S. aureus bacteraemia is a serious infection with a mortality of at least 20%; that is on average 20 in 100 people with S. aureus in their blood will die despite what we do to try and help. That’s a lot! And it gets even worse if you can’t find the cause of the bacteraemia, up to 60%. The mortality for S. aureus from an unknown source is so high for lots of reasons; patients are often frail or have underlying immunodeficiencies, but also because if you can’t find the “focus” you can’t easily undertake “source control”. If you can’t control the source, then the infection will persist…! Your patient might not!!!!
So, one of the key parts to dealing with S. aureus bacteraemia is finding the source or focus of infection so you can deal with it.
NOTE: Don’t forget to treat the patient whilst looking for the source. S. aureus is a serious pathogen, so don’t wait for your investigations or results… treat first and then ask the questions!
In my experience the most common causes of S. aureus bacteraemia fall into 4 categories… it is almost a mantra to me… skin and soft tissue, bone and joint, heart and finally intravascular devices.
- Skin and soft tissue – the most common infections caused by S. aureus are cellulitis and abscesses. Cellulitis is a bit of an odd one as not many people with cellulitis are bacteraemic, but cellulitis is so common that even though not many of these patients have positive blood cultures they still make up a big chunk of all S. aureus bacteraemias. Deep abscesses are a common cause of bacteraemia, especially abscesses in muscles e.g. psoas, known as pyomyositis.
- Bone and joint – septic arthritis, of both native and prosthetic joints, and osteomyelitis, are common causes of S. aureus bacteraemia and this can include “less obvious” bones and joints such as in the spine.
- Heart – approximately 10% of patients with S. aureus in their blood have infective endocarditis (infection of a heart valve), and this can overlap with the other categories here in that they can have both infective endocarditis and septic arthritis for example because the heart infection has led to the joint becoming infected through spread via the blood stream.
- Intravascular devices – S. aureus loves to stick to bits of plastic and other prosthetic materials. It is particularly fond of cannulas and central venous access catheters such as Hickman lines, Groshong lines and Portacaths, but any intravascular material will do.
In order to find the source of an infection you need to take a history from the patient, perform an examination and order some investigations. In my experience over 90% of the diagnosis is made on the history; the examination and investigations are there to prove your diagnosis and aid treatment.
This is the key skill of a doctor. Talk to the patient, let them tell you their tale, ask questions to clarify points, but let the story unfold.
Does the patient have any red areas on their skin, any new lumps or bumps, any painful lesions? Have they had any recent injections? Have they recently over exercised or “pulled” a muscle; pyomyositis usually occurs a few days after damaging a muscle even if that damage at first seems minor e.g. going for a long walk or straining to reach something on a high shelf. Skin and soft tissue infections like cellulitis hurt so the patient can usually pinpoint where that pain is coming from.
Can the patient move all of their joints? Have they had any operations to replace joints? Have they had any medicines injected into joints recently like steroids? Have they got back pain, has it recently got worse? Have they got any swollen joints? Again, infections of bones and joints are painful, and patients will be able to tell you this.
Does the patient already have a problem with their heart? Do they know they have abnormal heart valves that might allow bacteria to stick to them e.g. congenital valve abnormalities? Have they been getting increasingly short of breath when walking? Do they get short of breath lying down or wake at night short of breath? These might be symptoms of a heart valve that is being damaged by infection.
Does the patient have ANY prosthetic material in their body? You won’t believe the amount of times I’m told no prosthetic material only to pull up an x-ray and see a blindingly white “medically inserted foreign body”. Do they have cannulas or IV catheters? Do they have grafts in blood vessels? If they have cannulas and catheters do these hurt, is there any redness around the insertion site, is there any pus coming out of or around the device? Again, these types of infections are usually painful.
So, taking a history involves seeing the patient, not just reading the notes from someone else’s history taking! If there is nothing obvious from the oral history, or if the history has thrown up something that needs further investigation, we move on to examining the patient.
