Source: DGNews  |  Posted 2 years ago

By Crina Frincu-Mallos, PhD

BALTIMORE, Md -- October 16, 2009 -- Electrical impedance myography (EIM), an electrophysiologic technique used for assessing muscular health by measuring physical parameters such as resistance in response to applied electrical current, provides clinically relevant information in patients with spinal muscular atrophy (SMA), researchers stated here at the American Neurological Association (ANA) 134th Annual Meeting.

“Although still under refinement, the technique offers the promise of a useful, noninvasive method of neuromuscular disease evaluation,” said Seward Brian Rutkove, MD, Department of Neurology, Beth Israel Deaconess Medical Center and Children’s Hospital, Harvard Medical School, Boston, Massachusetts.

“One application for which EIM may be especially well-suited is the monitoring of muscle condition in children with SMA,” hence addressing a need to effectively assess disease burden, especially in children with type 2 or 3 SMA, he added.

In diseased muscle, EIM parameters such as tissue resistance, capacitance, and phase differ from those in healthy muscle: upon applying electrical current, the resistance increases and the capacitance decreases, causing the measured voltage to have a lower amplitude and, hence, to be less out-of-phase with the applied current, explained Dr. Rutkove.

The investigators aimed to evaluate whether EIM can discriminate between disease type.

“The fact that type 2 SMA patients can be effectively distinguished from type 3 supports the hypothesis that patients with more severe disease have more abnormal EIM spectra,” Dr. Rutkove remarked.

The study enrolled 21 patients (mean age 12.5 years) with genetically confirmed type 2 (n = 11) and type 3 SMA (n = 10) and 18 health controls (mean age 10.7 years).

Biceps and tibialis anterior on the dominant side were studied in all study participants, using EIM, ultrasound, and handheld dynamometry.

There were significant differences in the biceps strength between the type 2 and type 3 SMA patients (P < .001), with mean values of 1.48 kg (3.26 lbs) and 8.62 kg (19.0 lbs), respectively. The diseased muscles could be easily differentiated from the healthy muscles, with an average biceps strength in healthy controls of 11.88 kg (26.2 lbs).

Biceps data acquired at a single frequency value of 50 kHz indicate that EIM could differentiate between type 2 and type 3 SMA patients with 92.7% accuracy: the log-resistance slope was -30.1 +- 12.3 ohms/kHz in those with type 2 SMA, compared with -78.8 +- 36.8 ohms/kHz in patients with type 3 (P < .001).

Results were similar for tibialis anterior. For example, EIM could differentiate between type 2 and type 3 SMA patients using the phase slope values (92.2% accuracy; P = .001), with values of 5.09 +- 1.00 degrees/kHz in patients with type 2 SMA versus -1.08 +- 4.19 degrees/kHz in patients with type 3 SMA.

“Whereas the skin-fat thickness seems to impact the EIM data,” noted Dr. Rutkove, “the data appear to be strongly associated with strength.”

Dr. Rutkove indicated that the longitudinal data acquisition is expected to conclude by the end of 2010.

Funding for this study was provided by the Spinal Muscular Atrophy Foundation.

Presentation title: Electrical Impedance Myography for the Non-Invasive Assessment of Muscle in Spinal Muscular Atrophy. Abstract M-59