, corresponding to the contralateral side of the tremor limb. There was no
significant relationship between the tremor score and the rCBF of other regions. We found no positive correlation between the rCBF ratios in the thalamus and the tremor score (Table 13).
There was no difference in rCBF ratios between patients with or without a family history, patients’ age and duration of tremor
(Table 14, 15, 16).
n Discussion
Although the pathogenesis of ET remains unclear, several
studies have suggested that the olivo-cerebello-thalamo-cortical circuitry plays a pivotal role in its generation (25). This
assumption has been strengthened by PET and functional
magnetic resonance imaging (fMRI) studies, in which increased blood flow and neural activities have been detected in the
cerebellum, as well as basal ganglia (9, 26). In this current 99mTc ECD SPECT study, it was intriguing to find that the rCBF was significantly decreased in the thalamic region
contralateral to the more vigorous tremor limb. No significant alternation of the blood perfusion was detected in the hot spots as suggested by the PET or fMRI studies. Before jumping to conclusions, it must be said that, in the current study, the
patients were examined in a resting state instead of holding their hand in the air for tremor triggering, as in previous studies. Thus the current study may reflect the static (tremor-free) condition
rather than the dynamic (tremorous) condition of the brain.
Since there was no normal control in this study, one can not help but wonder whether this unique SPECT phenomenon could also appear in normal humans. However, this is unlikely, based on our limited experience with 99mTc ECD SPECT findings (unpublished observation) in subjects with diseases other than tremor, in whom no such asymmetric pattern has ever been observed.
In addition, the possibility that the hypoperfusion seen with the SPECT was caused by an organic lesion of the brain was also excluded by the normal MRI findings of these ET patients, If the current SPECT result actually reflects the stationary functional alternation of the thalamic region of the ET patients, is there a physiological assumption to explain the finding? Recently, deep brain stimulation (DBS) has been adopted to treat intractable ET by targeting the electrodes in the ventral intermediate (Vim) nucleus of the thalamus. This technique may benefit patients via such mechanisms as “ conduction blocking” (27), preferential activation of inhibitory axon terminals (28) and overriding
rhythmic firing frequencies over irregular firing patterns (29).
Also, Perlmutter has demonstrated the increased blood flow at the terminal fields of the thalamocortical projections after DBS of the Vim of thalamus by PET. It has been suggested that the net effect of DBS is to drive the efferent neurons of the thalamus (30). In keeping with the DBS-PET findings, the current finding
suggests that the thalamus plays a pivotal role in ET generation or modulation. In addition to the thalamus, the inferior olivary nucleus has also been speculated to play a crucial role in the generation of ET. This working hypothesis was deduced from the observation that in some of the ET patients, the tremor could be suppressed by alcohol consumption. The beneficial effect of alcohol was probably mediated through a reduction in cerebellar overactivity, which culminated in an increased afferent input to the inferior olivary nuclei. This functional alternation has been illustrated in a rCBF study with PET, in which the rCBF was found to be increased in the inferior olivary nuclei after oral alcohol administration in ET patients, but not in the controls (31). Because of the limitation of the SPECT, we were unable to
examine the olivary region in the current study. However, based on the finding gathered in the current study and in the previous reports of PET gathered, the olivo-cerebello-thalamic circuitry might be crucial for the generation of ET.
Technetium-99m-ethyl cysteinate dimer (ECD) is a relatively new 99m Tc-labeled lipophilic tracer that became available clinically for SPECT imaging studies of the cerebral area in the most recent decade (32). It has been adopted for the evaluation of several neurological conditions including cerebrovascular disease (33), dementia (34) and epilepsy (35). In all brain regions, 99mTc ECD SPECT showed a higher lesion contrast than that of 99mTc HMPAO SPECT. Thus it is generally agreed that 99mTc ECD is superior to 99mTc HMPAO in detecting brain lesions or functional alternations (36, 37). In addition, 99m Tc ECD does not have the problems of radiochemical instability and delayed imaging (38). With these benefits and the consistent findings gathered in the current study, we recommend ECD SPECT as an appropriate tool for the investigation of ET, apart
from being applied in the assessment of the aforementioned neurological diseases.
There is some variability of regional ECD brain perfusion patterns by 99mECD SPECT scan. It depends on patient’s age, gender, and hemisphere. On average, there was 1.4% higher in the right hemisphere for the whole study group. Global cerebral perfusion decreases with age. The age-related decline in
hemispheric blood flow in agreement with the finding of a 3.7%
decline per decade in healthy subjects. It was mostly found in the anterior cingulate gyrus, bilateral basal ganglia, left
prefrontal, left lateral frontal and left superior temporal and insular cortex (all P=0.001-0.02). There was an overall increase in right/left asymmetry with age, which was most pronounced in the frontal and temporal neocortex. Men had a significantly higher tracer uptake in the cerebellar hemispheres, left anterior temporal cortex and orbitofrontal cortex; otherwise women showed a higher tracer uptake bilaterally in the inferior parietal cortex in women. Average intersubject variability was 4.8%
while intrasubject reproducibility was 3.0%. However,
hemispheric asymmetry and intersubject variability of ECD perfusion pattern are relatively small in adults.
In addition to the examination of the correlation between the diminution of thalamic rCBF and the tremor severity, the
presence of tremor family history and the disease duration were also assessed to determine their impact on the alternation of the rCBF and the results were unremarkable.
In conclusion, the current study illustrated the alternation of rCBF in the thalamus region of ET patients and suggests that 99mTc ECD SPECT is a promising tool for the investigation of tremor.