The Most Cited Works in Essential Tremor and Dystonia.

BACKGROUND: The study of the most cited works in a particular field gives an indication of the important advances, developments, and discoveries that have had the highest impact in that discipline. Our aim was to identify the most cited works in essential tremor (ET) and dystonia.
METHODS: A bibliometric search was performed using the ISI Web of Science database using selected search terms for ET and dystonia for articles published from 1900 to 2015. The resulting citation counts were analyzed to identify the most cited works, and the studies were categorized.
RESULTS: Using the criterion of more than 400 citations, there were four citation classics for ET and six for dystonia. The most cited studies were those on pathophysiology followed by medical treatments, clinical classification, genetic studies, surgical treatments, review articles, and epidemiology studies. A comparison of the most cited articles for ET and dystonia showed that there was a divergence, with ET and dystonia having a higher number of epidemiologic and genetic studies, respectively. Whereas the peak period for the number of publications was 2000-2004 for ET, it was 1995-1999 for dystonia. DISCUSSION: Given the large number of patients with these disorders, there appears to be an unmet need for further research advances in both areas, but particularly for ET as the most common movement disorder.

Introduction

The study of the most cited works in a particular field gives an indication of the most impactful advances, developments, and discoveries that have been instrumental in driving research in that discipline.1,2 This analysis also offers insight into where the field has been, where the unmet needs are, and where the field may be heading.1 Such snapshots of the most important works have previously been generated for several areas of neurology and neurosurgery.1,3–8 A recent study examined citations related to essential tremor (ET),8 but a similar analysis has however not yet been carried out for dystonia. Importantly, no study has compared the literature in the two different but linked fields of ET and dystonia. Here we compared and contrasted these two research areas to produce a more comprehensive review and identify the most highly cited ET and dystonia articles.

Methods

We used a search strategy similar to one we have previously used for bibliometric analysis in other fields.1,9 A search was performed in January 2016 using the bibliometric database ISI Web of Science for articles published between 1900 and 2015 using the topic search terms “(essential OR benign OR familial) and tremor*” for ET. For dystonia, the search terms were “dystoni* OR meige syndrome OR writer cramp OR torticollis.” Alternative terms were also searched to identify additional citations: “professional cramp OR musician cramp OR eyelid opening apraxia OR Brueghel syndrome OR Oppenheim disease.” The “OR” is a Boolean operator to find records containing any of the terms, and the parentheses were used to define operator precedence such that the expression inside the parentheses is executed first. The asterisk was included as a wild card character. The search terms for ET returned 4,578 articles with the earliest published in 1903. The search terms for dystonia returned 18,839 articles with the earliest published in 1901. The two results were separately sorted by number of citations from highest to lowest and were manually examined to identify the top 100 cited articles related to the field. Articles were considered “citation classics” if they received 400 or more citations as previously defined.2

Results

We first conducted a broad search to identify the relative publication activity across each of the main movement disorders as identified by PubMed (Table 1). Relative to Parkinson’s disease (PD), the number of papers identified in PubMed on dystonia was approximately one fifth and on ET was approximately one twentieth.

Table 1

Number of Papers Available in PubMed for the Main Movement Disorders

Disorder(Search Term)1	Number of Citations in PubMed (up to December 31, 2015)
“Parkinson’s disease”	59,140
“Essential tremor”	2,930
“Dystonia”	12,978
“Myoclonus”	8,969
“Chorea”	6,742
“Tic”	5,952
“Tics”	3,502

Each search term was entered as indicated in PubMed for the period of January 1, 1900 to December 31, 2015.

The 100 most cited articles showed that the number of citations per article for ET ranged from 79 to 846 (median 100) compared to a range from of 137 to 560 (median 179) for dystonia. The articles were ranked by citations, and those articles that were ranked in the top 100 are shown in Table 2 for ET and Table 3 for dystonia. Out of these articles, there were four citation classics receiving 400 or more citations for ET10–13 and six citation classics for dystonia,14–19 representing 0.09% and 0.03% of all search results, respectively.

Table 2

Summary of Top 100 articles for Essential Tremor (Ranked in Order of Citations)

