Quantitative analysis of the "warm-up" phenomenon in myotonic dystrophy type 1.
To quantitate improvement in hand-grip myotonia and muscle strength (i.e., the "warm-up" phenomenon) in myotonic dystrophy type 1 (DM1), six successive, standardized maximum voluntary isometric contractions (MVICs) were recorded on 2 separate days using a computerized isometric hand-grip myometer in 25 genetically confirmed DM1 patients and in 17 normal controls. An automated computer program placed cursors along the declining (relaxation) phase of the MVICs at 90%, 50%, and 5% of peak force (PF) and calculated relaxation times (RTs) between these points. Mean 90% to 5% RT (a measure of myotonia) rapidly declined from 2.5 s in MVIC 1 to 0.8 s in MVIC 6 (warm-up = 1.7 s) in DM1; in controls, it remained 0.4 s for all six MVICs (warm-up = 0). In DM1, 70% of warm-up occurred between MVIC 1 and 2, almost exclusively in the terminal 50% to 5% phase of muscle relaxation. Day 1 warm-up was highly correlated with the severity of myotonia, and with day 2 warm-up. Improvement in myotonia was not accompanied by either transient paresis or improvement in PF. We conclude that, with this testing paradigm: warm-up of myotonia in DM1 can be reliably measured; is proportional to severity of myotonia; occurs rapidly, being most prominent between the first and second grips; mainly results from shortening of the terminal phase of muscle relaxation; and is not accompanied by significant warm-up in force output.
Logigian
E L
EL
Department of Neurology, University of Rochester Medical Center, Box 673, 601 Elmwood Avenue, Rochester, New York 14642, USA. eric_logigian@urmc.rochester.edu
Blood
C L
CL
Dilek
N
N
Martens
W B
WB
Moxley
R T
RT
Wiegner
A W
AW
Thornton
C A
CA
Moxley
R T
RT
D000328:Adult; D003936:Diagnosis, Computer-Assisted / Q000295:instrumentation / Q000592:standards; D005260:Female; D018737:Hand Strength; D006801:Humans; D007537:Isometric Contraction; D008297:Male; D008875:Middle Aged; D009223:Myotonic Dystrophy / Q000175:diagnosis* / Q000503:physiopathology*; D015203:Reproducibility of Results; D012720:Severity of Illness Index