Rest tremor suppression may separate essential from parkinsonian rest tremor

Rest tremor at 4–6 Hz is typical for classical rest tremor (PT) of Parkinson's disease (PD). But rest tremor also appears in other tremor syndromes and may therefore cause a misdiagnosis. In this study we evaluated if suppression of tremor during movement onset is a characteristic feature of Parkinsonian Tremor distinguishing PT from Essential tremor (ET) and if this sign can be reliably diagnosed.Clinically diagnosed patients with PT (n = 44) and ET (n = 22) with rest tremor were included. Video sequences were recorded according to a standardized protocol focusing on the change of tremor amplitude during transition from rest to posture (test 1) or to a target-directed movement (test 2). These videos were assessed for rest tremor suppression by 4 reviewers (2 specialists and 2 residents) blinded to the clinical diagnosis and were compared to the personal assessment of an unblinded movement disorder specialist.Rest tremor suppression was found in 39/44 PD patients and in 2/22 patients with ET during the personal assessment. Rest tremor suppression showed a high sensitivity (0.92–1.00) and an acceptable specificity (0.69–0.95) for PD tremor in both tests. The interrater-reliability of the video-sequences was good to very good (κ 0.73–0.91). Less than 3% of the video sequences were misclassified.We conclude that the assessment of the suppression of rest tremor during movement initiation is a simple and reliable tool to separate PT from rest tremor in ET also suggesting that the mechanisms of rest tremor in these two diseases are different.     

Olfaction in essential tremor patients with and without isolated rest tremor.

An olfactory deficit is present in patients with essential tremor (ET), but it is often milder than that in patients with Parkinson's disease (PD). In both, the deficit occurs early in the disease. Isolated rest tremor without other signs of parkinsonism can occur in patients with ET. If the rest tremor in these patients represents a manifestation of ET rather than early PD, we hypothesized that their University of Pennsylvania Smell Identification Test (UPSIT) scores would be similar to those of ET patients without rest tremor. The mean UPSIT score in 13 ET patients with isolated rest tremor did not differ from that of 58 ET patients without rest tremor (29.3 +/- 4.3 vs. 29.4 +/- 6.4; P = 0.69). Several ET patients with rest tremor had UPSIT scores that fell outside of the range that is seen in 95% of patients with PD. These data raise the possibility that some ET patients with isolated rest tremor may not have early PD and that the pathological process that is responsible for their ET is also involving the basal ganglia.     

Jaw tremor: prevalence and clinical correlates in three essential tremor case samples.

The spectrum of involuntary movements seen in essential tremor (ET) is limited. Jaw tremor is one such movement. The prevalence and clinical correlates of jaw tremor have not been studied in detail. The objective of this study was to estimate the prevalence and examine the clinical correlates of jaw tremor in ET using ET cases from three distinct settings (population, tertiary-referral center, brain repository). All ET cases underwent a videotaped tremor examination in which tremors (including limb, head, voice, and jaw) were assessed. The prevalence [95% confidence interval (CI)] of jaw tremor was lowest in the population sample (7.5%; 3.9%-14.2%), intermediate in the tertiary-referral center (10.1%; 6.8%-14.7%), and highest in the brain repository (18.0%; 12.3%-25.5%; P = 0.03). Jaw tremor was associated with older age (P < 0.001), more severe action tremor of the arms (P < 0.001), and presence of head and voice tremor (P < 0.001). Jaw tremor was present in 4/14 (28.6%) ET cases with consistent rest tremor vs. 15/193 (7.8%) cases without rest tremor (odds ratio = 4.8; 95% CI = 1.3-7.0; P = 0.009). The prevalence of jaw tremor was 7.5% to 18.0% and was dependent on the mode of ascertainment, being least prevalent in a population-based sample. ET cases with jaw tremor had a more clinically severe and more topographically widespread disorder. The association in our study between jaw tremor and rest tremor, along with the published observation that jaw tremor can occur in Parkinson's disease (PD), raises the question whether jaw tremor in ET is a marker for subsequent conversion to PD.     

