Skeletal muscle structure and function in relation to post-exertional malaise in patients with PASC and ME/CFS

Post-Acute Sequelae of SARS-CoV-2 Infection (PASC) and Myalgic
Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) are severely debilitating
diseases mainly characterized by chronic fatigue, post-exertional malaise (PEM)
and cognitive impairment with autonomic, neuro-endocrine, immunological and
cognitive involvement1-3. Particularly, skeletal muscle-related complaints,
such as extreme fatigue, muscle weakness and muscle pain, are symptoms that
tremendously decrease the patients quality of life, The key symptom, PEM,
distinguishes PASC and ME/CFS from other more common fatigue conditions and is
characterized by worsening or relapse of symptoms (including sleep dysfunction,
cognitive impairment and extreme skeletal muscle fatigue) after physical
activity, which can last for days or weeks1,3,4. We recently reviewed the
skeletal muscle alterations in both acute SARS-CoV-2 infection and patients
with Long COVID, and concluded that post-exertional malaise in patients with
Long COVID cannot be explained by current knowledge on skeletal muscle
structure and function1.

A possible lead to the understanding of PEM in Long COVID and ME/CFS
pathophysiology is provided by the observation that most cases appear to have
an infectious onset5-7. Following SARS-CoV-2 infection, 10-30% of patients
exhibit post-infectious fatigue syndrome, called Long COVID or post-COVID
condition8. PEM is the hallmark symptom of Long COVID and ME/CFS and a
criterion for CCC, ICCC and IOM case definitions. Little is known about the
underlying pathophysiology of skeletal muscle abnormalities, including PEM. No
treatment options are available, apart from the advice to patients to avoid
exercise or exercise below an (unknown) threshold.

Due to the heterogeneity of patients, these muscle-related symptoms may vary
dramatically, and is likely multifactorial in nature. Whilst long-term
consequences of hospitalization are known, the widespread incidence of
muscle-related symptoms suggests that skeletal muscle adaptations seen in
non-hospitalized patients with PASC and ME/CFS stand apart from those seen in
critical illness myopathy.

The muscle weakness can range from mild to severe, and such symptoms may
persist long after the viral infection has resolved. The high prevalence of
skeletal muscle-related symptoms hint towards structural and functional
alterations in skeletal muscle in patients with PASC and ME/CFS9-11. In this
study, we aim to study alterations in intracellular skeletal muscle structure
and function and the immune response that will help to explain why patients
with PASC and ME/CFS suffer from post-exertional fatigue.

To obtain insight whether skeletal muscle adaptations can explain
post-exertional fatigue malaise in patients with PASC and ME/CFS, the present
study will address the following objectives:

Primary objectives:
• To determine markers for skeletal muscle structure and function (such as
mitochondrial ultrastructure and function, viral infiltration, metabolite
concentrations, myokine secretion), the immune response and circulating
myokines (muscle-derived molecules), in non-hospitalized patients with PASC,
ME/CFS and healthy controls.

Secondary objectives:
• To determine each of the above variables both before and after induction of
post-exertional malaise, and assess the relationships between the measures
obtained from muscle biopsies and parameters of exercise tolerance.

Case-control study with assessments before and after an induction of
post-exertional malaise. 26 PASC patients, 26 ME/CFS patients and 30 matched
healthy controls who have recovered from a SARS-COV-2 infection will be
recruited for the study. Participants are required to visit the laboratory for
a total of 5 experimental sessions, for a total duration of approximately 8
hours. Four measurements will take place within a time window of 2 weeks. On
day 10, patients will fill in a digital questionnaire. The last time point will
be 1 year after the start of the study. See Figure 1 for an overview of study
design.

26 participants with PASC and 20 patients with ME/CFS (equal split between
sexes) between the ages of 18-65 yrs will be recruited for the study. 30
healthy participants matched for age BMI and preferably physical activity
levels will be included as a control group

Participants will be required to visit the laboratory on 4 separate occasions
over a 2 week period immediately prior to and following a maximal exercise
test. One year (± three months) we will require one additional visit. After the
first visit, participants will be asked to confirm dates for the subsequent
visits over the next 2 weeks face-to-face in the laboratory. Participants will
be instructed not to consume alcohol or perform strenuous exercise within the
24 h preceding each exercise test, and to abstain from caffeine consumption for
at least 3 h.

A muscle biopsy is an invasive procedure, however, the procedure is relatively
simple and poses little risk to the participants (Derry et al., 2009; Edwards
et al., 1983). For one, there is an extremely low risk of an allergic reaction
to the local anesthetic injection. During the needle biopsy (which takes about
5 seconds) participants may experience mild discomfort, such as a sensation of
pressure. The discomfort or experienced muscle stiffness after the needle
biopsy should resolve in one or two days after the procedure and is usually
well controlled with pain relievers. There is a very low risk of internal
bleeding at the biopsy site, which can result in more prolonged pain and
stiffness in the leg. However, subjects routinely start exercising at normal
capacity within one-to-two days after the biopsy. Local bruising and fainting
can occur, although these events occur infrequently. Although there is a very
rare risk (< 0.5%) of damage to a muscle tissue or a small nerve branch, which
could result in partial weakness of the m. vastus lateralis, this would likely
have no impact on daily activities. Nerve injuries usually resolve in 8-12
months, but there is a theoretical risk of mild leg weakness. Furthermore,
there can be a risk of wound infection with the procedure, although this risk
is minimal since needle biopsies are taken under sterile conditions. In most
cases the muscle biopsy will result only in a small scar, which usually
disappears within 12 months. To allow proper healing of the incision and
minimize any risk of infection, subjects should avoid prolonged submersion in
water for 4 days after the biopsy. Also, repeat biopsies are well tolerated and
therefore allow follow-up studies after an intervention (Edwards et al., 1983).
In addition, muscle needle biopsies do not entail detrimental effects or
additional risks on subsequent physical performance as has been shown by
earlier research (Altenburg et al., 2007; Babcock et al., 2012; Stathis et al.,
1994; Withers et al., 1991).