Absolute Number	Rank	Citations	Paper	Category
1	1	846	Benabid AL, Pollak P, Gervason C, et al. Long-term suppression of tremor by chronic stimulation of the ventral intermediate thalamic nucleus. Lancet 1991;337:403–406	Clinical: Surgery
2	2	809	Deuschl G, Bain P, Brin M. Consensus statement of the Movement Disorder Society on tremor. Mov Disord 1998;13 Supplement:2–23	Clinical: Classification
3	3	652	Benabid AL, Pollak P, Gao DM, et al. Chronic electrical stimulation of the ventralis intermedius nucleus of the thalamus as a treatment of movement disorders. J Neurosurg 1996;84:203–214	Clinical: Surgery
4	4	484	Schuurman PR, Bosch DA, Bossuyt PMM, et al. A comparison of continuous thalamic stimulation and thalamotomy for suppression of severe tremor. N Engl J Med 2000;342:461–468	Clinical: Surgery
5	5	294	Koller W, Pahwa R, Busenbark K, et al. High-frequency unilateral thalamic stimulation in the treatment of essential and Parkinsonian tremor. Ann Neurol 1997;42:292–299	Clinical: Surgery
6	6	291	Limousin P, Speelman JD, Gielen F, et al. Multicentre European study of thalamic stimulation in parkinsonian and essential tremor. J Neurol Neurosurg Psychiatry 1999;66:289–296	Clinical: Surgery
7	7a	220	Louis ED, Ottman R, Hauser WA. How common is the most common adult movement disorder? Estimates of the prevalence of essential tremor throughout the world. Mov Disord 1998;13:5–10	Review
8	7b	220	Critchley M. Observations on essential (heredo-familial) tremor. Brain 1949;72:113–139	Review
9	8	215	Benabid AL, Benazzouz A, Hoffmann D, et al. Long-term electrical inhibition of deep brain targets in movement disorders. Mov Disord 1998;13 Supplement:119–125	Clinical: Surgery
10	9	214	Bain PG, Findley LJ, Thompson PD, et al. A study of hereditary essential tremor. Brain 1994;117:805–824	Review
11	10	212	Blond S, Caparros-Lefebvre D, Parker F, et al. Control of tremor and involuntary movement-disorders by chronic stereotaxic stimulation of the ventral intermediate thalamic nucleus. J Neurosurg 1992;77: 62–68	Clinical: Surgery
12	11	212	Lou JS, Jankovic J. Essential tremor: clinical correlates in 350 patients. Neurology 1991;41:234–238	Clinical: Classification
13	12	210	Louis ED, Faust PL, Vonsattel JP, et al. Neuropathological changes in essential tremor: 33 cases compared with 21 controls. Brain 2007;130:3297–3307	Lab: Patho-physiology
14	13	198	Deuschl G, Raethjen J, Lindemann M, et al. The pathophysiology of tremor. Muscle Nerve 2001;24:716–735	Review
15	15	177	Jenkins IH, Bain PG, Colebatch JG, et al. A positron emission tomography study of essential tremor: evidence for overactivity of cerebellar connections. Ann Neurol 1993;34:82–90	Lab: Patho-physiology
16	15	174	Bain PG, Findley LJ, Atchison P, et al. Assessing tremor severity. J Neurol Neurosurg Psychiatry 1993;56:868–873	Clinical: Classification
17	16	172	Brooks DJ, Playford ED, Ibanez V, et al. Isolated tremor and disruption of the nigrostriatal dopaminergic system: an 18F-dopa PET study. Neurology 1992;42:1554–1560	Lab: Patho-physiology
18	17	169	Jankovic J, Cardoso F, Grossman RG, et al. Outcome after stereotaxic thalamotomy for parkinsonian, essential, and other types of tremor. Neurosurgery 1995;37:680–686	Clinical: Surgery
19	18	167	Rajput AH, Offord KP, Beard CM, et al. Essential tremor in Rochester, Minnesota: a 45-year study. J Neurol Neurosurg Psychiatry 1984;47:466–470	Epidemiology
20	19	166	Tasker RR. Deep brain stimulation is preferable to thalamotomy for tremor suppression. Surg Neurol 1998;49:145–153	Clinical: Surgery
21	20	164	Benito-León J, Bermejo-Pareja F, Morales JM, et al. Prevalence of essential tremor in three elderly populations of central Spain. Mov Disord 2003;18:389–394	Epidemiology
22	21a	158	Deuschl G, Wenzelburger R, Loffler K, et al. Essential tremor and cerebellar dysfunction: clinical and kinematic analysis of intention tremor. Brain 2000;123:1568–1580	Review
23	21b	158	Louis ED. Essential tremor. Lancet Neurol 2005;4:100–110	Review
24	22	154	Gulcher JR, Jonsson P, Kong A, et al. Mapping of a familial essential tremor gene, FET1, to chromosome 3q13. Nat Genet 1997;17:84–87	Lab: Genetic studies
25	23	149	Koller WC, Busenbark K, Miner K, et al. The relationship of essential tremor to other movement-disorders: report on 678 patients. Ann Neurol 1994;35:717–723	Epidemiology
26	24	142	Louis ED, Ferreira JJ. How common is the most common adult movement disorder? Update on the worldwide prevalence of essential tremor. Mov Disord 2010;25:534–541	Review
27	25	138	Elble RJ. Physiological and essential tremor. Neurology 1986;36:225–231	Clinical: Classification
28	26	131	Ondo W, Jankovic J, Schwartz K, et al. Unilateral thalamic deep brain stimulation for refractory essential tremor and Parkinson’s disease tremor. Neurology 1998;51:1063–1069	Clinical: Surgery
29	27a	128	Geraghty JJ, Jankovic J, Zetusky WJ. Association between essential tremor and Parkinson’s disease. Ann Neurol 1985;17:329–333	Epidemiology
30	27b	128	Winkler GF, Young RR. Efficacy of chronic propranolol therapy in action tremors of familial, senile or essential varieties. New Engl J Med 1974;290:984–988	Clinical: Medicine
31	28a	127	Higgins JJ, Pho LT, Nee LE. A gene (ETM) for essential tremor maps to chromosome 2p22–p25. Mov Disord 1997;12:859–864	Review
32	28b	127	Koller WC, Lyons KE, Wilkinson SB, et al. Long-term safety and efficacy of unilateral deep brain stimulation of the thalamus in essential tremor. Mov Disord 2001;16:464–468	Clinical: Surgery
33	29	126	Colebatch JG, Findley LJ, Frackowiak RSJ, et al. Preliminary-report: activation of the cerebellum in essential tremor. Lancet 1990;336:1028–1030	Lab: Patho-physiology
