00:00
How do we diagnose meningitis? Well it starts with a good history and a comprehensive
neurologic exam looking for signs of meningeal irritation and potentially signs that we're in a
dire strait with late complications like papilloedema, abducens nerve palsies, and Cushing
reflex. Then we need to do a laboratory assessment. Look at the complete blood count,
comprehensive metabolic profile, maybe coagulation test, and a pregnancy test. Complete
blood count is important for looking at the white blood cells. Is there evidence of a systemic
infectious process? Are metabolic profile looks at the kidney and the liver? Liver enzymes
can be elevated in viral processes. Kidneys can be dysfunctional in patients who are
dehydrated. And coagulation studies are not required but in selected patients are important
prior to doing a spinal tap. Sometimes we'll look for a source of a systemic infection with a
chest x-ray to look for pneumonia or blood cultures or urine cultures to look for some site of
an initial infection that then travelled to the brain. We see pneumonias that go to the brain
and urosepsis, UTIs that go to the brain and bacteremia that goes to the brain and to the CNS
space and so those are important places to look. But ultimately, usually the test of choice is
the lumbar puncture and we're looking for signs of infection and we're thinking about that
word pleocytosis. And that means too many cells in the spinal fluid, too many white blood cells
means infectious and really inpoints us in the direction of an infectious meningitis. Do we need
to do imaging before our spinal tap? Well, oftentimes patients get imaging. They come in to the
emergency department and a CT of the head is done and we know what that looks like before
the spinal tap but not always. And there are 5 indications for getting imaging, a CT of the
head, a computed tomography, a CAT scan of the head or an MRI prior to lumbar puncture.
01:53
And these are the 5. If the patient has a focal neurologic deficit, there could be a mass lesion
in the brain, that mass could cause a pressure differential and we wouldn't want to LP that
patient without looking at the imaging. If there is a new onset seizure. New onset seizure
means a mass lesion in the brain until proven otherwise, and if there is a mass lesion in the
brain there could be a pressure differential and we need imaging before evaluating that
patient. Papilloedema. Papilloedema means increased intracranial pressure until proven
otherwise, and that is coming from a mass, again until we have imaging that says that it's
not. If there is a mass lesion, there could be a pressure differential. We wouldn't want to LP
that patient unless we have imaging. Abnormal level of consciousness. We can't do a
comprehensive exam in many of those patients to look and be convinced that there is not a
focal deficit or some type of papilloedema. And so again, abnormal level of consciousness
impairs our ability to look on exam for a new mass lesion for a pressure differential and
those patients need imaging. And then the last indication for imaging before LP is an
immunocompromised state. Weird things happen, different things happen and patients don't
always present with the typical symptoms so it's harder for us to trust our history and exam
in patients who are immunocompromised and we need imaging to show that there is not a
pressure differential, that there is not a risk of herniation with the spinal tap and we get
imaging before spinal tap in those patients. So the 5 indications for imaging prior to LP.
03:25
Once we do the spinal tap, we need to know how to approach it and we ask a series of
questions to guide us in interpreting the finding. The first thing we want to know is "Is there
an infection?" And we look at the white blood cells. If the white blood cells are elevated, we
call that a pleocytosis and this indicates infection until proven otherwise. If the white blood
cells are not elevated, we need to think of other considerations in the differential for this
patient. If we're dealing with a pleocytosis, if white blood cells are elevated, we want to know
if it's bacterial, viral, or fungal. And there is a different signature for each of those. For
bacterial infections, we often see elevated polymorphonuclear cells and increased PMNs
should favor bacterial infection over viral, fungal, or some other type of organism. With viral
infections, we see monocytes, a lymphocytic predominance, not the polymorphonuclear cells
but lymphocytes often with mildly elevated protein and normal glucose and that signature
should point us in the direction of a viral infection. For fungal infections, we also see increased
monocytes, lymphocytes, that's what responds to a fungal infection but here we often see
low glucose, hypoglycorrhachia, and that signature should point us towards a fungal process.
04:44
So this table helps us to walk through the types of infectious organisms that we'd be
concerned about based on CSF findings. And as a reminder, when we look at CSF we're going to
think about the opening pressure, the white blood cells, how elevated they are or not, the
differential of those white blood cells, the glucose level, the protein, and in some cases the
presence of red blood cells. For bacterial infections, we see that the opening pressure is often
elevated, there is significant pleocytosis, increase in white blood cells, we see either sometimes
upwards of 200 white blood cells in the CSF. The differential on those white blood cells is
they are frequently polymorphonuclear cells, this is an acute infectious process and acute
inflammatory response to that. The glucose is low and there is not a normal value of glucose.
05:35
CSF glucose is based off the serum glucose and we see that CSF glucose and bacterial infections
is frequently less than 60% that in the serum, the protein level is very high and there can
be a few red blood cells. With the viral process, again, we can see slightly elevated opening
pressure, there is a pleocytosis but not as prominent as with bacterial infections usually less
than 200 cells though this is highly variable. It's monocytic, lymphocytic predominant with
normal glucose and normal protein unless there is a vigorous inflammatory process associated
with the virus. For fungi, we see normal to high and in some fungal infections significantly
elevated opening pressures with the pleocytosis but much milder than what we see with
bacterial infections, typically we see monocytic or lymphocytic predominant pleocytosis with
low glucose. This is an important difference between viral and fungal infections and often with
high protein or mildly elevated protein. And then with TB, an important atypical cause of
meningitis, we often see elevated opening pressure, a mild pleocytosis, lymphocytic
predominant, low glucose, and high protein. So here's a table that can guide us in evaluating
that most important spinal tap.