The Role of Head CT in the Diagnosis of Bacterial Meningitis

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August 22, 2017 by Casey Carr

To CT or Not to CT – That is the Question


By: Hai Le, MS4


Case Presentation

Mr. X is 42-year-old male with no significant past medical or past surgical history who presented to the emergency department with a 1-day history of slow-onset, diffuse, throbbing headache. His symptoms started at 5PM the previous day and worsened over the next four hours to 10/10 in severity. Movement, especially of his neck, worsened the pain. Associated symptoms included photophobia, subjective fever, nausea and vomiting, generalized fatigue, and possible decreased mental status from baseline that was difficult to assess. He denied ever having this type of pain before, denied a history of migraines, numbness, tingling, weakness, recent trauma, or sick contacts. He does not smoke or use recreational drugs and only drinks alcohol socially.

Significant physical exam finding include exacerbation of his headache on neck movement in all directions. A complete neurologic exam was otherwise non-focal. On admission, he had a fever of 101.5°F. His work-up was significant for a leukocytosis of 19.25 and a lactate of 2.5. Empiric antibiotics and IV dexamethasone were immediately started for suspected bacterial meningitis.


Images

A head / brain CT without contrast was obtained to rule out acute intracranial processes prior to a lumbar puncture (LP). The result suggested “mild diffuse cerebral edema” that was difficult to quantify given the lack of prior records for comparison. No evidence of intracranial mass effect or midline shift was noted.


Summary

1) In suspected bacterial meningitis without high-risk factors and no focal findings on exam, a head / brain CT WO contrast is not necessary prior to an LP.

2) Even in the context of an abnormal, non-focal CT without midline shift or mass effect, an LP is safe with only a theoretical risk of brain herniation.

3) Start empiric first line therapy as soon as you suspect acute bacterial meningitis!


Review of the Evidence

While first-line therapies were quickly initiated for suspected acute bacterial meningitis (ABM) given the presentation of fever, neck pain, headache, and possible altered mental status (AMS) [1] in an otherwise healthy individual, a diagnostic LP for cerebrospinal fluid (CSF) evaluation was necessary for a definitive diagnosis [2]. The role of CT imaging in this pathway remains controversial. In a prospective study of 301 adult ED patients with suspected ABM from 1995 – 1999 at the Yale-New Haven Hospital, 78% underwent a CT scan and 24% of those had abnormal findings on CT, which was correlated with age >60, immunocompromised state, history of CNS disease, AMS, focal neurological deficits, or recent seizures [3]. Of note, CT imaging prior to LP led to an approximate 2.3-hr delay in diagnosis and 1-hour delay in initiating therapy [3]. This is significant as delay in therapy has been associated with increased mortality in ABM in prior studies [4]. As such, without clinical features predictive of abnormal CT, imaging is not necessary prior to LP per the Infectious Disease Society of America recommendations [2, 3]. Similarly, ACEP (American College of Emergency Physicians) recommend that an LP could be performed without prior CT in patients without clinical signs of increased intracranial pressure (Level C) [5].

Even so, a lumbar puncture without a prior CT confers a theoretical risk of brain herniation, a feared and devastating complication [6]. In ABM, the evidence for adverse complications directly caused by a diagnostic LP is limited outside of several case reports that demonstrated only temporal relationships [7, 8]. Given Mr. X’s lack of focal clinical signs, which are superior to CTs in predicting risk of brain herniation in ABM [8], an LP was a reasonable next step in diagnosis even in the setting of “mild diffuse cerebral edema” on CT.


But, should Mr. X have gotten a CT at all?

Based on the current literature and standard of care, the answer is yes. Mr. X presented with altered mental status at presentation and unknown baseline.

Interestingly, AMS was removed as a contraindication for LP without CT for suspected ABM in Sweden in 2009. A subsequent retrospective study of more than 700 patients found that therapy was initiated 1.6 hours earlier, mortality decreased from 12% to 7%, and the risk of sequelae dropped from 49% to 38% [9]. Each hour of delay led to a relative increase in mortality of 12.6% [9]. Perhaps a paradigm shift is forthcoming.


References

  1. van de Beek D, de Gans J, Spargaard L, et al. Clinical features and prognostic factors in adults with bacterial meningitis. N Engl J Med. 2004; 351: 1849-59.
  2. Tunkel AR, Hartman BJ, Kaplan SL, et al. Practice guidelines for the management of bacterial meningitis. Clin Infect Dis. 2004; 39(9): 1267-84.
  3. Hasbun R, Abrahams J, Jekel J, et al. Computed tomography of the head before lumbar puncture in adults with suspected meningitis. N Engl J Med. 2001; 345: 1727-33.
  4. Proulx N, Frechette D, Toye B, et al. Delays in the administration of antibiotics are associated with mortality from adult acute bacterial meningitis. QJM. 2005; 98(4): 291-98.
  5. Edlow JA, Panagos PD, Godwin SA, et al. Clinical policy: critical issues in the evaluation and management of adult patients presenting to the emergency department with acute headache. Ann Emerg Med. 2008; 52(4): 407-436.
  6. Fitch MT and van de beek D. Emergency diagnosis and treatment of adult meningitis. Lancet Infect Dis. 2007; 7: 191-200.
  7. Archer BD. Computed tomography before lumbar puncture in acute meningitis: a review of the risks and benefits. CMAJ. 1993; 148: 961-65.
  8. Joffe AR. Lumbar puncture and brain herniation in acute bacterial meningitis: a review. J Intensive Care Med. 2007; 22: 194-207.
  9. Glimaker M, Johansson B, Grindborg O, et al. Adult bacterial meningitis: earlier treatment and improved outcome following guideline revision promoting prompt lumbar puncture. Clin Infect Dis. 2015; 60(8): 1162-69.

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