Tremor pattern differentiates drug-induced resting tremor from Parkinson disease

ObjectiveDAT-SPECT, is a well-established procedure for distinguishing drug-induced parkinsonism from Parkinson's disease (PD). We investigated the usefulness of blink reflex recovery cycle (BRrc) and of electromyographic parameters of resting tremor for the differentiation of patients with drug-induced parkinsonism with resting tremor (rDIP) from those with resting tremor due to PD.MethodsThis was a cross-sectional study. In 16 patients with rDIP and 18 patients with PD we analysed electrophysiological parameters (amplitude, duration, burst and pattern) of resting tremor. BRrc at interstimulus intervals (ISI) of 100, 150, 200, 300, 400, 500 and 750 msec was also analysed in patients with rDIP, patients with PD and healthy controls. All patients and controls underwent DAT-SPECT.ResultsRest tremor amplitude was higher in PD patients than in rDIP patients (p < 0.001), while frequency and burst duration were higher in rDIP than in PD (p < 0.001, p < 0.003, respectively). Resting tremor showed a synchronous pattern in all patients with rDIP, whereas it had an alternating pattern in all PD patients (p < 0.001). DAT-SPECT was normal in rDIP patients while it was markedly abnormal in patients with PD.ConclusionsIn the absence of DAT-SPECT, the pattern of resting tremor can be considered a useful investigation for differentiating rDIP from PD.     

Imaging essential tremor

To investigate over time changes in striatal dopamine transporter (DAT), we performed two sequential N‐ω‐fluoropropyl‐2β‐carbomethoxy‐3β‐(4‐iodophenyl) tropane single photon computed tomography (SPECT) scans in 20 subjects with essential tremor (ET), in 13 with Parkinson disease (PD) and in 23 healthy controls (HC, one scan only). We also performed an [^99mTc]ethyl cysteinate dimer bicisate SPECT exam for regional brain network analysis in 9 ET, in a second group of 18 PD (9 with tremor, tPD and 9 akinetic‐rigid dominant, arPD) and in 8 HC. PD subjects had a reduced DAT binding in comparison to ET and HC with an annual decline rate of 7.3% in the contralateral putamen. There were no mean uptake differences between ET and HC at baseline and no uptake loss over time in ET. A discriminant analysis grouped 30% (first scan) and 5% (second scan) of ET as PD and a partition analysis showed overlap between ET and PD for caudate nucleus uptake. Spatial covariance analysis revealed that the expression of the PD‐related regional pattern separated both tPD and arPD from ET and HC. In conclusion, PD and ET do not share a common pattern of dopaminergic loss over time. However, mild impairment of dopamine transporter in the caudate nucleus may contribute to tremor onset in ET. © 2010 Movement Disorder Society     

Assessment of rest tremor in Parkinson's disease

BACKGROUND AND PURPOSE: Rest tremor is the most frequent sign of Parkinson's disease (PD) after bradykinesia, occurring with various severity in about 75% of patients. An objective assessment of rest tremor is difficult. The aim of the study was to analyze rest tremor in PD with the three-dimensional gauging system CMS 10; more specifically, the impact of levodopa treatment on rest tremor, the influence of clinical factors, and the correlation between rest tremor and clinical scales were assessed. MATERIAL AND METHODS: Ninety-five patients with PD (mean age 67.6 years) and 30 healthy people in a control group (mean age 59.3 years) were examined. Clinical scales (UPDRS, Hoehn and Yahr, Schwab and England, as well as Webster scale) were used to assess severity of PD. The assessment of rest tremor was performed within the more and less affected upper limb with the three-dimensional gauging system CMS 10 (Zebris GmbH) before and 1-2 hours after levodopa intake. Frequency (Hz), amplitude (deg), velocity (deg/ms) and acceleration (deg/s2) of the tremor were evaluated. Results were compared with averaged results for left and right upper limb in the control group. RESULTS: The method used in this study objectively showed asymmetry in rest tremor. After levodopa intake, all evaluated parameters of rest tremor were decreased (mainly the amplitude and frequency, and to a lesser degree, velocity and acceleration). The motor part of UPDRS showed the best correlation with rest tremor. CONCLUSIONS: The three-dimensional measuring system CMS 10 is useful in the objective assessment of rest tremor in PD. Rest tremor in PD is under the influence of PD form, the intake of levodopa dose, the amount of levodopa, gender and level of education.     