The first thing to do in the examination is confirm the history. If the patient says they have a red patch of skin, take a look at it. If they have a painful joint, look at it! If there back hurts, look at it…!! you get the point. Confirm the story and see if it is enough to make the patient unwell.
If the history gave you no clues, then systematically examine the patient paying particular attention to:
- Skin and soft tissue– look at ALL of the skin, even if that means having to ask the patient to remove all their clothes down to their underwear. Make sure they can flex their hips without pain to rule out a psoas muscle abscess.
- Bone and joint – test functional movement such as walking, putting on shoes or reaching behind their back, gently press on the spine to look for tenderness, look for any swelling of the joints. Be thorough.
- Heart – look for signs of heart failure such as a high jugular venous pulse in the neck, oedema of the ankles and legs, crackles in the chest. Listen to the heart for murmurs which might suggest an abnormal heart valve. Look for evidence of clots flying from the heart to other parts of the body such as splinter haemorrhages (black lines on the nails… these can be normal so stop looking at your own fingers!) and painful lumps on the hands and feet (Janeway lesions). Look for evidence of immune mediated damage to body tissues, painless lumps on the hands and feet (Osler’s nodes) and retinal haemorrhages (Roth’s spots).
- Intravascular devices – look at all IV devices that you can see; making sure they are clean, painless and not red.
If the source is still unclear, or if you have a strong suspicion of a focus and need to prove it, then “targeted investigations” may be required.
The investigations can be split into three main groups: laboratory, cardiology, radiology.
Repeat the blood cultures at 24-48 hourly intervals to make sure the bacteraemia is being controlled. Not only is uncontrolled bacteraemia associated with much higher mortality, but ongoing bacteraemia can be a feature of infective endocarditis as the infection is in the blood stream itself and so the blood is hard to sterilise. Also do paired blood cultures from any lines plus a peripheral culture if a line is thought to be the source; if the line culture is positive >2 hours before the peripheral this proves the line is the source.
Get a urine sample and look for blood. This is a soft sign for renal damage due to glomerulonephritis and is a common finding in infective endocarditis.
Aspirate or biopsy any joints or bone that might be infected and culture the fluid to look for bacteria. Positive Gram films and cultures prove the diagnosis of septic arthritis or osteomyelitis.
Echocardiography is an ultrasound technique that looks at the structure and function of the heart. Every patient with S. aureus bacteraemia should really have an echo to rule out endocarditis, whatever the other possible focus of infection. This is because endocarditis in this group is relatively common, and without the correct treatment (prolonged antibiotics +/- surgery) it is almost always fatal; remember patients can have endocarditis in addition to their other focus of infection.
If the patient has any symptoms or signs suggesting a pyomyositis, bone or joint infection then TALK to a radiologist about the best way to investigate. Radiologists are the experts in these types of investigations, and they will do the right test for your patient; DO NOT tell them what you want, ask them what is best.
The radiologist might suggest a CT scan, MRI scan or even a PET CT scan to look for a focus. Take their advice.
I’m not a Radiologist but in my experience the types of tests they might suggest are:
- CT scan – to look for evidence of emboli to the kidneys and spleen or brain in endocarditis
- MRI scan – to look for pyomyositis or bone infections including the spine
- PET CT – when all else fails and you can’t find a focus; this can be done to look at the heart as well as other parts of the body, however very few hospitals have them and the waiting times are long (we have to send our patients to another hospital and the waiting list is usually about 2 weeks!)
If at the end of all of this, you still haven’t found the source then you will have to treat “blind” with AT LEAST 2 weeks of antibiotics. Two weeks is the MINIMUM course, anything less is associated with high relapse rates and higher mortality. And after treatment make sure the patient knows to come back if they feel unwell again… if they do then repeat the above, hopefully something will be found second time around….
Now although I seemed to have discovered my “focus” again…I probably still need to book an eye test, I can’t keep looking under my glasses to read patient notes! What was that Editor Chief in Charge, “am I just getting old – presbyopia, you say!”…well… you are closer to 50 than me…!!!!