Age / Sex : 43 / F

Age / Sex : 43/F

Chief complaint : Thigh pain (for one month after common cold)

               Muscle weakness(-), non-specific skin rash, not elevated serum creatinine kinase, normal EMG finding

Discussion

Findings: Right Vastus intermedius muscle shows intermediate singal intensity on T1WI,  diffuse homogeneous high signal intensity on T2WI and homogeneous enhancement on Gd-T1WI with synovial thickening and increased amount of joint fluid in right knee joint.

After NSAID drug treatement, muscle edema and synovitis improved.

Differential Diagnosis:

Dermatomyositis, Idiopathic inflammatory myositis

Diagnosis:  

Post-infectious myositis (Influenza-associated myositis, Myalgia Cruris Epidemica, Benign acute myositis)

Discussion:

 After the first description of benign acute myositis (BAM) in 1957, this process has been associated with influenza B virus infection and occasionally with other viruses (influenza A, parainfluenza, adenovirus, and herpes simplex). Post-infectious myositis (PIM) appears to be more common in children than in adults, but its age-specific incidence during influenza epidemics is unknown. As PIM has typically been associated with the influenza B virus, its incidence may depend on the nature of circulating strains during a given epidemic.

 The clinical picture is usually very characteristic: the syndrome is usually preceded by influenza, with upper respiratory symptoms such as fever, malaise, cough, sore throat and rhinorrhoea. The clinical manifestations that follow are of the sudden onset of muscle pain and  tenderness, and a gait disturbance. Serum creatinine kinase (CK), lactate dehydrogenase (LDH), GOT, GPT are elevated with leucopenia, mild neutropenia and lymphopenia. There is a spontaneous resolution of all symptoms and recovery is rapid, usually within 1 week.

 Imaging findings are non-specific and similar to dermatomyositis or other inflammatory muscle diseases: MRI reveals increased water content (edema) within involved muscles. This intramuscular edema is depicited as increased signal intensity on T2-weighted and STIR images. However there are no specific fidings of dermatomyositis such as skin rash (Gottron’s papules, heliotrope rash), muscle weakness and specific EMG findings.

References:

1. Koliou M, Hadjiloizou S, Ourani S, Demonsthenous A, Hadjidemetrious A. A case of benign acute childhood myositis associated with influenza A (H1N1) virus infection. Clin Microbiol Infect. 2010 Feb;16(2):193-5. Epub 2009 Oct 14

2. Belardi C, Roberge R, Kelly M, Serbin S. Myalgia cruris epidemica (benign acute childhood myositis) associated with a Mycoplasma pneumonia infection. Ann Emerg Med. 1987 May;16(5):579-81

3. Agyeman P, Duppenthaler A, Heininger U, Aebi C.Influenza-associated myositis in children. Infection. 2004 Aug;32(4):199-203