Distinguishing SWEDDs patients with asymmetric resting tremor from Parkinson's disease: A clinical and electrophysiological study

Approximately 10% of patients diagnosed clinically with early Parkinson's disease (PD) have normal dopaminergic functional imaging (Scans Without Evidence of Dopaminergic Deficit [SWEDDs]). An important subgroup of SWEDDs are those with asymmetric rest tremor resembling parkinsonian tremor. Clinical and pathophysiological features which could help to distinguish SWEDDs from PD have not been explored. We therefore studied clinical details including non‐motor symptoms in 25 tremulous SWEDDs patients in comparison to 25 tremor‐dominant PD patients. Blinded video rating was used to compare examination findings. Electrophysiological tremor parameters and also response to a cortical plasticity protocol using paired associative stimulation (PAS) was studied in 9 patients with SWEDDs, 9 with tremor‐dominant PD (with abnormal dopamine transporter single photon emission computed tomography findings), 8 with segmental dystonia, and 8 with essential tremor (ET). Despite clinical overlap, lack of true bradykinesia, presence of dystonia, and head tremor favored a diagnosis of SWEDDs, whereas re‐emergent tremor, true fatiguing or decrement, good response to dopaminergic drugs, and presence of non‐motor symptoms favored PD. A single tremor parameter could not differentiate between groups, but the combination of re‐emergent tremor and highest tremor amplitude at rest was characteristic of PD tremor. SWEDDs and segmental dystonia patients exhibited an abnormal exaggerated response to the PAS protocol, in contrast to a subnormal response in PD and a normal response in ET. We conclude that despite clinical overlap, there are features that can help to distinguish between PD and SWEDDs which may be useful in clinical practice. The underlying pathophysiology of SWEDDs differs from PD but has similarities with primary dystonia. © 2010 Movement Disorder Society     

震颤分析在帕金森病和原发性震颤鉴别诊断中的应用

目的 探讨震颤分析在帕金森病(parkinson's disease,PD)和原发性震颤(essential trem-or,ET)鉴别诊断中的应用价值.方法 选取2017年9月至2020年11月在福建省立金山医院门诊和住院确诊的PD患者27例(PD组)和ET患者23例(ET组),所有患者均至少有一侧上肢静止性或姿势性...     

Clinical correlates of action tremor in Parkinson disease

BACKGROUND: Action tremor is often noted in patients with Parkinson disease (PD), yet the clinical correlates of this type of tremor have been the focus of few studies. It is not clear whether this action tremor is a manifestation of the underlying basal ganglia disease. OBJECTIVE: To determine whether the severity of action tremor in PD is associated with age, age at disease onset, disease duration, levodopa dose, severity of rest tremor, or other motor (ie, bradykinesia, rigidity) and nonmotor manifestations of PD. METHODS: Patients with PD (N = 197) were ascertained as part of a familial aggregation study. All patients underwent a neurological examination. Rest tremor was rated with the Unified Parkinson Disease Rating Scale; and action tremor, with the Washington Heights-Inwood Genetic Study of Essential Tremor Rating Scale. RESULTS: Action tremor was present in 184 (93.4%) of 197 patients. Four patients (2%) met criteria for definite essential tremor. The action tremor score was not associated with age, age at onset, or disease duration. The action tremor score was associated with the rest tremor score (r = 0.37; P<.001), and more strongly with the ipsilateral than contralateral rest tremor score. The association between the action tremor score and the rest tremor score was diminished but still significant (r = 0.21, P<.02) even when we excluded these 63 patients with re-emergent tremor. Neither the action nor the rest tremor score was associated with the bradykinesia or rigidity scores, Hoehn and Yahr scale score, or modified Mini-Mental State Examination score. CONCLUSIONS: Action tremor was associated with rest tremor in PD, suggesting that, at least in part, action tremor is a manifestation of the underlying basal ganglia disease. Neither tremor was associated with other motor and nonmotor manifestations of PD. This in turn suggests that tremor in PD may represent an underlying pathophysiological process different from these other manifestations.     

Morphological differences in Parkinson’s disease with and without rest tremor

Background   Rest tremor is a hallmark of Parkinson’s disease (PD), but its pathogenesis remains incompletely understood. Nigro-striatal dopamine deficiency correlates best with bradykinesia, but not with tremor. Oscillating neurons in one or multiple localizations within the basal gangliathalamo-cortical loop may cause rest tremor, and an active contribution of the cerebellum and the cerebello-thalamo-cortical projections has been postulated. Objective   To compare the pattern of grey matter volume in PD patients with and without tremor to identify structural correlates of rest tremor. Methods   Voxel-based morphometry (VBM) of a high-resolution 3 Tesla, T1-weighted MR images, pre-processed according to an optimized protocol using SPM2, was performed in 24 patients with mild to moderate PD comparing local grey matter volume in patients with (n = 14) and without rest tremor (n = 10). Results   Grey matter volume is decreased in the right quadrangular lobe and declive of the cerebellum in PD with tremor compared to those without (PFDR < 0.05). Conclusions   These results demonstrate for the first time morphological changes in the cerebellum in PD patients with rest tremor and highlight the involvement of the cerebellum and cerebello- thalamo-cortical circuit in the pathogenesis of parkinsonian rest tremor.     

