viral studies show that influenza b is more likely than influenza a to cause myositis , likely due to the presence of nb protein in the membrane of influenza b , which is implicated in viral entry and may have myotrophic properties.[24 ] viral myositis is most commonly characterized as sudden onset of muscle weakness , pain and tenderness during the early recovery phase of the virus . symptoms are often isolated to the calf muscles , but other muscle groups are involved in one third of cases . it is typically self - limiting , with recovery within one week of the onset of symptoms , but there are reports of rhabdomyolysis with renal failure and compartment syndrome.[68 ] myositis occurs only in a small percent of those affected with influenza , and is most common in children ( mean age eight years ) , possibly because of virus tropism for immature muscle cells , which has been documented in animal studies . it is associated with mildly to moderately elevated serum creatine kinase ( ck ) level . however , it is unclear whether this is due to direct infection of the muscle by the virus , myotoxic cytokines or other immunologic processes , as the viral agent has not been consistently demonstrated in biopsy . the novel 2009 influenza a ( h1n1 ) virus was first identified in humans in april 2009 in mexico with the first confirmed case in utah on may third , 2009 . the virus rapidly spread throughout the world , and the world health organization ( who ) declared it a pandemic on june 11 , 2009 . epidemic levels of influenza like illness were reported in utah from october 2009 to january 2010 , and h1n1 was the only influenza in circulation in utah during this time . the symptoms include fever , cough , sore throat , runny nose , body aches , headache , chills and fatigue . the predicted prevalence of clinical influenza during the 2009 h1n1 epidemic was 12 - 30% of the population , compared with the typical rate of 5 - 15% for seasonal influenza . influenza associated hospitalizations in utah in 2009 - 2010 were highest in 5 - 24 year - olds followed closely by 25 - 49 year - olds , with 27.2% and 26.8% respectively . here we report three cases of myositis associated with influenza like illness during the pandemic , associated with distal upper extremity involvement . a 37-year old male surgeon presented with three days of progressive muscle pain and weakness . five days prior he had fever , myalgias and sore throat . his fever and sore throat resolved after two days , but he continued to have muscle pain that evolved into diffuse muscle weakness , greatest in his distal upper extremities . he also denied any medical history or medication use prior to his illness , except for ibuprofen three days prior to his presentation . he reported multiple sick contacts , including sick patients and his four - month old son . his forearm muscles and intrinsic muscles of the hand were tight and tender to touch . his neurological examination was notable for 5/5 strength throughout except bilateral wrist flexion and extension ( 4/5 ) , finger flexion ( 3/5 ) , finger extension ( 2/5 ) and thumb adduction ( 3/5 ) . laboratory values were generally unremarkable with the exception of ck ( 1,504 ng / ml ) . pertinent negative virologic tests included rsv by pcr , parvovirus b19 , ebv , csf bacterial culture , hiv , influenza a by pcr , influenza b by pcr , and csf influenza by pcr . ck decreased to 752 ng / ml and serum creatinine remained stable and within normal limits . nerve conduction and needle emg ( proximal , distal and paraspinal muscles ) studies were normal . a few days later he was able to return to work , including performing surgeries . a 25-year - old male presented with four days of progressive weakness and muscle pain . one day prior to presentation he noticed lower extremity weakness and pain , limiting his ability to ambulate . two weeks prior to presentation he had been prescribed amoxicillin for a sore throat and fever , however he was not tested for streptococcus or influenza at that time . he denied prior medical problems , trauma , routine medication use or recent strenuous exercise . strength was decreased bilaterally in upper and lower extremities , including wrist extension and flexion ( 4/5 ) , grip strength ( 4-/5 ) , hip flexion ( 3/5 ) , hip extension ( 5-/5 ) , knee flexion ( 4-/5 ) and knee extension ( 5-/5 ) . laboratory values were generally unremarkable with the exception of a ck of 3,575 ng / ml . pertinent negative virologic tests included csf bacterial culture , hiv , influenza a by dfa , influenza b by dfa , and csf influenza by pcr . needle emg study ( distal , proximal and paraspinal muscles ) showed only one run of positive waves in the thoracic paraspinal muscles . his strength returned to 5/5 except in his left upper extremity strength , which improved to 4+/5 in wrist flexion , wrist extension and finger extension . his ck decreased to 975 ng / ml and serum creatinine remained stable and within normal limits . a 45-year - old male presented with four days of progressive weakness and pain . the weakness in his hand progressed to the point that he was no longer able to perform his job as a cable installer . he also had a fever , cough and sore throat shortly after his wife was sick , but did not seek medical attention . he had otherwise been in his usual state of good health with no medication use or recent trauma . his neurological examination was notable for decreased strength to grade 4+/5 in bilateral upper extremities , hip flexors and hip extenders . laboratory values were generally unremarkable with the exception of a ck of 600 ng / ml . pertinent negative virologic tests included csf bacterial culture , influenza a by pcr , influenza b by pcr , and csf influenza by pcr . the day following admission his strength had returned to 5/5 and the muscle soreness had resolved . his ck declined to 410 ng / ml and serum creatinine remained stable and within normal limits . a 37-year old male surgeon presented with three days of progressive muscle pain and weakness . five days prior he had fever , myalgias and sore throat . his fever and sore throat resolved after two days , but he continued to have muscle pain that evolved into diffuse muscle weakness , greatest in his distal upper extremities . he also denied any medical history or medication use prior to his illness , except for ibuprofen three days prior to his presentation . he reported multiple sick contacts , including sick patients and his four - month old son . his forearm muscles and intrinsic muscles of the hand were tight and tender to touch . his neurological examination was notable for 5/5 strength throughout except