Objective: To substantiate a rationale for cognitive load (CL) assessment to differentiate functional tremor (FT) and organic tremor (OT) in the electrophysiologic study.

Background: FT is known to be a single manifestation of a functional movement disorder in 21.6% of patients, and in 40% of cases it occurs in combination with other disease phenotypes[1]. An electrophysiologic examination is included in the criteria for FT [2] and can provide additional important diagnostic information (e.g. the frequency peak width) [3, 4]. In the validated FT electrophysiological criteria, physical distractions are a major sign of FT [5] but mental/cognitive distractions are more commonly used in neurologic examination.

Method: The study included 17 patients with FT (14 women; 41, [38; 50] yrs.) and 15 patients with OT (11 women; 45, [41; 52] yrs.; patients were diagnosed with Parkinson’s disease [n=8], Wilson’s disease [n=4], essential tremor [n=1], neurodegeneration with iron accumulation [n=1], and generalized dystonia [n=1]). The electrophysiologic study included two-channel accelerometry and four-channel surface electromyography of both hands (flexors and extensors). The tremor was recorded in the outstretched arms position both in the absence of a CL and during a serial subtraction task as a CL. The frequency peak width of accelerometer oscillations and EMG activity were assessed by visual assesment with density compressed spectral array (DCSA) in a 30-second recording.

Results: Patients with FT were found to have an increased frequency peak width (p<0.001) both with and without CL compared to patients with OT while assessing accelerometer oscillations and EMG activity (Table 1). Notably, the introduction of CL resulted in a more prominent difference in peak width (p<0,001). Most patients with FT had an increase in frequency splitting in the density compressed spectral array, which, again, increased with the introduction of CL (Table 2).

Conclusion: Electrophysiological study with assessment of tremor frequency peak width and its change with CL can be used to differentiate between FT and OT.

Frequency characteristics of FT and OT

Frequency splitting estimation of the DSCA

References: 1. Lidstone SC, Costa-Parke M, Robinson EJ, Ercoli T, Stone J; FMD GAP Study Group. Functional movement disorder gender, age and phenotype study: a systematic review and individual patient meta-analysis of 4905 cases. J Neurol Neurosurg Psychiatry. 2022;93(6):609-616. doi: 10.1136/jnnp-2021-328462.
2. Gupta A, Lang AE. Psychogenic movement disorders. Curr Opin Neurol. 2009 Aug;22(4):430-6. doi: 10.1097/WCO.0b013e32832dc169.
3. Chou CZ, Ahlskog JE, Klassen BT, Coon EA, Ali F, Bower JH, Savica R, Hassan A. Utility of routine surface electrophysiology to screen for functional tremor prior to surgical treatment of essential tremor. Clin Park Relat Disord. 2022;7:100149. doi: 10.1016/j.prdoa.2022.100149.
4. Aybek S, Perez DL. Diagnosis and management of functional neurological disorder. BMJ. 2022;376:o64. doi: 10.1136/bmj.o64.
5. Schwingenschuh P, Saifee TA, Katschnig-Winter P, Macerollo A, Koegl-Wallner M, Culea V, Ghadery C, Hofer E, Pendl T, Seiler S, Werner U, Franthal S, Maurits NM, Tijssen MA, Schmidt R, Rothwell JC, Bhatia KP, Edwards MJ. Validation of “laboratory-supported” criteria for functional (psychogenic) tremor. Mov Disord. 2016;31(4):555-62. doi: 10.1002/mds.26525.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/cognitive-load-in-a-diagnostic-electrophysiologic-assessment-of-functional-tremor/