The following day the blood cultures were positive for a Staphylococcus aureus so the Microbiologist went to review the patient on the ward. In the notes was the result of the ultrasound scan of the hip which was normal with no effusion. Speaking to the patient she had taken out a new gym membership and started exercising in order to lose weight. The patient remembered “pulling a leg muscle” a week ago. The clinical team had been planning to stop her antibiotics and send her home with analgesia for a musculoskeletal pain but the Microbiologist was not so sure it was this simple, it’s that Staphylococcus aureus blood culture result you see...
So what was going on? Why was the patient in pain despite a normal hip? Was it just a minor musculoskeletal injury or was the Staphylococcus aureus relevant?
The history of recent new exercise, a high fever, pain on flexion of the hip and the presence of a Staphylococcus aureus in the blood culture all suggest the presence of an abscess in the psoas muscle. The pain is localised to the hip, and made much worse when the psoas muscle is used (my anatomy is not great but I do know that this is the main hip flexor muscle and that if it is inflamed then moving it will hurt!)
The normal sequence of events leading to a psoas abscess is that there is mild trauma to the psoas muscle at a time when the patient is or becomes bacteraemic with Staphylococcus aureus (perhaps from a minor abrasion or cut to the skin). The mild trauma is often due to someone starting exercising when they don’t normally do so (as with this patient), or someone who normally exercises and then suddenly “over exercises”. As blood enters the damaged muscle it takes the bacterium with it; the bacterium stays in the muscle and develops into an abscess. The psoas muscle is a big muscle with a rich blood supply so the infection usually causes systemic symptoms e.g. fever, sweats and rigors as well as a bacteraemia.
Antibiotics are only part of the treatment of a psoas abscess. If the abscess can be drained then it should be. This is the same principal as with any abscess and the old adage “if there is pus about let it out” still applies.
The choice of empirical antibiotic should be targeted at the most common cause of psoas abscess, Staphylococcus aureus, such as IV Flucloxacillin 2g QDS. If the patient is allergic to beta-lactams or is known to be colonised with Meticillin Resistant Staphylococcus aureus (MRSA) then they should be started on a glycopeptide antibiotic such as IV Teicoplanin or Vancomycin. Patients with a psoas abscess often need a long course of antibiotics. I would normally give 2-4 weeks of IV antibiotics followed by an oral agent based upon sensitivity testing, making a total of 6 weeks.
During the treatment I would recommend at least twice weekly white blood cell (WBC) counts and C-reactive protein (CRP) to monitor progress. If these markers do not settle quickly I would suggest further surgical or radiological exploration and drainage of pus.
“If there is pus about let it out”
Why should abscesses be drained? Surely if the patient is on the correct antibiotics then they will get better anyway won’t they? Not always! There are three very good reasons to drain abscesses:
- To reduce the number of bacteria present – the less bacteria present the more chance that the antibiotics will be able to overcome the remaining microorganisms; if there are too many bacteria present the antibiotics will just not be able to overpower them all
- Many antibiotics do not work well in the acidic environment of abscesses - antibiotics such as the beta-lactams are broken down by acids making them inactive. In addition some antibiotics e.g. Gentamicin, actually rely on the difference in electrical charge across the cell membranes or cell walls of bacteria compared to their outside environment and therefore these antibiotics do not work when the charge outside the bacteria is altered by the presence of acid
- Speed up recovery – drainage of the abscess reduces the bacterial burden, improving both the immune response and antibiotic efficiency, thereby speeding up the patients recovery
Many abscesses are now drained under radiological guidance, with Radiologists directly visualising the abscess using ultrasound or CT scans and carrying out the procedure under local anaesthetic. This has greatly improved patient management eliminating the need for a general anaesthetic and an open operation in order to wash out the pus.
So this patient underwent radiological drainage of her psoas abscess. The pus grew the same Staphylococcus aureus as in her blood cultures. She was treated with 2 weeks of IV Flucloxacillin and then changed to oral Flucloxacillin; 6 weeks of antibiotics in total. She made a full recovery from her infection… but I’ve no idea if she continued her gym membership!