Myths in EM: Is Head CT Needed Before LP in all Adults with Suspected Bacterial Meningitis?
- Apr 24th, 2017
- Michael D. April
Authors: Michael D. April, MD, DPhil, MSc (EM Staff physician at SAUSHEC) and Brit Long, MD (@long_brit, EM staff physician at SAUSHEC) // Edited by: Alex Koyfman, MD (@EMHighAK, EM Attending Physician, UT Southwestern Medical Center / Parkland Memorial Hospital)
A 22-year-old male presents with headache and fever for the last two days. When he woke this morning, his headache was much worse. The patient has no PMH or allergies, and his VS include T 38.2 C, HR 105, BP 132/78, RR 22, Sat 98% on RA. His neurologic exam is non-focal, and he will answer questions appropriately. However, his neck is quite stiff. You are strongly considering meningitis, and you realize the patient will need a lumbar puncture (LP). You put your orders in for CT and LP with IV ceftriaxone. But does the patient need a CT before his LP?
Bacterial meningitis is a deadly CNS infection. Mortality can reach 15-21% even in patients receiving antibiotic therapy.(1,2) A lumbar puncture (LP) with cerebrospinal fluid (CSF) analysis is considered gold standard.(3) If this is not completed quickly before or shortly after antibiotics, it can be difficult to obtain an organism on culture.(4) However, delay in antibiotics is dangerous and drastically affects survival.(5-7)
LP is essential for CSF in the evaluation for meningitis, but there is concern for brainstem herniation in the setting of brain edema and increased ICP.(8) CT provides a reliable evaluation for mass effect,9 and many assume that herniation can be prevented by abstaining from LP in the setting of mass effect.
Myth: Cerebral imaging with CT before LP improves survival in suspected meningitis through identifying patients who should not undergo LP due to increased ICP from mass lesions.
Is this important?
The gold standard diagnostic study for bacterial meningitis is CSF obtained by LP.(3) CSF is important in ensuring adequate treatment and diagnosis.(4) Rapid antibiotic is necessary to avoid patient morbidity and mortality,(5-7) and LP is a vital component of the evaluation.
One major concern that patients with brain edema or lesions causing intracranial mass effect may experience lumbar puncture-induced brainstem herniation.(8) CT provides physicians with a reliable means of identifying brain lesions causing mass effect.(9) The assumption is that by avoiding LP in patients with mass effect, herniation may be prevented.
Neuroimaging before LP optimizes survival for patients with suspected bacterial meningitis by identifying individuals with potential mass effect and herniation.
Why is it important?
Brainstem herniation can result in death or profound neurologic morbidity.(10) CT before LP in every patient with suspected bacterial meningitis could be considered safe practice. However this strategy has its own consequences, including radiation exposure and increased cost.(11,12) Even more important is the potential delay in antibiotics with head CT before LP.(5-7)
Several studies have tried to identify patients in need of pre-LP head CT.(13,14) The IDSA recommends a diagnostic pathway using selective CT in those patients with characteristics suggesting risk for post-lumbar puncture brainstem herniation.15 Yet these guidelines are highly controversial.(16) This post will provide an overview of the relevant primary literature concerning the need for head CT before LP in suspected meningitis.
A Little Lesson in History…
Most authors attribute the first description of the LP to Heinriche Quincke in 1891.(17) Papers published as early as 1896 reported peri-procedural deaths in patients with presumed intracranial lesions, though they note the challenges of distinguishing causation versus correlation.(18)
In 1938 Geoffrey Jefferson proposed a mechanism by which LP might precipitate brainstem herniation. He argued that in patients with intracranial lesions causing mass effect, the CSF exerts a protective upward buoying effect on the brainstem. He suggested that the brainstem may herniate downward through the foramen magnum if CSF is removed.(19) This paper was an important milestone by suggesting a causative relationship between LP and herniation.