34	30a	125	Rehncrona S, Johnels B, Widner H, et al. Long-term efficacy of thalamic deep brain stimulation for tremor: Double-blind assessments. Mov Disord 2003;18:163–170	Clinical: Surgery
35	30b	125	Wills AJ, Jenkins IH, Thompson PD, et al. Red nuclear and cerebellar but no olivary activation-associated with essential tremor: a positron emission tomographic study. Ann Neurol 1994;36:636–642	Lab: Patho-physiology
36	31	123	Zesiewicz TA, Elble R, Louis ED, et al. Practice parameter: Therapies for essential tremor – Report of the quality standards subcommittee of the American Academy of Neurology. Neurology 2005;64:2008–2020	Clinical: Classification
37	32a	121	Bucher SF, Seelos KC, Dodel RC, et al. Activation mapping in essential tremor with functional magnetic resonance imaging. Ann Neurol 1997;41:32–40	Lab: Patho-physiology
38	32b	121	Stolze H, Petersen G, Raethjen J, et al. The gait disorder of advanced essential tremor. Brain 2001;124:2278–2286	Clinical: Classification
39	33a	118	Benito-León J, Louis ED. Essential tremor: emerging views of a common disorder. Nat Clin Pract Neurol 2006;2:666–678	Review
40	33b	118	Dogu O, Sevim S, Camdeviren H, et al. Prevalence of essential tremor – Door-to-door neurologic exams in Mersin Province, Turkey. Neurology 2003;61:1804–1806	Epidemiology
41	34a	116	Benamer HTS, Patterson J, Grosset DG, et al. Accurate differentiation of Parkinsonism and essential tremor using visual assessment of [I-123]-FP-CIT SPECT imaging: The [I-123]-FP-CIT study group. Mov Disord 2000;15:503–510	Clinical: Classification
42	34b	116	Rautakorpi I, Takala J, Marttila RJ, et al. Essential tremor in a Finnish population. Acta Neurol Scand 1982;66:58–67	Epidemiology
43	35	115	Louis ED, Ottman R, Ford B, et al. The Washington Heights-Inwood Genetic Study of Essential Tremor: Methodologic issues in essential-tremor research. Neuroepidemiology 1997;16:124–133	Lab: Genetic studies
44	36	113	Critchley E. Clinical manifestations of essential tremor. J Neurol Neurosurg Psychiatry 1972;35:365–372	Clinical: Classification
45	37a	112	Lombardi WJ, Woolston DJ, Roberts JW, et al. Cognitive deficits in patients with essential tremor. Neurology 2001;57:785–790	Clinical: Classification
46	37b	112	Benito-León J, Louis ED, Bermejo-Pareja F. Population-based case-control study of cognitive function in essential tremor. Neurology 2006;66:69–74	Epidemiology
47	38	102	Jankovic J, Schwartz K, Clemence W, et al. A randomized, double-blind, placebo-controlled study to evaluate botulinum toxin type A in essential hand tremor. Mov Disord 1996;11:250–256	Clinical: Medicine
48	39a	101	Wilms H, Sievers J, Deuschl G. Animal models of tremor. Mov Disord 1999;14:557–571	Review
49	39b	101	Bain P, Brin M, Deuschl G, et al. Criteria for the diagnosis of essential tremor. Neurology 2000;54:S7–S7	Clinical: Classification
50	39c	101	Sydow O, Thobois S, Alesch F, et al. Multicentre European study of thalamic stimulation in essential tremor: a six year follow up. J Neurol Neurosurg Psychiatry 2003;74:1387–1391	Epidemiology
51	40a	99	Asenbaum S, Pirker W, Angelberger P, et al. [I-123]beta-CIT and SPECT in essential tremor and Parkinson’s disease. J Neural Transm (Vienna) 1998;105:1213–1228	Clinical: Classification
52	40b	99	Growdon JH, Shahani BT, Young RR. Effect of alcohol on essential tremor. Neurology 1975;25:259–262	Lab: Patho-physiology
53	40c	99	Alesch F, Pinter MM, Helscher RJ, et al. Stimulation of the ventral intermediate thalamic nucleus in tremor dominated Parkinson’s disease and essential tremor. Acta Neurochir (Wien) 1995;136:75–81	Clinical: Surgery
54	41	98	Hornabrook RW, Nagurney JT. Essential tremor in Papua, New Guinea. Brain 1976;99:659–672	Epidemiology
55	42a	97	Rajput AH, Rozdilsky B, Ang L, et al. Clinicopathological observations in essential tremor: report of 6 cases. Neurology 1991;41:1422–1424	Lab: Patho-physiology
56	42b	97	Lee RG, Stein RB. Resetting of tremor by mechanical perturbations – a comparison of essential tremor and Parkinsonian tremor. Ann Neurol 1981;10:523–531	Clinical: Medicine
57	43	96	Marshall J. Observations on essential tremor. J Neurol Neurosurg Psychiatry 1972;25:122–125	Clinical: Classification
58	44a	95	Gironell A, Kulisevsky J, Barbanoj M, et al. A randomized placebo-controlled comparative trial of gabapentin and propranolol in essential tremor. Arch Neurol 1999;56:475–480	Clinical: Medicine
59	44b	95	Louis ED, Zheng W, Jurewicz EC, et al. Elevation of blood beta-carboline alkaloids in essential tremor. Neurology 2002;59:1940–1944	Lab: Patho-physiology
60	44c	95	Singer C, Sanchezramos J, Weiner WJ. Gait abnormality in essential tremor. Mov Disord 1994;9:193–196	Clinical: Classification
61	44d	95	Benito-León J, Bermejo-Pareja F, Louis ED. Incidence of essential tremor in three elderly populations of central Spain. Neurology 2005;64:1721–1725	Epidemiology
62	44e	95	Raethjen J, Lindemann M, Schmaljohann H, et al. Multiple oscillators are causing Parkinsonian and essential tremor. Mov Disord 2000;15:84–94	Lab: Patho-physiology
63	44f	95	Stefansson H, Steinberg S, Petursson H, et al. Variant in the sequence of the LINGO1 gene confers risk of essential tremor. Nat Genet 2009;41:277–279	Lab: Genetic studies
64	45a	94	Busenbark KL, Nash J, Nash S, et al. Is essential tremor benign? Neurology 1991;41:1982–1983	Epidemiology
65	45b	94	Louis ED, Shungu DC, Chan S, et al. Metabolic abnormality in the cerebellum in patients with essential tremor: a proton magnetic resonance spectroscopic imaging study. Neurosci Lett 2002;333:17–20	Lab: Patho-physiology
66	45c	94	Hariz MI, Shamsgovara P, Johansson F, et al. Tolerance and tremor rebound following long-term chronic thalamic stimulation for parkinsonian and essential tremor. Stereotact Funct Neurosurg 1999;72:208–218	Clinical: Surgery