Essential tremor and tremor in Parkinson's disease are associated with distinct 'tremor clusters' in the ventral thalamus

Different tremor entities such as Essential Tremor (ET) or tremor in Parkinson's disease (PD) can be ameliorated by the implantation of electrodes in the ventral thalamus for Deep Brain Stimulation (DBS). The exact neural mechanisms underlying this treatment, as well as the specific pathophysiology of the tremor in both diseases to date remain elusive. Since tremor-related local field potentials (LFP) have been shown to cluster with a somatotopic representation in the subthalamic nucleus, we here investigated the neurophysiological correlates of tremor in the ventral thalamus in ET and PD using power and coherence analysis. Local field potentials (LFPs) at different recording depths and surface electromyographic signals (EMGs) from the extensor and flexor muscles of the contralateral forearm were recorded simultaneously in twelve ET and five PD patients. Data analysis revealed individual electrophysiological patterns of LFP-EMG coherence at single and double tremor frequency for each patient. Patterns observed varied in their spatial distribution within the Ventral lateral posterior nucleus of the thalamus (VLp), revealing a specific topography of 'tremor clusters' for PD and ET. The data strongly suggest that within VLp individual tremor-related electrophysiological signatures exist in ET and PD tremor.     

Cortical control of saccades in Parkinson disease and essential tremor

A number of studies suggest that some features of essential tremor (ET) and Parkinson disease (PD) overlap. Besides tremor, also some cognitive features have been implicated in ET and PD. There is recent evidence that a common genetic mutation occurs in ET and PD. Saccadic eye movements could provide an easily quantifiable procedure to help in the differential diagnosis in early PD and ET. Being able to distinguish early on the two diseases may help in tailoring therapy. Cortical control of saccades and antisaccades as they pertain to the potential discrimination of PD and ET is reviewed. Imaging and electrophysiological studies are highlighted; however, there are still few studies. Hopefully this review will stimulate further research, in particular in the direction of differences and similarities in the neural circuits involved in PD and ET.     

Cohort study of prevalence and phenomenology of tremor in dementia with Lewy bodies

To study prevalence, specific patterns and response to treatment of tremor in dementia with Lewy bodies (DLB), in comparison with other tremulous disorders prevalence, qualitative and quantitative features of tremor were studied in an incident cohort of 67 dopaminergic treatment naive DLB, 111 Parkinson’s Disease (PD) and 34 Essential Tremor (ET) patients. Tremulous DLB patients (tDLB) were compared with tremulous PD (tPD) and ET patients and followed for 2 years. Double blind placebo-controlled acute drug challenge with l-Dopa and alcohol was performed in all ET, 24 tDLB and 27 tPD. Effects of dopaminergic chronic treatment in all tDLB and tPD patients and primidone in 8 tDLB were also assessed. Tremor occurred in 44.76 % of DLB patients. The tDLB patients presented a complex pattern of mixed tremors, characterized by rest and postural/action tremor, including walking tremor and standing overflow in 50 % tDLB. Standing tremor with overflow was characteristic of tDLB (p < 0.001). Head tremor was more frequent in tDLB than tPD and ET (p = 0.001). The tDLB tremors were reduced by acute and chronic dopaminergic treatments (p < 0.01) but not by alcohol or primidone. Tremor occurs commonly in DLB patients with a complex mixed tremor pattern which shows a significant response to acute and chronic dopaminergic treatments. Recognizing that there is a clinical category of tremulous DLB may help the differential diagnosis of tremors.     