Post-Lumbar Puncture Brainstem Herniation in Patients with Mass Effect Lesions
Several observational studies published since the 1950s evaluated outcomes in patients thought to have elevated ICP who underwent LP. These papers are often cited when discussing risk of LP,(8) but these studies require several considerations. The first major point is these studies are before CT was commonly used, and authors relied on clinical suspicion of tumor,(20,21) histological verification of tumor by CSF analysis or autopsy,(20,22) or papilledema.(20,21,23-25) These criteria comprise patients with many different pathologies, and papilledema can occur in conditions without mass effect (like idiopathic intracranial hypertension, a disease in which LP is needed for diagnosis and treatment).(26)
The risk of post LP neurologic decline thought to be due to herniation varied from 0 to 6.2% in these studies.(23-25) Time to neurologic decline also varied, from 5 minutes(24) to 24 hours, which calls into question the causative relationship of LP and decline with herniation.(20-22,24) Patients not undergoing LP may also experience neurologic decline with presumed intracranial mass.(21)
As these studies predate CT use in today’s medical era, extrapolating these data evaluating the risk of LP and herniation with mass effect to today is difficult. When CT became more readily available for use, physicians accepted the premise that patients with potential intracranial lesions were at higher risk for LP complications based on older literature. Thus, more definitive study was difficult, with data largely consisting of case series.(9)
Bottom Line: While data are limited to historical observational studies from the pre-CT period, there may be a small risk (~1-2%) that LP may be associated with neurologic decline in patients with intracranial mass effect lesions.
Post-Lumbar Puncture Brainstem Herniation in Bacterial Meningitis
Patients with bacterial meningitis may be at higher risk than those with mass effect lesions for herniation if they undergo LP. One cases series from 1956-1962 reported herniation in 9 of 175 patients who underwent LP. Four of these patients demonstrated cerebral edema, but no other anatomic abnormalities in the other patients were found.(27) Another cohort of 86 patients with meningitis who underwent LP reported herniation in 8.1% of patients.(28) These results question whether neuroimaging can identify patients with bacterial meningitis at greatest risk for herniation after LP. Several pediatric studies discuss cases of post-LP herniation in patients with normal imaging.(29,30)
Can deferral of LP prevent brainstem herniation? While this seems likely on first inspection, the literature suggests the relationship of procedure and poor outcome is not clear cut. One patient in the case series discussed above experienced herniation before LP.(27) Another study evaluating a head imaging decision rule derivation found 2 patients herniated even though they never underwent LP.(14) The pediatric literature again provides several cases of patients with meningitis who experienced herniation before LP.(29)
Patients with bacterial meningitis may be at risk for herniation, though imaging may not be able to identify those patients at risk. Bacterial meningitis is rare (1.7% of ED patients who undergo LP),(14) especially with the pneumococcal conjugate vaccine.(31)
Bottom Line: Patients with bacterial meningitis may be at higher risk than patients with intracranial mass effect lesions for brainstem herniation (8.1%), but it is not clear if deferring LP in patients with CT-diagnosed cerebral edema and mass effect decreases this risk.
Decision Rule Derivation Studies
Many professional societies continued to recommend CT before LP to evaluate for mass effect lesions in the early 2000’s, despite the limitations in data.(9,32,33) Two studies sought to balance radiation exposure(12) and delay in antibiotics(5-7) versus identifying patients with potential risk for post LP herniation due to mass effect from intracranial lesion. These studies sought to derive decision rules that selectively image patients with clinical features associated with abnormalities found on CT.
Gopal et al. investigated consecutive ED patients requiring LP in the ED for any indication. Investigators enrolled 111 of 113 consecutive patients, all with preceding head CT.(13) They identified three predictors for abnormality on head CT: altered mental status, focal neurologic abnormality, and papilledema. Authors stressed that one of these alone would not be accurate in predicting head CT abnormality, but the three items together could serve as a screening tool. Patients with none of these characteristics had a negative likelihood ratio (LR-) for any CT abnormality of 0.0 (95% CI 0-0.6). The presence of one or more characteristics yielded a positive likelihood ratio (LR+) of 1.6 (95% CI 1.2-1.9). Interestingly, authors also asked physicians to predict the likelihood of intracranial lesion based on clinical suspicion, which demonstrated LR+ 18.8 (95% CI 4.8-43) and LR- 0.0 (95% CI 0.0-0.7), suggesting clinical gestalt may be better than a decision rule.(13)
A study conducted by Hasbun et al. focused on ED patients undergoing evaluation for suspected bacterial meningitis.(14) They enrolled 301 patients regardless if they underwent CT or LP. 235 (78.1%) underwent head CT, and this group is what the authors used to derived their decision rule. They found age >59 years, immunocompromised state (HIV/AIDS, immunosuppressive therapy, transplant), history of CNS disease (known mass lesion, stroke, focal infection), seizures in the past week, and various neurologic examination abnormalities to predict any abnormality on head CT. The absence of any of these criteria resulted in LR- 0.1, while the presence of one or more criteria provided LR+ 2.0.(14)
Bottom Line: Patients with absence of any high-risk characteristics (age >59 years, immunocompromised state, history of central nervous system disease, seizures in the past week, and various neurologic examination abnormalities) have a LR- of 0.1 for any CT abnormality.