67	46a	93	Kumar R, Lozano AM, Sime E, et al. Long-term follow-up of thalamic deep brain stimulation for essential and Parkinsonian tremor. Neurology 2003;61:1601–1604	Clinical: Surgery
68	46b	93	Boecker H, Wills AJ, Ceballos-Baumann A, et al. The effect of ethanol on alcohol responsive essential tremor: a positron emission tomography study. Ann Neurol 1996;39:650–658	Lab: Patho-physiology
69	47a	92	Louis ED, Barnes L, Albert SM, et al. Correlates of functional disability in essential tremor. Mov Disord 2001;16:914–920	Clinical: Classification
70	47b	92	Hubble JP, Busenbark KL, Wilkinson S, et al. Deep brain stimulation for essential tremor. Neurology 1996;46:1150–1153	Clinical: Surgery
71	47c	92	Rajput A, Robinson CA, Rajput AH. Essential tremor course and disability – A clinicopathologic study of 20 cases. Neurology 2004;62:932–936	Lab: Patho-physiology
72	47d	92	Bermejo-Pareja F, Louis ED, Benito-León J. Risk of incident dementia in essential tremor: A population-based study. Mov Disord 2007;22:1573–1580	Epidemiology
73	48	91	Louis ED, Ottman R. How familial is familial tremor? The genetic epidemiology of essential tremor. Neurology 1996;46:1200–1205	Review
74	49a	90	Louis ED, Marder K, Cote L, et al. Differences in the prevalence of essential tremor among elderly African-Americans, whites, and Hispanics in northern Manhattan, NY. Arch Neurol 1995;52:1201–1205	Epidemiology
75	49b	90	Findley LJ, Koller WC. Essential tremor: a review. Neurology 1987;37:1194–1197	Review
76	49c	90	Hellwig B, Haussler S, Schelter B, et al. Tremor-correlated cortical activity in essential tremor. Lancet 2001;357:519–523	Lab: Patho-physiology
77	50a	88	Helmchen C, Hagenow A, Miesner J, et al. Eye movement abnormalities in essential tremor may indicate cerebellar dysfunction. Brain 2003;126:1319–1332	Lab: Patho-physiology
78	50b	88	Hallett M, Dubinsky RM. Glucose-metabolism in the brain of patients with essential tremor. J Neurol Sci 1993;114:45–48	Lab: Patho-physiology
79	50c	88	Louis ED, Ford B, Frucht S, et al. Risk of tremor and impairment from tremor in relatives of patients with essential tremor: a community-based family study. Ann Neurol 2001;49:761–769	Epidemiology
80	51a	87	Jeanneteau F, Funalot B, Jankovic J, et al. A functional variant of the dopamine D-3 receptor is associated with risk and age-at-onset of essential tremor. Proc Natl Acad Sci U S A 2006;103:10753–10758	Lab: Genetic studies
81	51b	87	Jefferson D, Jenner P, Marsden CD. Beta-adrenoreceptor antagonists in essential tremor. J Neurol Neurosurg Psychiatry 1979;42:904–909	Clinical: Medicine
82	52a	86	Benito-León J, Louis ED, Bermejo-Pareja F. Elderly-onset essential tremor is associated with dementia. Neurology 2006;66:1500–1505	Clinical: Classification
83	52b	86	Deuschl G, Elble R. Essential tremor–neurodegenerative or nondegenerative disease towards a working definition of ET. Mov Disord 2009;24:2033–2041	Review
84	53a	86	Shahed J, Jankovic J. Exploring the relationship between essential tremor and Parkinson’s disease. Parkinsonism Relat Disord 2007;13:67–76	Review
85	53b	86	Leehey MA, Munhoz RP, Lang AE, et al. The fragile X premutation presenting as essential tremor. Arch Neurol 2003;60:117–121	Lab: Genetic studies
86	54a	85	Koller W, Biary N, Cone S. Disability in essential tremor: effect of treatment. Neurology 1986;36:1001–1004	Clinical: Medicine
87	54b	85	Deng H, Le W, Jankovic J. Genetics of essential tremor. Brain 2007;130:1456–1464	Review
88	55a	84	Gasparini M, Bonifati V, Fabrizio E, et al. Frontal lobe dysfunction in essential tremor: a preliminary study. J Neurol 2001;248:399–402	Lab: Patho-physiology
89	55b	84	Shill HA, Adler CH, Sabbagh MN, et al. Pathologic findings in prospectively ascertained essential tremor subjects. Neurology 2008;70:1452–1455	Lab: Patho-physiology
90	56	83	Koller WC, Vetere-Overfield B. Acute and chronic effects of propranolol and primidone in essential tremor. Neurology 1989;39:1587–1588	Clinical: Medicine
91	57a	82	Jain S, Lo SE, Louis ED. Common misdiagnosis of a common neurological disorder : how are we misdiagnosing essential tremor? Arch Neurol 2006;63:1100–1104	Clinical: Classification
92	57b	82	Jankovic J. Essential tremor: a heterogeneous disorder. Mov Disord 2002;17:638–644	Clinical: Classification
93	57c	82	Troster AI, Woods SP, Fields JA, et al. Neuropsychological deficits in essential tremor: an expression of cerebello-thalamo-cortical pathophysiology? Eur J Neurol 2002;9:143–151	Lab: Patho-physiology
94	57d	82	Axelrad JE, Louis ED, Honig LS, et al. Reduced Purkinje cell number in essential tremor. Arch Neurol 2008;65:101–107	Lab: Patho-physiology
95	58	81	Brin MF, Lyons KE, Doucette J, et al. A randomized, double masked, controlled trial of botulinum toxin type A in essential hand tremor. Neurology 2001;56:1523–1528	Clinical: Medicine
96	59a	80	Wills AJ, Jenkins IH, Thompson PD, et al. A positron emission tomography study of cerebral activation-associated with essential and writing tremor. Arch Neurol 1995;52:299–305	Lab: Patho-physiology
97	59b	80	Louis ED, Ford B, Lee H, et al. Diagnostic criteria for essential tremor: a population perspective. Arch Neurol 1998;55:823–828	Clinical: Classification
98	59c	80	Kralic JE, Criswell HE, Osterman JL, et al. Genetic essential tremor in gamma-aminobutyric acidA receptor alpha 1 subunit knockout mice. J Clin Invest 2005;115:774–779	Lab: Genetic studies
99	60a	79	Cleeves L, Findley LJ, Koller W. Lack of association between essential tremor and Parkinson’s disease. Ann Neurol 1988;24: 23–26	Epidemiology
100	60b	79	Tan EK, Matsuura T, Nagamitsu S, et al. Polymorphism of NACP-Rep1 in Parkinson’s disease: An etiologic link with essential tremor? Neurology 2000;54: 1195–1198	Lab: Genetic studies

