Bacterial Meningitis: Diagnosis & CSF Findings

00:02 It's essential to get some spinal fluid in a patient who has meningitis.

00:10 And when you get some spinal fluid, during the lumbar puncture, you will note that the opening pressure is elevated.

00:20 When you microscopically examine the spinal fluid, you will find that there are too many white cells.

00:28 Now, normally, there should be no more than five white cells in spinal fluid and they should be all mononuclear, lymphocyte or round cells.

00:42 So, it's abnormal if a spinal tap shows even one white cell that’s a neutrophil.

00:49 So, neutrophils are abnormal in spinal fluid.

00:53 There are some exceptions.

00:55 Because of the pace of meningococcal meningitis, the patient may not have had time to develop much in the way of white cells.

01:04 This little girl that I told you about had just one or two neutrophils in her spinal fluid.

01:12 And in infants under four weeks of age, they may not have white cells in the spinal fluid of that magnitude.

01:23 Now, if they have white cells, there is a neutrophil predominance, more than 80% are neutrophils.

01:31 Once again, the exceptions to that would be a newborn where they had the cause being something like E. coli They may not have a neutrophil predominance.

01:43 And also with Listeria.

01:45 Now, if you remember the name of Listeria is Listeria monocytogenes.

01:51 And that implies that the organism can cause meningitis with mononuclear cells in the spinal fluid.

02:01 So, that's where it got that name.

02:02 Here is a last example: in tuberculous meningitis, you also find lymphocytosis rather than neutrophil predominance.

02:09 Now, obviously, when you get some spinal fluid, you’re going to culture it and you'll get a positive culture in about 80% of individuals unless they’ve been given antibiotics.

02:25 And think how many patients go to the physician and get empirical antibiotics for fever.

02:32 I wonder what's up with that.

02:35 But that may obscure the diagnosis of bacterial meningitis and the cultures may be negated or reduced to 60% or 70% positive if antibiotics had been given.

02:52 It's also important to measure the spinal fluid glucose.

02:58 You'll find that the spinal fluid glucose is lower than it’s supposed to be.

03:02 It'll be usually less than 40 mg/dL in at least 60% of individuals.

03:10 And if you compare the spinal fluid glucose to the serum glucose, it's less than 31% of the corresponding serum glucose.

03:23 And oftentimes, it's less than 18 mg/dL.

03:27 It's usually very low.

03:29 And if it's that low, it's going to be highly predictive of bacterial meningitis.

03:35 The protein is usually elevated, sometimes quite elevated to 500 mg/dL And a Gram stain of spinal fluid is essential.

03:48 And you'll find that it's positive.

03:50 It'll predict what you need to treat with in 60% to 90% of patients.

03:57 You can choose antibiotics on the basis of the Gram stain findings.

04:01 We also can test for bacterial antigens.

04:06 And the best use of it is when somebody's been given prior antibiotics.

04:11 The bacterial antigens may still be around, even though the bugs have been partially killed.

04:18 And so, it's used when the Gram stain is negative.

04:22 Unfortunately, it's not particularly helpful to do bacterial antigens in urine or serum in patients with bacterial meningitis.

04:33 Now if we compare these findings to viral meningities, in the later you would find normal glucose levels, not so high protein levels and lymphocytic predominence.

04:42 Now, I did mention that spinal fluid should be examined in everybody who's got a suspicion of meningitis, but there are certain physical findings that may have - make you delay doing a lumbar puncture because if the patient has evidence of increased intracranial pressure, you may kill them with a lumbar puncture.

05:07 Because if they have markedly elevated intracranial pressure, if you do a lumbar puncture and relieve some of that pressure below, they may actually herniate, and so the herniation can kill them.

05:23 So, what you need to do is, if you think there is clinical evidence of an increased intracranial pressure, you need to verify that with imaging.

05:34 And so, you have to delay therefore your examination of spinal fluid.

05:40 Where you find Gram-positive diplococci along with a lot of neutrophils? Is it Haemophilus influenzae, a very pleomorphic Gram-negative coccobacillus? Or is it that classic Neisseria meningitidis? Or is it Listeria monocytogenes, which are small Gram-positive rods? And the value of the Gram stain is that it suggests an etiology before your cultures come back.

06:12 The cultures may not come back for up to 72 hours, though time to positive blood and CSF cultures can be as quick as 24 hours.

06:19 So, if you've got an idea of what to treat with the Gram stain, you can design your antibiotic regimen.

06:26 Unfortunately, the Gram stain is less useful if the patient has received antibiotics and it may be negative.

06:35 One thing it doesn't change is the spinal fluid formula.

06:40 The protein still up, the white count is still up, the glucose is still low.

06:46 That doesn't usually change with antibiotics.

06:49 Blood cultures are positive in a large number of patients.

06:57 But as you could imagine, the blood cultures are often negative if antibiotics have been given.

07:03 So, blood cultures are useful when a spinal tap is contraindicated.

07:11 So, you have evidence that the patient has increased intracranial pressure.

07:15 You’ve got to get cultures from somewhere, and so getting them from the blood.

07:19 If you get an organism, you know what to treat.

07:22 There are some rapid tests that we can do on spinal fluid.

07:27 The one that was in vogue when I was training was the latex agglutination.

07:34 This is no longer used because of the number of false positives.

07:38 We now use immunochromatographic tests, which are rapid, they can be done in less than two minutes and give you an idea of what to treat.

07:50 For example, in a study showing 450 specimens of cerebral spinal fluid, 122 of them had Streptococcus pneumoniae in them.

08:04 And they were identified by culture in 87 individuals and 35 were identified by PCR.

08:16 But the bottom line was that all the patients that had Streptococcus pneumoniae were positive, so this immunochromatographic test had a sensitivity in this study of 100%.

08:29 It’s probably not quite that good, but it's pretty good.

08:33 And we can also do PCR, targeting the 16S ribosomal RNA gene, which is highly conserved among bacteria.

08:43 So, you can demonstrate that the patient has a bacterial meningitis with PCR.

08:49 It's a little more labor-intensive and it’s not available immediately.

08:54 But in a study of 206 CSF specimens tested, 17 pathogens were identified by either culture or PCR and three of them were culture negative, but PCR positive.

09:10 So, it's showing great promise.

09:12 So, the rapid tests ultimately have a sensitivity of about 100% and a specificity of more than 95%, but we still need more studies dealing with these rapid tests.