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⚃ 1.2.4.1 Introduction:
⚃ 1.2.4.2 Myositis-specific auto-antibodies (MSAs):
⚃ 1.2.4.1 Introduction:
⚄ 1.2.4.1.1 Muscle diseases have traditionally been put
under the umbrella of
myopathy:
a disease of the muscle that causes weakness of the muscle (
not a disease involving the nerves
).
≻ As discoveries are made, the classification evolves, but there is
no currently accepted international classification system for muscle
diseases.
≻ As a group, these diseases are sometimes referred to as muscular
dystrophy diseases.
≻ Inclusion body myositis is usually listed under the umbrella of
muscular dystrophy diseases.
≻ A second Umbrella is the idiopathic inflammatory myopathies (IIM):
a group of conditions characterised by inflammation of muscles (myositis)
and other body systems.
≻≻ This is where IBM has traditionally been classified along
with polymyositis (PM) and dermatomyositis (DM), but the features of IBM
are
clearly distinct
from these other two types.
Figure 1.2.4.1.1a
.
Figure 1.2.4.1.1b
⚄ 1.2.4.1.2 Myositis versus myopathy.
Figure 1.2.4.1.2
⚃ Notes on Figure 1.2.4.1.2
⚄ IBM is an abbreviation for 'inclusion body myositis' not
'inclusion body myopathy' (Greenberg, 2019, Milone, 2017).
≻ The abbreviation IBM
should
refer clearly and only to the single disease inclusion body myositis, not
to the hereditary inclusion body myopathies (“hIBMs”).
≻ On this page, IBM will only refer to inclusion body myositis and I
will not specify sporadic.
⚄ The hereditary, myopathy diseases are clearly distinct from inclusion body myositis: these are different diseases.
⚄ Lilleker et al. (2024) recommend in the future researchers
and doctors only use “IBM” to specify inclusion body myositis,
and drop the terms “sporadic” and “familial.”
≻ Further, “hIBM” should be dropped and the names of
each specific disease should be used e.g. Myofibrillary myopathy; GNE
myopathy; UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase;
MSP-1; MSP-2; etc..
.
⚃ 1.2.4.2 Myositis-specific auto-antibodies
(MSAs):
The classification of muscle diseases has been evolving over time as more
and more research discoveries are made.
≻ Different diseases are being recognized and categories adjusted.
≻ Muscle diseases have been classified mainly based on their
clinical presentation.
≻ Today, the recognition of antibodies associated with different
muscle diseases has impacted classification.
≻ “A major advancement in the field of myositis was the
discovery of auto-antibodies that are
specific for myositis,
called
myositis-specific auto-antibodies (MSAs);
(present in up to 60% of patients with IIM), which are helpful in
establishing a diagnosis of IIM (Lundberg et al., 2021).
≻ Other patients may have auto-antibodies that are also present in
other autoimmune disorders, such as systemic lupus erythematosus (SLE),
systemic sclerosis, or Sjögren syndrome; these auto-antibodies are named
myositis-associated auto-antibodies (MAAs).
Figure 1.2.4.2.1a
Figure from Ashton, C., Paramalingam, S., Stevenson, B., Brusch, A., & Needham, M. (2021). Idiopathic inflammatory myopathies: A review. Internal Medicine Journal, 51 (6), 845-852. Source.
.
Figure 1.2.4.2.1b
Figure 1.2.4.2.1b is based on: Mariampillai, K., Granger, B., Amelin, D., Guiguet, M., Hachulla, E., Maurier, F., … Benveniste, O. (2018). Development of a new classification system for idiopathic inflammatory myopathies based on clinical manifestations and myositis-specific autoantibodies. JAMA Neurology, 75 (12), 1528. JAMA Neurol. 2018;75(12):1528-1537. doi:10.1001/jamaneurol.2018.2598
Notes on Figure 1.2.4.2.1b
^ trigger: statin use
* trigger: malignancy
**
CN1A [Anti-NT5c1A] – [anti-cN1A]:
Found in approximately 50 to 60% of IBM patients. Also found in about 25
percent of patients with juvenile myositis and in about 12 percent of
healthy children (Yeker et al., 2018).
≻ According to
Greenberg (2019),
"Anti-cN1A antibodies
are
highly specific to IBM
and are seen in 90-95% of patients with IBM compared with 5-10% of
patients with polymyositis, dermatomyositis or non-immune neuromuscular
diseases."
≻ However, these antibodies have
only moderate diagnostic sensitivity,
ranging from 37% to 76%.
≻ Varying sensitivities may be related to the different methods of
testing that have been used.
≻ In other words, these antibodies are very specific to IBM –
if you have them, you probably have IBM but, they are not that sensitive
to IBM – only about 50 to 60% of IBM patients will have these
antibodies – if you
do not
have them, you may still have IBM.
⚄ Anti-NT5C1A autoantibodies were detected in 71 (61%) of 117 patients with IBM, 2 (5%) of 42 patients with PM, 2 (5%) of 42 healthy volunteers, 24 (15%) of 159 patients with DM, 10 (23%) of 44 patients with Sjögren's syndrome, and 13 (14%) of 96 patients with systemic lupus erythematosus" (Lloyd et al., 2016).
⚄
Anti-cN-1A autoantibodies
are demonstrated in one third of the patients with sIBM and in less than
5% with other IIMs or neuromuscular diseases.
≻ A recent study demonstrated that positive
anti-cN-1A
sIBM patients are included in a more severe sIBM subtype and represent a
homogeneous group as exhibiting higher mortality risk, less proximal upper
limb weakness (not typical of sIBMs) and a cytochrome oxidase deficiency
in muscular fibers, when compared to negative patients" (Palterer et
al., 2018).
⚄ For information on myositis-associated autoantibodies see: https://understandingmyositis.org/myositis-antibody-testing/
.