[<Superscript>123</Superscript>I]-FP-CIT SPECT and olfaction test in patients with combined postural and rest tremor

Summary. Mixed-type tremors pose a clinical diagnostic challenge. The aim of the study was to better characterize patients with combined postural and rest tremor. Patients were categorized into four groups: essential tremor (ET) (n = 7), combined rest + postural tremor (n = 17), PD (n = 17), and control subjects (n = 9). All underwent the University of Pennsylvania Smell Identification Test (UPSIT). The mixed-tremor group was also evaluated with SPECT imaging using the dopamine transporter (DaT) ligand ¹²³I-labeled FP-CIT. There was no significant difference in olfaction scores between the mixed tremor and essential tremor groups (23.2 ± 6.6 vs 21.7 ± 4.9) or between these groups and controls (27.2 ± 5.0). The patients with PD had significantly lower scores than all the other groups (13.7 ± 5.4, p < 0.001). Of the 12 patients with mixed tremor evaluated by SPECT, 9 had normal findings. This study suggests that rest tremor is part of the spectrum of ET, even in patients with long-standing disease. However, in a minority of patients, there might be transformation of ET–PD. Correspondence: Ruth Djaldetti, Rabin Medical Center, Department of Neurology, Beilinson Campus, Petah Tiqwa 49100, Israel     

Isolated tremor and disruption of the nigrostriatal dopaminergic system: an 18F-dopa PET study.

We measured striatal 18F-dopa influx constants (Ki) for 20 patients with isolated, predominantly postural, tremor (eight familial, 12 sporadic) and 11 with predominantly rest tremor. Results were compared with 30 controls and 16 Parkinson's disease (PD) patients. The eight familial essential tremor (ET) patients had normal striatal 18F-dopa uptake. Two of the 12 sporadic postural tremor patients had subnormal putamen 18F-dopa Ki, one (who later became akinetic) falling in the PD range. The mean putamen 18F-dopa uptake of the 11 rest tremor patients was reduced to PD levels (51% of normal). Our findings argue against an association between ET and PD, but support the existence of a "benign" tremulous variant of PD. The presence of low-amplitude rest tremor, cogwheel rigidity, reduced arm swing, and short tremor duration was not a useful predictor of nigral dysfunction in patients with postural tremor. In contrast, patients with predominantly rest tremor, particularly with onset in the leg, consistently showed reduced putamen 18F-dopa uptake.     

Long duration asymmetrical postural tremor is likely to predict development of Parkinson's disease and not essential tremor: clinical follow up study of 13 cases

BACKGROUND: Patients presenting with asymmetrical postural tremor with or without mild rest tremor may be diagnosed as having essential tremor (ET), although there is considerable diagnostic uncertainty as to the long term outcome of these patients. OBJECTIVE: In this study, retrospective observations were made on 13 patients presenting originally with asymmetrical postural tremor, initially thought to have ET based on tremor characteristics, alcohol responsiveness, and family history but who subsequently met the criteria of Parkinson's disease (PD). METHODS: The patients were observed and followed up clinically with ancillary imaging using dopamine transporter SPECT scan or levodopa challenge tests in some cases. The diagnosis at original presentation with postural tremor was made with retrospective case note review. RESULTS: After a variable and long latent period all patients developed additional signs suggesting a clinical diagnosis of PD although picking up an initial label of ET. CONCLUSIONS: We suggest exercising caution regarding a diagnosis of ET in patients presenting with late onset asymmetrical postural tremor even if there is no rest tremor. Alcohol sensitivity of tremor, family history of tremor, or responsiveness to beta blockers may not be helpful in diagnosing ET in these cases and some may develop PD in the long term.     

Exploring the relationship between essential tremor and Parkinson's disease

Although essential tremor (ET) and Parkinson's disease (PD) are considered distinct disorders, there is overlap in some clinical features. In some PD patients, a long-standing postural tremor in the hands may precede the onset of parkinsonian features by several years or decades. Furthermore, large families with both ET and PD phenotypes have been described and autopsy studies have demonstrated Lewy body pathology in brains of ET patients. Functional neuroimaging suggests that some ET patients have dopaminergic deficit. We examine here the evidence for and against an association between ET and PD, and critically review data supporting the notion that a subset of ET patients is predisposed to developing PD.     

Nonlinear dynamic analysis of oscillatory repetitive movements in Parkinson's disease and essential tremor