Limits of Decision Rules
Using a decision rule for evaluation in patients with bacterial meningitis has been advocated in the literature.(33-35) The IDSA recommends the rule by Hasbun et al. in determining the need for head CT before LP.(15) However, the Hasbun and Gopal studies have important limitations.(13,14)
1. Both rules demonstrate poor specificity, as the Hasbun et al. rule has a specificity of 52%, and importantly, this specificity refers to detecting any head CT abnormality. However, physicians should be concerned with head CT abnormalities possessing mass effect.(8) The cohort in the Hasbun study identified 4 patients in whom providers deferred LP, and all were decision rule positive. If specificity is recalculated based on these patients, specificity decreases to 41.6%, which suggests over half of patients not requiring head CT before LP will undergo imaging if the physician relies on this rule.
2. A second issue is the calculation of test characteristics using only patients who had head CT. Most physicians will apply the decision rules to broader population of patients who undergo CT. While most patients who did not receive CT in these studies were likely well appearing and may have not met study criteria, we do not know based on the reported data.
3. The Hasbun and Gopal investigations are derivation studies, and neither has been validated to this point. Both should have undergone validation in a different site and patient population. Validation studies ideally rely on measurement efficacy performed by clinicians, not investigators.(36) Just as an example, the literature discusses important expectations for validation studies for other disease processes, such as subarachnoid hemorrhage.(37)
Bottom Line: There as several limits to these decision rules that seek to determine the need for head CT before LP, and they have not been validated.
Head Imaging and Patient Survival
No study has explored decision rule impact on definitive patient outcomes. Conducting studies evaluating decision-rule based imaging against CT before LP or LP alone would be difficult due to expense and rarity of these poor patient outcomes in today’s medical era. Gopal et al. found 2 of 3 patients had positive CSF cultures and did not receive antibiotics prior to LP and CT.(13) Hasbun et al. found patients receiving head CT had a delay in antibiotics as well (close to one hour greater time to receipt).(14) Importantly, delay in antibiotics may be associated with 30% increase in mortality per hour in patients with meningitis.(7)
Swedish guidelines in 2009 were revised by removing the “impaired consciousness” portion of the criteria for obtaining a head CT before LP. Since that time, a reduction in door-to-antibiotic times and reduction in mortality (11.7% to 6.9%) have occurred.(16,38) These highlight the importance of antibiotics if meningitis is considered, especially if CT will be obtained.
Bottom Line: Swedish data reports shortened door-to-antibiotic times and improved survival among patients with bacterial meningitis after modifying criteria for CT before LP.
You decide to perform a LP without CT, as the patient does not meet criteria for head CT. You provide 2g ceftriaxone IV after the LP is completed. The CSF results are concerning for meningitis, and you admit the patient.
– The best available evidence suggests some association between LP and brainstem herniation in patients with intracranial mass effect lesions.
– However, this risk is small, and there is no evidence that deferring LP prevents herniation in patients with these lesions.
– Decision rules that attempt to identify patients most likely to have intracranial lesions with risk for post-LP herniation have several limitations and have not been validated.
– Patients with significant pre-test probability for intracranial mass effect lesion or bacterial meningitis based on clinical assessment should receive head CT.
– Antimicrobials are required if meningitis is suspected.
– Concern for meningitis warrants antibiotics before CT if imaging will be obtained.
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