All pressure is created equal

That moment when an idea strikes you. Something stutters the laminar flow of the mind. A mental murmur, perhaps. It’s that hiccup that alerts you that something’s up, something needs to be reassessed.

 

This happened to me at Janus General, where Grock works. Neurology clinic referred a patient to the ED for evaluation of headache and bilateral papilledema. The paperwork reflected: “rule-out idiopathic intracranial hypertension (IIH, pseudo-tumor).” On sign-out, we acknowledged her and said, “Oh, she’s waiting for a lumbar puncture.”

 

After my emotional inflammation waned (why would neuro clinic send this lady to the ED? Don’t they routinely do LPs in their clinic? And they found papilledema in their clinic one week ago…what’s the emergency today?), I was struck. She has elevated intracranial pressure as evidenced by her swollen discs. Isn’t that a contraindication to tap? Does she need a head CT prior to lumbar puncture in spite of her papilledema? Remembering that one of the treatment modalities for IHH is a therapeutic tap, I wondered what makes the increased ICP in IHH different than the ones we worry about…

 

We are all taught that one of the feared complications of lumbar puncture is herniation. It is routine practice to get a CT head prior to lumbar puncture, regardless of the clinical question. Why? Where does this come from? What’s the data?

 

It comes from 1969, from a retrospective series of end-stage brain tumor patients.[i] All had focal-neurologic findings and altered mental status. None had CT scans, as none were yet performed. All got lumbar punctures. Some died quickly “after LP,” others died shortly thereafter. Sounds like pretty powerful causation, no?

 

A review from 2007[ii] acknowledges that the association between lumbar puncture and herniation is not supported by data, but is, however plausible, on a “pathophysiologic” basis. I’m not sure what to make of that.

 

The highest quality evidence I could find came from 2001, from the NEJM.[iii] A retrospective series of 301 patients presenting with suspected meningitis and was intended to identify clinical-features that predicted a low risk of abnormal CT findings. Although its ability to define causation is limited, the findings are interesting on several counts.

 

First, ~80% of patients received CT prior to LP, reflecting typical practice among emergency physicians. 84% of patients had normal mental statuses and neurologic exams. 56 of 301 patients had abnormal head CTs. Lumbar puncture was only avoided in 4 patients: 2 of 4 died of herniation during that hospitalization. Here’s the meat: of 56 patients with abnormal CT scans, 52 underwent LP, including 7 patients with CT-evidence of mass effect.  At follow-up one week later, none had herniated.

 

With regards to IIH, there exist case reports of herniation. In the anesthesia literature, a recent review[iv] acknowledge that safe of “neuroaxial” techiniques, ie. Spinal and epidural anesthesia, for patients with IIH. “Uncal herniation has not been shown to occur in IIH.”

 

I have a pathophysiologic theory: increased pressure doesn’t know the etiology from whence it comes. Let’s take a page from the pseudotumor data. Increased ICP is not a contraindication to LP.

Take-away: 1. Scary things are scary. If your patient has focal neurologic signs and/or a progressively deteriorating neurologic exam, they’re sick and high risk for herniation regardless what you do to them.  2. There is no evidence to suggest that LP has a causative role in herniation.

 

 

[i] Duffy GP. Lumbar puncture in the presence of raised intracranial pressure. Br Med J 1969; 1:407.

[ii] Joffe AR. Lumbar puncture and brain herniation in acute bacterial meningitis: a review. J Intensive Care Med 2007; 22:194.

[iii] Hasbun R, Abrahams J, Jekel J, Quagliarello VJ. Computed tomography of the head before lumbar puncture in adults with suspected meningitis. N Engl J Med 2001; 345:1727.

Klauer, K. LPs: Stop Managing by Myth. Emergency Physician’s Monthly. May, 2011. http://www.epmonthly.com/departments/cme/cme-archive/lps-stop-managing-by-myth/

[iv] Karmaniolou I, Petropoulos G, Theodoraki K. Management of Idiopathic Intracranial Hypertension in Parturients: Anesthetic Considerations. Can J Anaesth 2011. 58(7): 650-7