NEUROLOGY / NEUROSURGERY Flashcards
CT for SAH
Diagnostic sensitivity and time since symptom onset
•90 - 96% sensitive overall
•< 6 hours - reported as 100%
- has a 95% chance of being > 98%
- based on consultant radiologist reporting
- specificity > 99.5%
•< 12 hours 98-100%
-95% chance of being > 94%
•< 24 hours 92-95%
•< 48 hours 80-86%
•> 72 hours 75%
•> 1 week 50%
•therefore a normal result after 6 hours may not exclude diagnosis
Decreased sensitivity when
- few clinical signs present
- small aneurysm
- Hb < 10 g/dL
- inexperienced radiologist
-approximately half of ‘missed’ SAH on CT is detected by more experienced radiologists
LP for SAH
Lumbar puncture
Investigation without CT
•CT generally recommended prior to LP in patients being evaluated for possible SAH
-however neuroimaging may not be immediately available
•ACEP and British Society of Neurological Surgeons believe LP may be performed without prior CT when
- normal mentation present
- no neck stiffness
- no focal findings or signs or raised ICP
- CT not readily available
LP not indicated
•in normal risk patients if a high quality CT scan has been performed within 6 hours of symptom onset and reported as negative
- approximately 2000 LPs would need to be performed to detect a single true positive result
- not performing LP in scans obtained up to 12 hours is also reasonable
LP may be indicated
•very high risk patients (previous SAH, PCK, multiple relatives with SAH)
- the risk of harm from LP is estimated to be greater than the potential benefit in suspected SAH unless the post CT probability of SAH is > 4%
- this may apply to a patient with a much higher than average pre CT probability of SAH (normally 5-10% pre CT)
- CT performed > 6-12 hours after symptom onset
- if other diagnoses diagnosed by LP are being considered
- meningitis
- benign intracranial hypertension
•LP may not detect other potentially serious causes of headache, e.g., carotid / vertebral dissection
Blood
- > 100,000 RBCs indicative of SAH
- > 10,000 RBCs
- present in > 90% of cases
- LR+ 6
- > 2,000 RBCs and/or visual xanthochromia is close to 100% sensitive
- < 100 RBCs rare
- < 65% change in counts between 1st and 3rd tube LR+ 3.6
- > 65% change in counts LR- 0.10
- however differentiation from a traumatic tap may be extremely difficult
Traumatic tap
- traumatic tap occurs in 20% of LP’s
- traumatic tap and SAH may co-exist
- none of the following criteria are 100% accurate
- normal CSF pressure
- clearing of blood in tubes 1-3 / marked decrease in RBC count in tubes 1 to 3
- proportion of white cells to red cells the same as in blood - i.e. RBC:WBC = 500 - 1,000:1
- clot formation
- absence of xanthochromia
Xanthochromia
- xanthochromia should not occur in a bloody tap as the blood has not been exposed to the enzymes in the CSF that breakdown Hb
- spectrophotometry for detection of CSF bilirubin is slightly more sensitive than visual inspection (85% vs 70%)
- concerns regarding low specificity of spectrophotometry
- specificity as low as 35% in one series
- specificity is approximately 85% for spectrophotometry and closer to 95% for visual inspection
- however, few hospitals perform formal spectrophotometry - many rely only on visual inspection
- a systemic review concluded there is insufficient evidence to conclude spectrophotometry is superior to visual inspection
- other limitations of spectrophotometry
- time and labour intensive
- not often performed in smaller centres
- CSF samples must be wrapped in aluminium foil to protect them from light prior to analysis, otherwise results are invalid
•said to not be present in traumatic tap
-one in vitro study demonstrated early development of xanthochromia in heavily blood stained traumatic taps
SAH - CT Angiography
CT angiography
- widely available
- may identify aneurysm and negate need for LP
- 90% sensitive in detecting aneurysms > 3 mm diameter
- although the aneurysm may be an incidental finding, that may lead to unnecessary investigation
•may also detect
- AVM
- carotid or vertebral artery dissection, not seen on plain CT
Limitations
- 15% of SAHs are due to perimesencephalic haemorrhage so may be missed if CTA used instead of LP
- does not usually provide any additional information about other causes of headache
- is usually less cost effective than LP
indications
- LP is indicted but not able to be performed
- patient does not consent or prefers CTA
- technically not possible
- if cervical vascular dissection is also being considered (neck pain)
-the most common missed diagnosis in patients with suspected SAH