Table 3

Summary of Top 100 Articles on Dystonia Citations for Dystonia (Ranked in Order of Citations)

Absolute Number	Rank	Citations	Paper	Category
1	1	560	Ozelius LJ, Hewett JW, Page CE, et al. The early-onset torsion dystonia gene (DYT1) encodes an ATP binding protein. Nat Genet 1997;17:40–48	Lab: Genetic studies
2	2	537	Burke RE, Fahn S, Marsden CD, et al. Validity and reliability of a rating-scale for the primary torsion dystonias. Neurology 1985;35:73–77	Clinical: Classification
3	3	523	Ichinose H, Ohye T, Takahashi E, et al. Hereditary progressive dystonia with marked diurnal fluctuation caused by mutations in the GTP cyclohydrolase-I gene. Nat Genet 1994;8:236–242	Lab: Genetic studies
4	4	491	Berardelli A, Rothwell JC, Hallett M, et al. The pathophysiology of primary dystonia. Brain 1998;121:1195–1212	Lab: Patho-physiology
5	5	453	Tsui JKC, Stoessl AJ, Eisen A, et al. Double-blind-study of botulinum toxin in spasmodic torticollis. Lancet 1986;2:245–247	Clinical: Medicine
6	6	438	Vidailhet M, Vercueil L, Houeto JL, et al. Bilateral deep-brain stimulation of the globus pallidus in primary generalized dystonia. N Engl J Med 2005;352:459–467	Clinical: Surgery
7	7	391	Ridding MC, Sheean G, Rothwell JC, et al. Changes in the balance between motor cortical excitation and inhibition in focal, task specific dystonia. J Neurol Neurosurg Psychiatry 1995;59:493–498	Clinical: Medicine
8	8	363	Burke RE, Fahn S, Jankovic J, et al. Tardive dystonia: late-onset and persistent dystonia caused by anti-psychotic drugs. Neurology 1982;32:1335–1346	Lab: Patho-physiology
9	9	362	Kupsch A, Benecke R, Mueller J, et al. Pallidal deep-brain stimulation in primary generalized or segmental dystonia. N Engl J Med 2006;355:1978–1990	Clinical: Surgery
10	10	320	Sheehy MP, Marsden, CD. Writers cramp -a focal dystonia. Brain 1982;105: 461–480	Clinical: Classification
11	11	312	Risch N, de Leon D, Ozelius L, et al. Genetic-analysis of idiopathic torsion dystonia in Ashkenazi Jews and their recent descent from a small founder population. Nat Genet 1995;9:152–159	Lab: Genetic studies
12	12	311	Byl NN, Merzenich MM, Jenkins WM. A primate genesis model of focal dystonia and repetitive strain injury 1. Learning-induced dedifferentiation of the representation of the hand in the primary somatosensory cortex in adult monkeys. Neurology 1996;47:508–520	Lab: Patho-physiology
13	13	292	Vitek JL, Chockkan V, Zhang JY, et al. Neuronal activity in the basal ganglia in patients with generalized dystonia and hemiballismus. Ann Neurol 1999;46:22–35	Lab: Patho-physiology
14	14	282	Berardelli A, Rothwell JC, Day BL, et al. Patho-physiology of blepharospasm and oromandibular dystonia. Brain 1985;108:593–608	Lab: Patho-physiology
15	15	279	Siebner HR, Tormos JM, Ceballos-Baumann AO, et al. Low-frequency repetitive transcranial magnetic stimulation of the motor cortex in writer’s cramp. Neurology 1999;52:529–537	Clinical: Medicine
16	16	259	Jankovic J, Orman J. Botulinum-a toxin for cranial-cervical dystonia: a double-blind, placebo-controlled study. Neurology 1987;37:616–623	Clinical: Medicine
17	17a	255	Elbert T, Candia V, Altenmuller E, et al. Alteration of digital representations in somatosensory cortex in focal hand dystonia. Neuroreport 1998;9:3571–3575	Lab: Patho-physiology
18	17b	255	Jankovic J, Schwartz K, Donovan DT. Botulinum toxin treatment of cranial-cervical dystonia, spasmodic dysphonia, other focal dystonias and hemifacial spasm. J Neurol Neurosurg Psychiatry 1990;53:633–639	Clinical: Medicine
19	17c	255	Coubes P, Roubertie A, Vayssiere N, et al. Treatment of DYT1-generalised dystonia by stimulation of the internal globus pallidus. Lancet 2000;355:2220–2221	Clinical: Surgery
20	18	248	Zimprich A, Grabowski M, Asmus F, et al. Mutations in the gene encoding epsilon-sarcoglycan cause myoclonus-dystonia syndrome. Nat Genet 2001;29:66–69	Lab: Genetic studies
21	19	246	Brin MF, Fahn S, Moskowitz C, et al. Localized injections of botulinum toxin for the treatment of focal dystonia and hemifacial spasm. Mov Disord 1987;2:237–254	Clinical: Medicine
22	20	242	Nutt JG, Muenter MD, Aronson A, et al. Epidemiology of focal and generalized dystonia in Rochester, Minnesota. Mov Disord 1988;3:188–194	Epidemiology
23	21	234	Greene P, Kang U, Fahn S, et al. Double-blind, placebo-controlled trial of botulinum toxin injections for the treatment of spasmodic torticollis. Neurology 1990;40:1213–1218	Clinical: Medicine
24	22	233	Jankovic J, Leder S, Warner D, et al. Cervical dystonia: clinical findings and associated movement-disorders. Neurology 1991;41:1088–1091	Epidemiology
25	23	231	Greene P, Fahn S, Diamond B. Development of resistance to botulinum toxin type-a in patients with torticollis. Mov Disord 1994;9:213–217	Clinical: Medicine
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52	46	176	Muenter MD, Sharpless NS, Tyce GM, et al. Patterns of dystonia (I-D-I and D-I-D) in response to l-dopa therapy for Parkinson’s disease. Mayo Clin Proc 1977;3:163–174	Clinical: Medicine
53	47	175	Marsden CD, Harrison MJ. Idiopathic torsion dystonia (dystonia musculorum deformans). A review of forty-two patients. Brain 1974; 97:793–810	Review
54	48	174	Kumar R, Dagher A, Hutchison WD, et al. Globus pallidus deep brain stimulation for generalized dystonia: clinical and PET investigation. Neurology 1999;53:871–874	Lab: Patho-physiology
55	49	173	Goodchild RE, Kim CE, Dauer WT. Loss of the dystonia-associated protein torsinA selectively disrupts the neuronal nuclear envelope. Neuron 2005;48:923–932	Lab: Genetic studies
