What is BACM?

Patient Presentation
A 7-year-old female came to clinic with a history of sudden onset of lower leg pain in the afternoon. She had stayed home the past few days because of an influenza-like illness and had been improving. After a short nap she woke up and complained that her calf muscles hurt and she was crying. She would walk but this also increased the pain. Her mother tried to massage her legs but this made the pain worse. The mother put her in a warm bath and used warm compresses that helped the child some. In clinic about 4 hours after the onset of the symptoms, the mother said that she thought it was very strange because as they were getting ready to come to the office, her pain had gone away. She was walking normally and now denied any pain. Both mother and daughter said that the change was quite dramatic. She had no bowel or bladder problems and her urine continued to be a normal color and amount. She denied any neurological problems including any changes in sensation or mentation. She denied any pain elsewhere in her body. The patient had not traveled and influenza was circulating in the community. The past medical history was positive for pressure equalizing ear tubes at age 3. The family history was negative for any muscle, neurological or kidney disease. The review of systems was positive for rhinorrhea and cough that were both improving. Her last fever was 2 days previous.

The physical examination showed a normal appearing female in no distress. Vital signs were normal including growth parameters of ~75%. HEENT showed clear rhinorrhea. Lungs were clear and abdomen was soft. Palpation of the lower extremities was normal. Vigorous dorsiflexion of the foot did not elicit pain. There was normal range of motion in all lower extremity joints including the toes. No warmth, redness, or swelling was noted. Strength and tone were normal. DTRs were +2/+2 with down going Babinski reflexes bilaterally. There were normal pulses and sensation in the lower extremities. The patient had a normal gait. Her back also had normal range of motion and did not elicit any pain.

The diagnosis of a transient acute myositis was made. The family was counseled that this can occur at times usually due to viruses. They were to continue to monitor her and make sure she kept hydrated. They were also to monitor her urine output and report if there were any changes in the quantity or coloring. “Usually as the virus goes away, so do the muscle pains, but you keep watching her closely and call if things change,” the pediatrician counseled.

Leg pain in children has a broad differential diagnosis and includes more innocuous problems such as growing pains and transient synovitis but also more serious problems including neurological diseases or malignancy. Occult or self-limited trauma are also common causes. A review can be found here. Limping can be painful or painless and has its own differential diagnosis which can overlap with leg pain. A differential diagnosis for limping can be found here. Muscle weakness also has its’ own differential diagnosis which can be found here.

Learning Point
Benign acute childhood myositis (BACM) has other names including viral myositis, acute myositis, influenza-induced myositis, or myalgia cruris eidemica. It is a benign acute myositis that occurs usually during the late winter or early springtime. School age children predominate but the range is ~3-14 years, with males being more common than females. Exact incidence as well as the exact disease mechanism are unknown.

Infectious agents that have been linked to it include influenza A (including H1N1), influenza B (most common overall), adenovirus, coxsackie, dengue, parainfluenza type 1, respiratory syncytial virus and Mycoplasma pneumoniae. The clinical course is that the patient has a viral prodrome for several days and the fever resolves. The patient then has sudden onset of lower extremity (particularly calf muscles) moderate to severe pain with refusal to walk/bear weight. The onset is often after a rest period such as a nap. Patients who are walking will try to minimize the muscle movement and may toe-walk or have a wide-based, stiff-legged gait (i.e. “Frankenstein gait”).

Physical examination shows pain usually in the gastronemius-soleus muscle groups, however rarely other groups can be involved predominantly in the lower extremities and very rarely in the upper extremities. The patient has normal strength, tone, deep tendon reflexes and normal neurological examination including sensory examination of the lower extremities. Laboratory testing is usually not needed but there is an elevated serum creatinine kinase level. Creatinine kinase level does not correlate with symptoms. There may also be elevated aspartate aminotransferase and alanine aminotransferase levels as well as leukopenia.

BACM is benign but Guillain-Barré, osteomyelitis and deep vein thrombosis may present acutely too. Rhabdomyolysis is also a rare complication of BACM and can be reviewed here. Compartment syndrome should be considered in the proper context also. Other problems such as dermatomyositis and muscular dystrophy usually have more chronic symptoms. Polio has not been eradicated from the world and with the appropriate history such as travel to Pakistan, Afghanistan or Nigeria, should also be a consideration.

The natural history of BACM is self-limited with resolution in about 1 week without sequelae. Recurrences are rare. Patients should be treated symptomatically.

Questions for Further Discussion
1. What are indications for a muscle biopsy?
2. What are indications for admission for leg pain?

3. What are indications for radiological imaging for leg pain?

Related Cases

To Learn More
To view pediatric review articles on this topic from the past year check PubMed.

Evidence-based medicine information on this topic can be found at SearchingPediatrics.com, the National Guideline Clearinghouse and the Cochrane Database of Systematic Reviews.

Information prescriptions for patients can be found at MedlinePlus for these topics: Myositis and Muscle Disorders.

To view current news articles on this topic check Google News.

To view images related to this topic check Google Images.

To view videos related to this topic check YouTube Videos.

Jain S, Kolber MR. A stiff-legged gait: benign acute childhood myositis. CMAJ. 2009 Nov 10;181(10):711-3.

Terlizzi V, Improta F, Raia V. Simple diagnosis of benign acute childhood myositis: Lessons from a case report. J Pediatr Neurosci. 2014 Sep-Dec;9(3):280-2.

Rosenberg T, Heitner S, Scolnik D, Levin Ben-Adiva E, Rimon A, Glatstein M. Outcome of Benign Acute Childhood Myositis: The Experience of 2 Large Tertiary Care Pediatric Hospitals. Pediatr Emerg Care. 2016 Aug 20. pp. 1-3.

Cavagnaro S M F, Aird G A, Harwardt R I, Marambio Q CG. Benign acute childhood myositis: Clinical series and literature review. Rev Chil Pediatr. 2017 Apr;88(2):268-274.

Magee H, Goldman RD. Viral myositis in children. Can Fam Physician. 2017 May;63(5):365-368.

Szenborn L, Toczek-Kubicka K, Zaryczanski J, Marchewka-Kowalik M, Miskiewicz K, Kuchar E. Benign Acute Childhood Myositis During Influenza B Outbreak. Adv Exp Med Biol. 2018;1039:29-34.

Donna M. D’Alessandro, MD
Professor of Pediatrics, University of Iowa