Tropical Pyomyositis Diagnosis by a Modern Medical Imaging – A Case Report and Review Of Literature
MRI: Magnetic Resonance Imaging;
AST: Aspatate Transaminase;
MRSA: Methiciliin Resistant Staphylococcus aureus;
VISA: Vancomycin Intermediate Staphylococcus aureus
A 53 year old diabetic man presented to the general surgical unit with intermittent fever and left lower limb pain of two weeks duration. Pain was of non mechanical type and there was no history of preceding trauma. On examination he was febrile and a swelling was noted in the medial and posterior aspect of the left thigh affecting the adductor muscle group, with overlying skin induration. Regional lymph nodes were not enlarged. His initial blood counts were within normal limits but the ESR and CRP were high. Liver function tests were within the normal range except for a marginally elevated Aspatate Transaminase (AST) level. An initial diagnosis of thrombophlebitis was made and the patient was treated with non steroidal anti inflammatory drugs. However the patient continued to be symptomatic, with increasing swelling of the thigh. A duplex scan of the thigh was performed which excluded deep vein thrombosis. Ultra sound scan of the abdomen was performed to exclude a psoas deep seated abscess. The patient continued to deteriorate despite antibiotic treatment. He rapidly progressed to a state of severs sepsis with oliguric renal failure requiring haemodialysis. An MRI scan of the left thigh was then performed which showed a multiloculated area of low signal intensity on T1 weighted images and uniform high signal density on T2 weighted images, with Gadolinium enhancement in the periphery, affecting the Left adductor group of muscles including adductor magnus and extending into hamstrings. Underlying bone was normal (Figure 1 and 2). A provisional diagnosis of tropical pyomyositis with systemic sepsis was made. Treatment with intra venous cloxacillin was started after obtaining blood cultures. Incision and drainage of the abscess was performed under general anesthesia, and pus as well as tissue biopsies were sent for culture.
Patient’s condition improved steadily following drainage and institution of antibiotics. Pus and blood cultures failed to grow any organism, although tissue biopsy culture became positive for Staphylococcus
Figure 1 and 2 Magnetic Resonance (MR) images of the left thigh showing the extent of the multiloculated area suggestive of pyomyositis.
Tropical pyomyositis is characterized by single or multiple micro abscesses within a large skeletal muscle group of the body. Although initially described in the tropics, increasing number of cases have been reported from the temperate climates as well [2, 3]. Due to its prevalence in the tropics [13,14], it is also known as Myositis tropicans, tropical skeletal muscle abscess, or tropical myositis. This is illustrated by the fact that, in certain tropical countries, pyomyositis accounts for nearly four percent of hospital admissions [15, 16]. Recent data suggests that up to 75% of topical pyomyositis patients have some form of underlying immunosuppression such as diabetes, leukemia or asplenia . Vigorous exercise or trauma has also been implicated as a predisposing factor for tropical pyomyositis [18,19].
Clinically pyomyositis commonly presents as a swelling in large skeletal muscle groups such as quadriceps, glutei, pectoralis major, serratus anterior, biceps, iliopsoas, gastrocnemius, abdominal and spinal muscles , as was the case with our patient. Unusual locations such as hip abductors and obturator externus muscle  has also been reported. It is more common among males. Usually a single muscle group is involved but on occasion multiple muscle groups are affected simultaneously or contiguously [14, 16]. Clinical picture is characterized by three stages- invasive, suppurative and late stage.
The Invasive stage is characterized by variable fever, painful and firm swelling of a muscle group with or without erythema, and minimal systemic symptoms. As the inflammatory process is diffuse, aspiration at this stage does not reveal any pus. High spiking fever with systemic signs herald the onset of the suppurative stage. Even though pus may be aspirated at this stage, classical signs of an abscess may be absent due to the overlying tense muscle. As the disease progresses, the late stage is characterized by systemic sepsis with acute renal failure and sometimes complicated by metastatic abscess formation. Deviating from the classical presentation pyomyositis has also been noted to present with acute spinal cord compression , or even cervicobrachial neuralgia .
