One study showed that 40% of patients with a diagnosis of idiopathic intracranial hypertension (IIH) who are referred for a sub-specialty opinion with a neuro-ophthalmologist don't actually have IIH!
This is really high. Whenever I see a new patient I think it is important to take some time and confirm the original diagnosis before we jump in to treatment questions.
There are probably a few things going on.
Sometimes physicians have diagnostic biases. Just because a woman is obese and has a headache, doesn't mean she's got IIH.
There are also a lot of false positives on testing. Sometimes we rely too much on tests like brain scans.
MRI scan is an essential part of diagnosing IIH. We must rule out a mass like a tumor or clot. We can also look for signs of pressure on the MRI. These signs of pressure are getting more and more attention and I think sometimes we over-rely on those findings. In reality, a lot of normal people show some of those same signs. I've seen another study which looked at 10 possible signs of intracranial pressure:
1 empty sella (flattened pituitary gland)
2 widened Meckel's caves (enlarged space behind the eye sockets)
3 tortuous or twisted optic nerves
4 fluid in the optic nerve sheath behind the eye
5 optic nerve head enhancement (contrast signal in the tip of the optic nerves)
6 protrusion of the disc into the globe (actually seeing the swollen nerve inside the eye on the scan)
7 posterior scleral flattening (a flat spot on the back of the eye)
8 cerebellar tonsillar ectopia (the bottom of the brain pushes down toward the neck)
9 encephalocele (defect in the skull)
10 transverse sinus stenosis (narrowed veins inside the head)
Many normal patients had some of these findings. If a patient had 4 or more, they probably did have IIH but fewer than that and it might be a false positive.
False positives lead to over diagnosis of IIH.
I often meet patients and have to explain that just because their scan "showed high pressure", it doesn't mean that they have IIH. Lots of normal people have some of those findings, too.
The lumbar puncture can be over-interpreted, too. You would think that would be pretty reliable, right? If the patient has a lumbar puncture that showed high pressure, then they must have high pressure... unless the procedure was done incorrectly. Positioning is important. If the head is elevated during the procedure, then the pressure will be as elevated as the head is. If the patient is too tense or holding their breath, we can see errors in the pressure. Sometimes there is a difference in opinion about the interpretation of the results. For a long time normal pressure was defined as 20 cm H2O. However, since then there have been more information showing up to 25 cm H2O is normal, sometimes doctors are using different standards for what is normal.
The other common issue is that papilledema is difficult to identify accurately. I like to think I can reliably see papilledema (or swollen optic nerves) on exam. However, I have been fooled in the past. Everyone has. The exam is hard to do. There is a lot of variability of what is normal. Other features like drusen (calcium deposits) or gliosis (nerve scar) can look confusingly like papilledema.
So, whenever I see a new consult for IIH, I recognize that probably is the correct diagnosis, but I also recognize that frequently it is over-diagnosed. It is important to look at the whole picture and look at the quality of all the evidence before confirming a diagnosis of IIH. Once we confirm the diagnosis it is appropriate to address treatment.