56	50	171	Cohen LG, Hallett M. Hand cramps – clinical-features and electromyographic patterns in a focal dystonia. Neurology 1988;38:1005–1012	Clinical: Classification
57	51	170	Blitzer A, Brin MF, Stewart CF. Botulinum toxin management of spasmodic dysphonia (Laryngeal dystonia): A 12-year experience in more than 900 patients. Laryngoscope 1998;108:1435–1441	Clinical: Medicine
58	52a	169	Dauer WT, Burke RE, Greene P, et al. Current concepts on the clinical features, aetiology and management of idiopathic cervical dystonia. Brain 1998;121:547–560	Review
59	52b	169	Irani SR, Michell AW, Lang B, et al. Faciobrachial dystonic seizures precede Lgi1 antibody limbic encephalitis. Ann Neurol 2011;69:892–900	Lab: Patho-physiology
60	52c	169	Gelb DJ, Lowenstein DH, Aminoff MJ. Controlled trial of botulinum toxin injections in the treatment of spasmodic torticollis. Neurology 1989;39:80–84	Clinical: Medicine
61	53	167	Burke RE, Fahn S, Gold AP. Delayed-onset dystonia in patients with static encephalopathy. J Neurol Neurosurg Psychiatry 1980;43:789–797	Clinical: Classification
62	54a	165	Waddy HM, Fletcher NA, Harding AE, et al. A genetic-study of idiopathic focal dystonias. Ann Neurol 1991;29:320–324	Lab: Genetic studies
63	54b	165	Krauss JK, Pohle T, Weber S, et al. Bilateral stimulation of globus pallidus internus for treatment of cervical dystonia. Lancet 1999;354:837–838	Clinical: Surgery
64	55	164	Odergren T, Hjaltason H, Kaakkola S, et al. A double blind, randomised, parallel group study to investigate the dose equivalence of Dysport (R) and Botox (R) in the treatment of cervical dystonia. J Neurol Neurosurg Psychiatry 1998;64:6–12	Clinical: Medicine
65	56a	163	Simpson DM, Gracies JM, Graham HK, et al. Assessment: Botulinum neurotoxin for the treatment of spasticity (an evidence-based review): report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology. Neurology 2008;70:1691–1698	Review
66	56b	163	Bressman SB, de Leon D, Brin MF, et al. Idiopathic dystonia among Ashkenazi Jews: evidence for autosomal dominant inheritance. Ann Neurol 1989;26:612–620	Lab: Genetic studies
67	57a	162	Jin H, May M, Tranebjaerg L, et al. A novel X-linked gene, DDP, shows mutations in families with deafness (DFN-1), dystonia, mental deficiency and blindness. Nat Genet 1996;14:177–180	Lab: Genetic studies
68	57b	162	Nygaard TG, Marsden CD, Fahn S. Dopa-responsive dystonia: long-term treatment response and prognosis. Neurology 1991;41:174–181	Clinical: Medicine
69	58a	161	Rupniak NMJ, Jenner P, Marsden, CD. Acute dystonia induced by neuroleptic drugs. Psychopharmacology (Berl) 1986;88:403–419	Lab: Patho-physiology
70	58b	161	Vidailhet M, Vercueil L, Houeto JL, et al. Bilateral, pallidal, deep-brain stimulation in primary generalised dystonia: a prospective 3 year follow-up study. Lancet Neurol 2007;6:223–229	Clinical: Surgery
71	58c	161	Newton MR, Berkovic SF, Austin MC, et al. Dystonia, clinical lateralization, and regional blood-flow changes in temporal-lobe seizures. Neurology 1992;42:371–377	Epidemiology
72	58d	161	Aguiar PD, Sweadner KJ, Penniston JT, et al. Mutations in the Na+/K+-ATPase alpha 3 gene ATP1A3 are associated with rapid-onset dystonia Parkinsonism. Neuron 2004; 43:169–175	Lab: Genetic studies
73	58e	161	Silberstein P, Kuhn AA, Kupsch A, et al. Patterning of globus pallidus local field potentials differs between Parkinson’s disease and dystonia. Brain 2003;126:2597–2608	Lab: Patho-physiology
74	59	160	Lew MF, Adornato BT, Duane DD, et al. Botulinum toxin type B: A double-blind, placebo-controlled, safety and efficacy study in cervical dystonia. Neurology 1997; 49:701–707	Clinical: Medicine
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77	61b	157	Kotagal P, Lüders H, Morris HH, et al. Dystonic posturing in complex partial seizures of temporal-lobe onset: a new lateralizing sign. Neurology 1989:39:196–201	Lab: Patho-physiology
78	62a	155	Jankovic J, Vuong KD, Ahsan, J. Comparison of efficacy and immunogenicity of original versus current botulinum toxin in cervical dystonia. Neurology 2003;60:1186–1188	Clinical: Medicine
79	62b	155	Vercueil L, Pollak P, Fraix V, et al. Deep brain stimulation in the treatment of severe dystonia. J Neurol 2001;248:695–700	Clinical: Surgery
80	63a	154	Kothary R, Clapoff S, Brown A, Campbell R, Peterson A, Rossant J. A transgene containing lacz inserted into the dystonia locus is expressed in neural tube. Nature 1988;335:435–437	Lab: Genetic studies
81	63b	154	Byrnes ML, Thickbroom GW, Wilson SA, et al. The corticomotor representation of upper limb muscles in writer’s cramp and changes following botulinum toxin injection. Brain 1998;121:977–988	Clinical: Medicine
82	64a	152	Rajput AH, Gibb WR, Zhong XH, et al. Dopa-responsive dystonia – pathological and biochemical observations in a case. Ann Neurol 1994;35:396–402	Clinical: Medicine
83	64b	152	Krack P, Pollak P, Limousin P, et al. From off-period dystonia to peak-dose chorea – The clinical spectrum of varying subthalamic nucleus activity. Brain 1999; 122:1133–1146	Clinical: Classification
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For further analysis of the most cited works, each article was classified into one of seven categories: epidemiology, laboratory genetic studies, laboratory pathophysiological studies, clinical classification, clinical medical therapies, clinical surgical therapies, and review articles. The numbers of articles in each category by field of study (ET and dystonia) are shown in Figure 1.