Uncertainty exists on whether Parkinson's disease (PD) and essential tremor (ET) patients have similar degree of impairment during motor tasks. We investigated this problem by analyzing nonlinear dynamics of repetitive movements in 21 control subjects, 33 mild‐moderate PD patients, and 18 ET patients. Accelerometer signals were recorded during finger tapping and unbounded forearm movements between two points, and processed with moving average filtering to generate a new signal consisting of the temporal distance between consecutive cycles. We calculated: mean interpeak interval (slowness), interpeak interval variability (irregularity), and beat decay (BD) of the auto mutual information (AMI) value, which estimates signal predictability by measuring the loss of signal information over a timescale. Both PD and ET had longer interpeak interval (except for finger tapping), higher interpeak interval variability, and higher BD‐AMI values than controls (P ≤ 0.007, all comparisons). ET patients had higher BD‐AMI values than PD (P = 0.003). BD‐AMI was the parameter that discriminated better between subjects (diagnosis accuracies about 80%). No differences existed between PD patients with and without tremor or between PD or ET patients with different disease stages, for any parameter. Evaluation of nonlinear dynamics of oscillatory repetitive movements is a feasible and promising tool for studying movement physiology. Movement performance is more predictable in PD and ET than in controls, even in early disease stages. Slowness and irregularity of movement in PD and ET cannot be fully explained by tremor. Some common pathogenic mechanisms leading to bradykinesia may contribute to this impairment. © 2010 Movement Disorder Society     

伴有快动眼睡眠行为障碍帕金森患者的震颤特征及多巴反应性

目的研究伴与不伴快动眼睡眠行为障碍(rapid eye movement sleep behavior disorder,RBD)的帕金森病(Parkinson disease,PD)患者的震颤特征及多巴反应性。方法根据2014年国际睡眠障碍分类第三版RBD的临床最低诊断标准,本研究采用RBD筛查问卷(RBD screening questionnaire,RBDSQ)量表来诊断临床很可能RBD(clinically probable RBD,cpRBD),将PD患者分为伴有cpRBD的PD(PD+cpRBD)与不伴有cpRBD的PD(PD-cpRBD)两组。对入组患者进行一般资料的收集,采用修订的H-Y分级、统一帕金森评分量表3.0版运动检查部分(UPDRS-Ⅲ)、MDS-UPDRS震颤量表对患者的运动功能进行评估,并且分别对两组患者首发侧肢体姿势性震颤、动作性震颤及静止性震颤的幅度进行评分,比较两组患者一般资料及震颤特征的差异性。对所有患者行急性左旋多巴冲击试验,将两组患者UPDRS-Ⅲ及MDS-UPDRS震颤量表评分最大改善率进行比较。结果共纳入42例伴有震颤的PD患者,PD+cpRBD组19例,PD-cpRBD组共23例,两组患者在性别、年龄、发病年龄、病程、关期UPDRS-Ⅲ评分及H-Y分级方面均无明显差异(P0.05)。与PD-cpRBD组相比,PD+cpRBD组关期震颤评分明显增高(t=2.379,P=0.022),震颤症状由首发侧肢体进展至对侧肢体的时间短(u=-2.133,P=0.033),首发侧肢体静止性震颤幅度大(u=-2.956,P=0.003),动作性震颤幅度大(u=-2.657,P=0.008)。口服左旋多巴/苄丝肼(200/50 mg)后,PD-cpRBD组的UPDRS-Ⅲ及震颤评分最大改善率均明显高于PD+cpRBD组(UPDRS-Ⅲ最大改善率u=-3.134,P=0.002;震颤评分最大改善率t=-3.189,P=0.003)。结论本研究表明,伴有cpRBD的PD患者震颤程度相对较重,以静止性震颤和动作性震颤为主,由首发侧肢体进展至对侧肢体的时间相对较短,对左旋多巴的反应性较差。     

H-reflex recovery curves differentiate essential tremor, Parkinson's disease, and the combination of essential tremor and Parkinson's disease.

The purpose of this study was to examine H-reflex parameters among the pathophysiologic conditions of essential tremor (ET), Parkinson's disease (PD), combined essential tremor with Parkinson's disease (ETPD), and a control group. H-reflex latencies, amplitude of maximum H-reflex to maximum M-response ratio (H:M), vibration H-reflex to control H-reflex (Hv:Hc), and H-reflex recovery curves (HRRCs) were recorded and compared between a control group and patient groups with ET, early-stage PD, and with ETPD. No statistically or clinically significant differences were found between the patient groups and the control group for latency, H:M ratio, or Hv:Hc ratio. Significantly greater ratio values were observed for the PD group over the other groups for the HRRC tests at each interstimulus interval between 200 and 300 msec (p < 0.05), but values were not different between PD and ETPD patients for intervals between 350 and 1,000 msec. Patients with ET, PD, and ETPD apparently have different underlying pathologies. HRRC tests do not distinguish ET patients from normal, but differentiates specifically between PD and ETPD, and normal individuals. HRRC testing may be a useful method for evaluating pathologies between ET, PD, and ETPD patients.