Post-streptococcal myalgia and myositis are extremely rare autoimmune phenomena occurring approximately two weeks (median 13 days, range 5–42 days) after upper respiratory tract infections with group A β-haemolytic streptococci [3].
Until today only 26 scientific reports about post-streptococcal myalgia and myositis exist and the pathogenesis remains largely unclear [3, 9,10,11]. The streptococcal M-Protein shares structural similarities with soft tissue antigens, such as myosin, and therefore antibody-mediated cross-reactivity or molecular mimicry is hypothesized [2, 12]. PSM occurs in children and young adults and most often affects the limbs, though several reports on isolated ocular muscle affection (7 out of 26 reported cases) exist [3, 9,10,11]. Among patients with PSM myalgia seems more frequent than myositis.
The diagnostic approach to PSM includes an accurate medical history and physical examination and an awareness for other post-streptococcal complications such as glomerulonephritis.
In most patients elevated acute phase reactants such as CRP or erythrocyte sedimentation rates were detected along with elevated ASL antibodies. Elevated Creatine kinase suggesting muscle damage only has been reported in one other case with recurrent episodes of incapacitating myalgia of limbs [3].
MRI is the most important modality for evaluation of myositis, including orbital muscle affection [13, 14]. In three patients with PSM an MRI of the affected muscles was performed. Findings are described as a “diffuse edema” of the affected muscles and “fluid between the muscular fasciae in STIR and fat-saturated gadolinium enhanced T1-weighted sequences” [11], “patchy muscular and fascial edema” [15] and “signs of myositis that were restricted to the vastus medialis of the left rectus femoris muscle” [3]. In summary, the mentioned MRI changes of PSM are all consistent with inflammatory myositis, as is the MRI of our patient [7, 13]. Until now, there seem to be no distinctive characteristics that allow discrimination from other inflammatory myopathies.
Our patient shows typical features of PSM, which are: age, the acute onset of muscular symptoms one week after an upper respiratory tract infection with streptococci, the elevated CRP and ASL antibodies as well as the affected muscles and the rapid therapeutic response. The elevated CK indicates a more severe course of disease.
We obtained unique images in the muscular ultrasound of the left thigh, considering that no scientific reports on ultrasonographic appearances of PSM exist.
The domain of muscular ultrasound in general lies in diagnosing neuromuscular disorders whereas its role in inflammatory myositis is not yet established. General acute inflammatory myositis usually presents with increased muscle echogenicity and with progression into chronic phase muscle thickness decreases [16]. The focal hyperechogenicity and loss of regular architecture within the muscle is most likely related to an infiltration by inflammatory cells and edema between the muscle fibres [17]. Although these changes are non-specific, focal changes as seen in this case can be considered a sonographic sign of muscle tissue pathology and, given their fluctuance, are a suitable tool for follow-up exams [6]. Our findings are therefore consistent with acute focal inflammatory myositis, in this case caused by an autoimmune response after a streptococcal infection [7, 8].
Since ultrasound is widely available, it may prove helpful for the diagnosis of this probably under-recognised disorder, along with clinical and laboratory findings. More case studies of such rare occurrences are needed to gain more information for diagnostic and therapeutic purposes.
PSM has a favourable prognosis, and most patients usually fully recovery within one to four weeks, such as this patient [3]. Treatment options include antibiotics, non-steroidal anti-inflammatory drugs and prednisolone [3].
In summary, post-streptococcal myalgia and myositis are rare occurrences following streptococcal infections. Muscular ultrasound may be of value in identifying inflammatory changes in affected muscles and in diagnosing such uncommon disorders as post-streptococcal myositis.
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