Mainstay of the diagnosis is high degree of clinical suspicion supported by positive blood culture, and later confirmed with a pus culture, as well as tissue biopsy for culture and sensitivity testing. Blood cultures may be positive in 5%–10% of the cases in the tropics to 20%–30% in temperate regions [23, 24]. The muscle enzymes Creatine phospo Kinase, Aldolase, and Aminotransferase may be normal or only slightly elevated, despite extensive muscle involvement [25,26].
Commonest cause of the abscesses in pyomyositis is staphylococcus- 90% in tropics and 75% in temperate climates . However Pneumococcus, Neisseria, Haemophilus, Aeromonas, Serratia, Yersinia, Pseudomonas, Klebsiella, and Escherichiahave all been uncommonly implicated [3,27]. Rarely, Salmonella, Citrobacter, Fusobacterium, Anaerobes, and Mycobacteriumhave been implicated [28-31].
Pus cultures are negative in around 30% of cases in the tropics and similarly, 90-95% patients have sterile blood cultures[ 23, 27].In temperate countries up to 30% of blood cultures may become positive .
Interestingly, skeletal muscle is intrinsically resistant to microbial proliferation under normal circumstances, as evidenced by the very low incidence of abscess in muscles in autopsy proven cases of Staphylococcal septicaemia . Skeletal muscle becomes especially vulnerable, when it is traumatized or following vigorous exercise, and this is evidenced by the fact that 20 to 50% of pyomyositis cases have a preceding history of blunt trauma or vigorous exercise. Some studies have revealed that in susceptible individuals, neutrophils are not adequately primed against staphylococcal during the course of infection. Intravenous drug abuse is also a common risk factor for developing pyomyositis [36,37]. It is increasingly being reported among patients with HIV [37,38].
Due to its nonspecific presenting features, the differential diagnosis for pyomyositis is wide, and includes muscle contusion, osteomyelitis septic arthritis, cellulitis, muscle haematoma, deep vein thrombosis, muscle rupture, leptospirosis and polymyositis.
Despite advances in treatment and diagnosis, mortality of pyomyositis remains around 0.5% to 2% . Early and appropriate use of medical imaging aid differentiating among the extremely broad differential diagnosis for this uncommon albeit lethal condition, reducing morbidity and mortality.
Although MRI is considered the gold standard of diagnosing pyomyositis , dynamic compression ultrasound used at the bedside or emergency department might demonstrate loculated collection of pus, facilitating early diagnosis and institution of prompt treatment . It is suggested that ultrasound is best utilized during the purulent stage of pyomyositis, where it may demonstrate diffuse muscle hyperechogenicity and diffuse hyperemia [40,41]. However the reader must be cautioned that despite the presence of fluid,intramuscular abscess may not exhibit the more typical ultrasound features seen in superficial collections, necessitating dynamic compression ultrasound for accurate diagnosis .
Hyper intense signal in the muscles of concern on T2-weighted images is thought to correlate with a MRI diagnosis of pyomyositis [42-44].
Once the diagnosis is established, prompt surgical debridement as well as aggressive antimicrobial therapy is essential, eg;Cloxacillin. For Methiciliin Resistant Staphylococcus aureus (MRSA) organisms, vancomycin or teicoplanin are suitable alternatives. For Vancomycin Intermediate Staphylococcus aureus (VISA), Linezolid is a suitable alternative. Treatment should continue until the wound is clean, the leukocyte count becomes normal, or the patient is afebrile for 7–10 days. If the patient is a nasal carrier of staphylococci, eradication with topical Mupirocin or oral Rifampacin is recommended .
We believe that increased awareness with early appropriate medical imaging led prompt diagnosis and treatment are the cornerstones for reducing the mortality from this rare albeit lethal infection.
Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor- in-Chief of this journal.
The authors declare that they have no competing interests.
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