There is yet another paper to further elucidate who should get the CT / LP work up for subarachnoids. The authors looked at six EDs over 5 years, and encompassed 2,248 patients – of which 1898 had suitable LPs for analysis (insufficient sample, exposed to light [?], blood contamination, incorrect storage or transport [?], or they just plain lost the sample). CT reads were done by on-call radiologists in training (I imagine that means residents), or by board-certified radiologists. Images were then reviewed by a board-certified neuro-radiologist OR by a board-certified general radiologist for a final report. There was no “time limit” from onset of headache for exclusion (ie, patients did not need to be scanned within six hours).
92 patients of the 1898 patients were positive for blood via spectrophotometry – 4.8%. Nine of these positives ended up having a subarachnoid hemorrhage (9.8% of their LPs, or 0.47% of patients). What do these nine patients generally have in common? Six of them had a headache for a week or longer. One had a previously coiled embolism. One had a negative CT performed within 3 hours of onset. All of these nine patients got coiled or clipped.
So, what do we make of these? If you present within 6 hours with a negative CT (or, dare I say, less than a week!), you have a 0.053% of having a subarachnoid. If you still do the LP in those <6hours from onset (or, <1 week from onset), the baseline risk of a false positive is 10% vs 1.1% chance of true SAH.
Time for some well-informed shared decision making, and perhaps a higher threshold for those that present with a prolonged headache. Further good news to take away from this study- it does not seem as neuro-radiologists need to make the call and that the <6 hour proposal can be safely extended to the community setting at this point.
(First link is to free text of article. Pubmed did not have a working link to the paper at time of post. If the first link does not work, try here)