Figure 1

Plot Showing the Number of Articles by Category. Essential Tremor (shaded grey) and dystonia (hatched lines).

Epidemiology

Studies were included in this category if they described the epidemiology of the condition including prevalence, incidence, comparisons, and trends over time. For ET, there were 15 studies in this category. Out of these, eight studies investigated the incidences of ET in different regions or countries. The remaining seven examined the association of ET with dementia, PD, and other functional measures. By comparison, there were only four dystonia articles in this category. These studies investigated the prevalence of dystonia in different population subgroups.

Clinical: classification

This broad category included new description of the disease, new diagnostic features, its classification into subtypes, as well as condition-specific measures and rating scales. There were 18 articles in this category, including one citation classic for ET.11 Similarly, there were 13 articles for dystonia and one citation classic.16

Laboratory: genetic studies

Studies were included in this category if they were genetic studies. There were seven studies for ET, including investigations in the familial essential tremor gene FET1, fragile X premutation, variants in the sequences of the LINGO1 gene and the dopamine D-3 receptor, GABA(A) alpha 1 receptor knockout, polymorphism of NACP-Rep1, and methodologic issues in ET research. For dystonia, there were 19 studies including two citation classics.14,15 These two genetic studies were the first and third most highly ranked articles by citations in the dystonia field. There were five studies on the early onset dystonia gene (DYT1),;four on idiopathic dystonia; two on the genetics of dopa-responsive dystonia; one on Leber optic atrophy and dystonia; one on lacZ transgene insertion; and one each on mutations of the deafness/dystonia peptide (DDP) gene, GTP cyclohydrolase-1 gene, epsilon sarcoglycan gene, dystonia musculorum gene, Na+/K+-ATPase alpha 3 gene, and phospholipase A2 group VI (PLA2G6) gene.

Laboratory: pathophysiology

Studies investigating the pathophysiologic mechanisms underlying ET and dystonia were included in this category. There were 21 studies for ET, including 10 that employed imaging techniques. There were four studies on the physiology of ET, five on pathology, and the remaining studies on pharmacology or biochemical analysis of cerebrospinal fluid. For dystonia, there were 20 studies with 13 on neurophysiology, six on imaging, and one on pharmacology.

