Accurate assessment of endpoints that are clinically meaningful to the patient is essential for the regulatory approval of a candidate therapeutic. Many of the endpoints used in clinical trials for myotonic dystrophy (DM) type 1 (DM1) thus far do not meet this requirement and thus do not represent adequate registration endpoints. Such registration endpoints are the holy grail for DM. Tools must be validated to assess the diverse factors that contribute to fatigue in order to develop clinical trial endpoints and effective therapies.

Baldanzi and colleagues (University of Pisa) have published an evaluation of several instruments in a cohort of 26 subjects with the genetic and clinical diagnosis of DM1 and proposed a paradigm to assess central and peripheral fatigue. They defined central fatigue as a decrement in voluntary muscle activation during exercise related to cognitive/behavioral function. By contrast, peripheral fatigue was characterized as the consequence of altered transmission at the neuromuscular junction or muscular dysfunction. The authors suggest a protocol for evaluation as an assessment of fatigue in DM1. 

Fatigue is an important contributor to patient-reported burden of disease in DM1. However, across neuromuscular diseases, there has been considerable debate around both defining and measuring fatigue.

Objectively, fatigue is defined as a decrease in power (work performed over time). Fatigue may arise from having to operate at or near one’s maximal motor functional capacity—diminishment of that capacity leads to early onset of fatigue. Yet, another important disease burden in DM2, pain, limits the performance of work and thus the measurement of fatigue is confounded as patients may not want to exert maximal effort due to the discomfort it causes. 

Since patients are able to detect even subtle changes in fatigue, patient reported outcome measures (PROMs) have potential as clinical trial endpoints. Fatigue can have both central and peripheral origins, and its dual origin may impact therapy development strategies. Thus, for a multi-systemic disease like DM, it is particularly important that we have tools to quantitatively evaluate fatigue regardless of origin and, optimally, gain some insights into peripheral and central contributions. A PROM scale such as the Modified Fatigue Impact Scale (MFIS) has three subscales (physical, cognitive, and psychosocial functioning) that may, in part, help understand fatigue. Another commonly used scale, the Multidimensional Assessment of Fatigue (MAF), is valuable in assessing four dimensions of fatigue—degree and severity, distress caused, timing and impact on activities of daily living.

Because fatigue is multifactorial, studies are needed to evaluate and validate measures of fatigue. The Myotonic Dystrophy Health Index (MDHI) is a PROM that has been incorporated into many recent clinical studies and trials and includes separate question banks that assess fatigue, sleep, and cognition. Its fatigue component is thought to focus on muscle fatigue, muscle endurance, and “tiredness” arising from muscle. The sleep and cognitive components have been linked to CNS-based fatigue, including motivation and concentration. 

The complex etiology of fatigue in DM1 makes it difficult for individual instruments to dissect peripheral and central components of fatigue. Given its key role in the burden of DM, it is critical that validated measures of fatigue be incorporated into natural history studies and clinical trials. 

Reference:

The proposal of a clinical protocol to assess central and peripheral fatigue in myotonic dystrophy type 1.
Baldanzi S, Ricci G, Bottari M, Chico L, Simoncini C, Siciliano G.
Arch Ital Biol. 2017 Jul 1;155(1-2):43-54. doi: 10.12871/000398292017125.