Journal of Neurology

, Volume 254, Issue 1, pp 46–52 | Cite as

The “enhanced N35” somatosensory evoked potential: its associations and potential utility in the clinical evaluation of dystonia and myoclonus

ORIGINAL COMMUNICATION

Abstract

In median nerve somatosensory evoked potentials, the cortical N35 amplitude sometimes exceeds the P25 amplitude (C3’/C4’ referred to Fz; “enhanced N35” feature). Six hundred consecutive patient median nerve SEPs were retrospectively analysed and compared with 27 controls. The feature was more often present in patients with dystonia (62%) than in patients with other disorders (22%; relative risk for the condition 2.8; Fisher’s exact p = 0.003) or control subjects (7.4%; odds ratio 20; p = 0.0006). Similarly, the feature was more often present in patients with myoclonus (38%) than in patients with other disorders (22%; relative risk 1.7; p = 0.02) or control subjects (odds ratio 7.5; p = 0.006).There was no clear relationship of the feature to short latency SEP abnormalities except in cases of myoclonus. Further comparison was made of the characteristics of 72 patients each, with and without the feature, whose short latency SEP components were normal. The relationship of the feature to dystonia or myoclonus held true in this case-controlled arm of the study. The sensitivity and specificity were 65% and 78% respectively for any form of dystonia; 43% and 79% respectively for any form of myoclonus. The feature was even more specific in both conditions when compared with controls (93%). Most cases of dystonia with an identifiable cause in this study were of secondary forms. It is known that this feature often occurs in association with “giant” SEPs in some myoclonic conditions. However, its occurrence in dystonia may be a useful new finding in an established test, helping to identify a condition where there is increasing evidence for disordered sensorimotor integration.

Keywords

somatosensory evoked potential N35 myoclonus dystonia multiple sclerosis 

References

  1. 1.
    Dawson G (1947) Investigations on a patient subject to myoclonic seizures after sensory stimulation. J Neurol, Neurosurg Psychiatry 10:141–162CrossRefGoogle Scholar
  2. 2.
    Shibasaki H, Yamashita Y, Neshige R, Tobimatsu S, Fukui R (1985) Pathogenesis of giant somatosensory evoked potentials in progressive myoclonic epilepsy. Brain 108(Pt 1):225–40PubMedCrossRefGoogle Scholar
  3. 3.
    Recommendations for the Practice of Clinical Neurophysiology (1999) In: Guidelines of the International Federation of Clinical Neurophysiology. 2nd ed. Supp 52 to Electroencephalogr Clin NeurophysiolGoogle Scholar
  4. 4.
    McDonald WI, Compston A, Edan G, Goodkin D, Hartung HP, Lublin FD, McFarland HF, Paty DW, Polman CH, Reingold SC, Sandberg-Wollheim M, Sibley W, Thompson A, van den Noort S, Weinshenker BY, Wolinsky JS (2001) Recommended diagnostic criteria for multiple sclerosis: guidelines from the International Panel on the diagnosis of multiple sclerosis. Ann Neurol 50(1):121–7PubMedCrossRefGoogle Scholar
  5. 5.
    Jones SJ (1981) An ‘interference” approach to the study of somatosensory evoked potentials in man. Electroencephalogr Clin Neurophysiol 52(6):517–30PubMedCrossRefGoogle Scholar
  6. 6.
    Allison T, McCarthy G, Wood CC, Jones SJ (1991) Potentials evoked in human and monkey cerebral cortex by stimulation of the median nerve. A review of scalp and intracranial recordings. Brain 114(Pt 6):2465–503PubMedCrossRefGoogle Scholar
  7. 7.
    Nakashima K, Rothwell JC, Day BL, Thompson PD, Shannon K, Marsden CD (1989) Reciprocal inhibition between forearm muscles in patients with writer’s cramp and other occupational cramps, symptomatic hemidystonia and hemiparesis due to stroke. Brain 112(Pt 3):681–97PubMedCrossRefGoogle Scholar
  8. 8.
    Lidsky TI, Manetto C, Schneider JS (1985) A consideration of sensory factors involved in motor functions of the basal ganglia. Brain Res 356:133–46PubMedGoogle Scholar
  9. 9.
    Tinazzi M, Priori A, Bertolasi L, Frasson E, Mauguiere F, Fiaschi A (2000) Abnormal central integration of a dual somatosensory input in dystonia. Evidence for sensory overflow. Brain 123:42–50PubMedCrossRefGoogle Scholar
  10. 10.
    Kaji R (2003) Dystonia. In: Hallett M (ed) Handbook of Clinical Neurophysiology. Movement disorders. Vol 1. Elsevier, Amsterdam, pp 451–461Google Scholar
  11. 11.
    Rossini PM, Bassetti MA, Pasqualetti P (1995) Median nerve somatosensory evoked potentials. Apomorphine-induced transient potentiation of frontal components in Parkinson’s disease and in parkinsonism. Electroencephalogr Clin Neurophysiol 96(3):236–47PubMedCrossRefGoogle Scholar
  12. 12.
    Reilly JA, Hallett M, Cohen LG, Tarkka IM, Dang N (1992) The N30 component of somatosensory evoked potentials in patients with dystonia. Electroencephalogr Clin Neurophysiol 84:243–7PubMedCrossRefGoogle Scholar
  13. 13.
    Kanovsky P, Streitova H, Dufek J, Rektor I (1997) Lateralization of the P22/N30 component of somatosensory evoked potentials of the median nerve in patients with cervical dystonia. Mov Disord 12:553–60PubMedCrossRefGoogle Scholar
  14. 14.
    Mazzini L, Zaccala M, Balzarini C (1994) Abnormalities of somatosensory evoked potentials in spasmodic torticollis. Mov Disord 9:426–30PubMedCrossRefGoogle Scholar
  15. 15.
    Grissom JR, Toro C, Tretteau J, Hallett M (1995) The N30 and N140–P190 median nerve somatosensory evoked potentials waveforms in dystonia involving the upper extremity. Neurology 45(Suppl. 4):A458Google Scholar
  16. 16.
    Barba C, Frot M, Guenot M, Mauguiere F (2001) Stereotactic recordings of median nerve somatosensory-evoked potentials in the human pre-supplementary motor area. Eur J Neurosci 13:347–56PubMedCrossRefGoogle Scholar
  17. 17.
    Schrag A, Lang AE (2005). Psychogenic movement disorders. Curr Opin Neurol 18(4):399–404PubMedCrossRefGoogle Scholar

Copyright information

© Steinkopff Verlag Darmstadt 2007

Authors and Affiliations

  1. 1.Dept. of Clinical NeurophysiologyThe National Hospital for Neurology and NeurosurgeryLondonUK
  2. 2.Dept. of Neurology and NeurophysiologyRoyal North Shore HospitalSydneyAustralia

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