Clinical: medical therapies

Only those studies dealing with the application of medical and nonsurgical treatments were included in this category. For ET, there were eight articles on medical treatment with two studies on propranolol and primidone, and one study each on gabapentin and propranolol, on propranolol alone, on alcohol, on botulinum toxin, and on mechanical displacement. For dystonia, there were 30 articles including one citation classic.18 The effect of botulinum toxin was investigated in 19 out of the 30 articles. The remaining articles included five studies on transcranial magnetic stimulation; three studies on levodopa; and one study each on baclofen, lidocaine, and anticholinergic treatment.

Clinical: surgical therapies

Only those studies that dealt primarily with surgical treatment were included in this category. In several studies, the primary aim of the surgical treatment was for tremor whose etiology could either be due to PD or ET; these were also included. For ET, there were 16 articles with three citation classics.10,12,13 There was one article on the effect of thalamotomy and two articles comparing thalamotomy with deep brain stimulation (DBS); the 13 remaining articles investigated the effects of thalamic stimulation of the ventral intermediate nucleus. For dystonia, there were nine articles on surgical therapies, all of which were studies of globus pallidus internus (GPi) DBS.

Review articles

There were 15 review articles for ET: seven on the general aspects of ET and the remainder reviewing the treatment, pathophysiology, and effect of electrical stimulation. For dystonia, there were five reviews of pathophysiology or diagnostic features.

Time trends and journals

The publication year of the most cited articles are summarized in Figure 2. This showed that there was a peak of the most cited articles for ET for articles published between 2000 and 2004, and for dystonia between 1995 and 1999. The most cited articles were published in 38 journals. The top 10 journals by numbers of articles published and numbers of citations per articles are shown in Figure 3. The top 10 journals accounted for 162 (81%) of the combined 200 most cited articles.

Figure 2

Number of Most Cited Articles in 5-Year Time Periods.

Figure 3

Top 10 Journals with the Highest Numbers of Most Cited Articles for Essential Tremor and Dystonia. The size of the bubble represents the 5-year journal impact factor for 2015.

Discussion

The most highly cited articles in the fields of ET and dystonia were identified (Tables 2 and 3). By category, the most cited studies were those on pathophysiology followed by medical treatments, clinical classification, genetic studies, surgical treatments, review articles, and epidemiology studies.

Comparing ET and dystonia, there are very similar proportions of articles in the most cited on pathophysiology and clinical classification categories. Nevertheless, the two fields diverge in other subfields because ET is characterized by a larger number of studies describing the epidemiology of the condition, whereas dystonia has a much higher representation of genetic studies. This is likely an indication of the greater understanding and contribution of genetic factors to dystonia compared to ET, which is—by contrast—a much more common condition but probably characterized by a wide spectrum of possible etiologies, making genetic studies very difficult. Moreover, there is a predominance of surgical treatment for ET, whereas medical treatment predominates for dystonia.

Trends over time showed that the peak period of when the most cited papers were published was between 1995 and 2005, with dystonia peaking 5 years before ET. This is in line with our previous work looking at the most cited papers in the functional neurosurgery literature, which showed a similar peak in the 1990s.6 The proposed reasons for the peak during this time are likely similar, namely that this period was particular active and productive with significant success in elucidating the causes and diagnosis of these conditions, and effective treatments such as deep brain stimulation or botulinum toxin were developed for dystonia. Another possible reason for the proposed peak may be a fundamental feature of contemporary research whereby older publications are no longer cited because they have been replaced by new studies that have replicated the findings and superseded them.6 At the same time, more recent studies have not had time to become established as a most cited article, for example the more recent discovery of the DYT6 gene.20

The majority of the most cited articles were generally published in specialized journals such as Neurology, Annals of Neurology, Movement Disorders, Brain, Journal of Neurology, Neurosurgery, and Psychiatry, among others (Figure 3). However, articles published in the more general medical journals such as the New England Journal of Medicine or Lancet tend to receive more citations per article published. This also corresponded to their higher overall journal impact factor (Figure 3). An exception were articles published in the Journal of Neurosurgery, which received on average 356 citations per articles, more than would be expected by the journal’s impact factor. These were attributable to contributions from three studies on deep brain stimulation.12,21,22 This suggests that the publication of important clinical therapeutic studies in a specialized journal is also able to achieve significant impact.

Limitations

The choice of ISI Web of Science, which indexes over 15,000 journals, over Google Scholar, which indexes a wider range of academic documents may also have had an effect on our findings. However, our previous reviews in other fields yielded very similar results using these two search engines when the study field was small.6,9 Another limitation is that the search terms may not have yielded all possible results despite our inclusion of broad terms. For the ISI Web of Science search engine, it is possible to perform either a title- or topic-based search. The former would only search for the search term in the title of the article, whereas a topic-based search would also include the abstract. Given the small size of the ET and dystonia fields, a topic-based search was selected to retrieve all possible results as described in the methods section. This is particularly important for ET, as many of the studies would also include patients with parkinsonian tremor. By contrast, a citation analysis study on ET using a title-based search would yield fewer results.8 Finally, the list of most cited papers changes with time and is therefore a snapshot of the current state of research.

Conclusions

We performed an analysis to identify the most cited ET and dystonia papers. There are fewer citation classics compared to PD, confirming that this is a smaller field of research. Compared to dystonia, areas of research in ET such as genetics, neurophysiology, and medical treatment are underrepresented. The peak of citations for ET is also lagging by about 5 years. These findings suggest that further work remains to be carried out to improve our understanding of the basic science of ET.