REDUCTIONS IN MUSCLE QUALITY AND QUANTITY IN CHRONIC INFLAMMATORY DEMYELINATING POLYNEUROPATHY PATIENTS ASSESSED BY MAGNETIC RESONANCE IMAGING

Introduction: Weakness in patients with chronic inflammatory demyelinating polyneuropathy (CIDP) may be caused by decreases in muscle quantity and quality, but this has not been explored. Methods: Twelve patients with CIDP (mean age 61 years) and 10 age-matched (mean age 59 years) control subjects were assessed for ankle dorsiflexion strength, and two different MRI scans (T1 and T2) of leg musculature. Results: Isometric strength was 36% lower in CIDP patients compared with controls. Tibialis anterior muscle volumes of CIDP patients were smaller by 17% compared with controls, and non-contractile tissue volume was 58% greater in CIDP patients. When normalized to total muscle or corrected contractile volume, strength was 29% and 18% lower, respectively, in CIDP patients. Discussion: These results provide insight into the structural integrity of muscle contractile proteins and pathologic changes to whole-muscle tissue composition that contribute to impaired muscle function in CIDP. Muscle Nerve 000:000–000, 2018 Chronic inflammatory demyelinating polyneuropathy (CIDP) is an inflammatory peripheral nerve disorder with sensory disturbances and progressive muscle weakness of the proximal and distal extremities. Most studies on neuroinflammatory diseases, such as multiple sclerosis and inflammatory myelopathies, have focused on the detection of specific neurologic lesions using MRI. Indeed, nerve lesions in CIDP have been shown by MRI of the lumbar nerve roots (L4 and L5), tibial nerve, and brachial plexus. In one study of the lumbar nerve roots in both treated and untreated CIDP patients and in those with polyneuropathy due to other etiologies, MRI was useful in identifying nerve root hypertrophy, and as an adjunct test to identify CIDP in situations in which the diagnosis was uncertain. Using another MRI technique, Sinclair et al. demonstrated that the magnetization transfer ratio (MTR) in the posterior leg muscles of CIDP patients was significantly lower compared with controls, indicating that hydration changes or subtle biochemical disparities within the leg musculature had occurred in the chronic degenerative disease stage of CIDP. Moreover, their study also established that lower MTR values in the anterior compartment of the leg correlated strongly with diminished ankle dorsiflexion strength. The loss of strength in muscles of those with CIDP is most severe distally, can be profound, and likely has a direct impact on functional impairment. However, very few data exist regarding the limb musculature in persons with CIDP, specifically muscle quantity and quality. A few studies have reported correlations between MRI-detected nerve impairments and clinical findings (e.g., muscle strength and impairment). Although force (strength) is strongly associated with the quantity of muscle tissue that is accessible for measurement, there are essential qualitative muscle properties involved that may also severely affect muscle strength. Muscle quality can be defined as strength per unit of muscle mass as assessed by MRI methods, and is often assessed by normalizing strength to cross-sectional area or to total muscle volume. When muscle strength is normalized to tissue amount and strength deficits persist, this indicates deficiencies in contractile quality. Transverse magnetization (T2)-relaxation time is a quantitative MRI sequence sensitive to small changes in proton density of a tissue. For example, when skeletal muscle atrophies with aging or disease, a concurrent increase in non-contractile tissue, such as connective tissue and adipose tissue, is deposited intramuscularly. Thus, in aging, T2-relaxation Additional supporting information can be found in the online version of this article. Abbreviations: CIDP, chronic inflammatory demyelinating polyneuropathy; CMAP, compound muscle action potential; CSF, cerebrospinal fluid; ET, echo time; IVIg, intravenous immunoglobulin; MTR, magnetization transfer ratio; MVC, maximum voluntary contraction; ROI, region of interest; SNAP, sensory nerve action potential; T2, transverse magnetization; TA, tibialis anterior; TR, repetition time