This case is inspired by a Twitter conversation started by Dr Brent May ( @DocBrent ) – he is an Anaesthetist and motorsport doc in Australia.
Brent asked the simple question: “SAH Q – no CT available. What is the “usefulness” of RBC count on LP? Lab “unable to do xanthochromia”. This is a real problem in many hospitals – especially the smaller places with no CT or specific lab crew. So lets look at a case and try to get to the bottom of this one!
You are working in a remote clinic with a small Emergency area. It is a slow day. You are 400 km from the closest hospital which has a CT and laboratory services.
Jan is a 45 year old local woman. She presents with a bad headache – this is not usual for her, she is not usually a “headachy” person. No PMHx of migraine or other headache syndrome. It woke her from sleep this AM – about 4 hours ago. Maximal intensity from the onset. She says it has not responded to simple analgesia and resting at home. She has had no syncope or neurological symptoms. The headache has remained relatively constant. She is complaining of neck stiffness – though has normal ROM in her neck. She has no fever, vomiting or photophobia.
On examination she has no neurological signs. Her obs are all normal, BP = 140/85, afebrile. You cannot reproduce any objective neck stiffness.
So – first question. Based on that clinical info – what would you estimate Jan’s “probability” of having a subarachnoid haemorrhage ? Have a Gestalt guess… 2%, 5%, 10%, 50 %???
How low would you want your “probability” to be to avoid a transfer and subsequent work-up? i.e.. what is your lower test threshold for SAH? 1:100, 1: 1000, less?
Here is a copy of Perry & Steill’s landmark paper in the BMJ 2011 – check out tables 1 & 2. Jan is pretty much a “average Jan” for the cohort of 3000 pts in their paper.
About 7.7 % ended up having a subarachnoid bleed – and another 2 – 3 % had another non-benign cause for their headache. So 10% is a reasonable “pretest probability” for badness in Jan’s case.
Now a 10% risk of serious intracranial pathology. That would mandate a work up in most places. Unfortunately your hospital only has rudimentary lab gear – you can do an LP and get red cell counts, but not xanthochromia. A CT would be helpful if you can do it in less than 6 hours – but even if you called for transfer now – it would be well past 6 hours by the time Jan made it into the doughnut 400 km away.
So here we are. You can do an LP and get red cell counts on the CSF. Xanthochromia is probably not helpful as it is early (less than 12 hours) and simple visual inspection for yellow change is inaccurate. [Best read by non-colour blind female lab techs apparently! Can you ask for that on the forms?] Most Australian labs use automated spectroscopy to analyse for xanthochromia – and this is usually only available in larger, tertiary labs.
The accepted gold standard diagnostic features of SAH are variable – but the Canadian group who do a heap of research use the following:
“Subarachnoid haemorrhage was defined by:-
– subarachnoid blood on unenhanced computed tomography of the head, OR
– xanthochromia in the cerebrospinal fluid, OR
– red blood cells (>5×106/l) in the final sample of cerebrospinal fluid,
PLUS an aneurysm or arteriovenous malformation evident on cerebral angiography.”
So now the big question. Is there any way that you can effectively “rule out” SAH in Jan without transferring her 400 km to the next hospital?
Does a low CSF red cell count allow us to “exclude” SAH? Can we use any other tests to decrease the risk for Jan?
Tough yet common problem in many rural areas in Australia. I would love to hear your thought and comments below. Does it depend on the timing? Can we watch and wait if the initial LP is clear? Or do we need to get imaging and a review for xanthochromia on all the Jan’s we see in ED?
If you are interested in the diagnostic dilemma that is the “SAH work up” then I highly recommend listening to David Newman’s SMART EM episode on the topic from way back in 2010. (WARNING: it is nearly 2 hours of podcast!)
Let me know what you think