bilateral wrist flexion and extension ( 4/5 ) , finger flexion ( 3/5 ) , finger extension ( 2/5 ) and thumb adduction ( 3/5 ) . laboratory values were generally unremarkable with the exception of ck ( 1,504 ng / ml ) . pertinent negative virologic tests included rsv by pcr , parvovirus b19 , ebv , csf bacterial culture , hiv , influenza a by pcr , influenza b by pcr , and csf influenza by pcr . ck decreased to 752 ng / ml and serum creatinine remained stable and within normal limits . nerve conduction and needle emg ( proximal , distal and paraspinal muscles ) studies were normal . a few days later he was able to return to work , including performing surgeries . a 25-year - old male presented with four days of progressive weakness and muscle pain . one day prior to presentation he noticed lower extremity weakness and pain , limiting his ability to ambulate . two weeks prior to presentation he had been prescribed amoxicillin for a sore throat and fever , however he was not tested for streptococcus or influenza at that time . he denied prior medical problems , trauma , routine medication use or recent strenuous exercise . strength was decreased bilaterally in upper and lower extremities , including wrist extension and flexion ( 4/5 ) , grip strength ( 4-/5 ) , hip flexion ( 3/5 ) , hip extension ( 5-/5 ) , knee flexion ( 4-/5 ) and knee extension ( 5-/5 ) . laboratory values were generally unremarkable with the exception of a ck of 3,575 ng / ml . pertinent negative virologic tests included csf bacterial culture , hiv , influenza a by dfa , influenza b by dfa , and csf influenza by pcr . needle emg study ( distal , proximal and paraspinal muscles ) showed only one run of positive waves in the thoracic paraspinal muscles . his strength returned to 5/5 except in his left upper extremity strength , which improved to 4+/5 in wrist flexion , wrist extension and finger extension . his ck decreased to 975 ng / ml and serum creatinine remained stable and within normal limits . the weakness in his hand progressed to the point that he was no longer able to perform his job as a cable installer . he also had a fever , cough and sore throat shortly after his wife was sick , but did not seek medical attention . he had otherwise been in his usual state of good health with no medication use or recent trauma . his neurological examination was notable for decreased strength to grade 4+/5 in bilateral upper extremities , hip flexors and hip extenders . laboratory values were generally unremarkable with the exception of a ck of 600 ng / ml . pertinent negative virologic tests included csf bacterial culture , influenza a by pcr , influenza b by pcr , and csf influenza by pcr . the day following admission his strength had returned to 5/5 and the muscle soreness had resolved . his ck declined to 410 ng / ml and serum creatinine remained stable and within normal limits . these cases have many of the classic features of influenza myositis , including ; acute onset of muscle weakness , pain and tenderness during the recovery phase of the virus , laboratory abnormality of mildly to moderately elevated ck , and complete resolution occurring within one week . however these cases of viral myositis also have several unique characteristics that may be unique to h1n1 ; influenza - related myositis more commonly affects a younger population and typically has isolated bilateral calf involvement , while all of our patients had four extremity involvement , with more distal than proximal upper extremity involvement . while these cases only represent a small portion of those with myositis related to h1n1 , the presentation pattern is stereotyped and notably benign . acute myositis is associated with a large number of viruses , including h1n1.[71618 ] the cases described in this report were not virologically - confirmed influenza , but all met the world health organization ( who ) definition for possible h1n1 influenza - like illness and occurred when h1n1 influenza was highly prevalent in utah during the fall and winter of 2009 . additionally , h1n1 disproportionately affects younger and healthier patients when compared to typical influenza , which is consistent with the cases described here . the median duration of viral shedding by pcr is 5 - 9 days for h1n1 , and given that pcr on serum for influenza was preformed 5 - 21 days after symptom onset , a false negative pcr is not unexpected.[2123 ] according to the cdc , the sensitivity of rapid influenza testing for detecting h1n1 infection is as low as 10% . therefore , a negative rapid influenza result , as in case number two , does not exclude h1n1 infection . additionally , influenza by pcr on csf is insensitive , even in cases of influenza - related encephalopathy or myositis.[2426 ] for these reasons , h1n1 diagnosis during the pandemic was generally based on cdc clinical criteria in geographic regions with a heavy disease burden . according to monto et al . , the positive predictive value of typical viral symptoms ( as demonstrated in all of our cases ) during an influenza epidemic is about 80% . muscle biopsy is not routinely recommended in the workup of viral myositis as a negative muscle biopsy does not exclude viral myositis and was not performed due to the improvement of symptoms in all three cases . given the patient profiles , symptoms , and timing during the peak of the h1n1 pandemic , we postulate that these cases were h1n1-associated viral myositis . recognition of this benign pattern of h1n1 myositis in adults may aid in the cost - effective and appropriate evaluation and management of future patients . these cases also serve as a reminder that influenza - related myositis may have atypical characteristics depending on the strain of influenza , which should be kept in mind in when new outbreaks occur .
acute viral myositis is a rare condition that occurs during the recovery phase of an illness , most commonly influenza . 
 it is characterized by muscle pain and weakness with an isolated laboratory finding of elevated serum creatine kinase ( ck ) . 
 we describe three previously healthy patients who were hospitalized after developing myositis following influenza - like illness during the 2009 influenza a ( h1n1 ) virus pandemic . 
 all experienced myalgias and weakness in all four extremities , including distal upper extremities , associated with an elevated ck level that resolved along with their myalgias and weakness within one week with supportive care . 
 these cases serve as a reminder that influenza - related myositis may have atypical characteristics depending on the strain of influenza , and clinicians should be open to this possibility when new outbreaks occur .