KL Kwong, WK Chiu

Abstract

Lumbar puncture is important in the investigation of suspected CNS infection. However, there is genuine concern and controversy about the risk of cerebral herniation after lumbar puncture. CT scan brain is widely performed before lumbar puncture whenever raised intracranial pressure is suspected. The pros and cons of interposition of CT scanning before lumbar puncture pose a clinical dilemma and have led to confusion among paediatricians and even a delay in initiation of therapy. CT scan brain only provides structural information and is considered insensitive in the exclusion of raised intracranial pressure. Clinical signs, not a CT scan brain, are the best indicators of when to or when not to do lumbar puncture. To exercise caution, relevant questions need to be addressed before performing a lumbar puncture. Literature is reviewed regarding lumbar puncture and cerebral herniation. The aim is to formulate a rational approach in managing children with suspected CNS infection.

Introduction

Cerebral herniation after lumbar puncture is a nightmare for every paediatrician. The event is catastrophic and often fatal. As a consequence of this concern, CT scan brain has been widely recommended before lumbar puncture, whenever raised intracranial pressure is suspected. However, the pros and cons of interposition of CT scanning before lumbar puncture pose a clinical dilemma and have led to confusion among paediatricians and might even result in a delay in initiation of therapy. Relevant literature regarding cerebral herniation and lumbar puncture are appraised with the aim to formulate a rational approach in the management of children with suspected CNS infection.

Lumbar puncture has been used extensively in the investigation of many neurological conditions. Cerebrospinal fluid analysis and culture is the definitive method of diagnosing CNS infection. Lumbar puncture is mandatory in any patients in whom meningitis is suspected.¹ However, there is an increasing anxiety that this procedure carries risk in cerebral herniation. Knowledge of facts relevant to disease process leading to cerebral herniation is helpful. The seven questions that paediatricians need to address before performing lumbar puncture are: (1) Is lumbar puncture indicated? (2) Are there any contraindications for lumbar puncture? (3) What are the precautions when one performs lumbar puncture? (4) Is neuro-imaging required before the procedure? (5) What is the risk of cerebral herniation in CNS infection and its relationship to lumbar puncture? (6) How can one recognise cerebral herniation? If cerebral herniation does occur, what are the subsequent measures? (7) If lumbar puncture is not considered safe, what should be done?

(1) Is Lumbar Puncture Indicated?

The basic elements of clinical features of bacterial meningitis are fever, inconsolable irritability, findings of meningeal irritation such as neck stiffness and a dull sensorium. However, the "classical signs" of meningitis are often absent in infants. Lumbar puncture is advocated for any infant who is drowsy or ill without awaiting development of meningeal signs.² Lumbar puncture is recommended (Evidence rating: Level A) in the evaluation of viral encephalitis.³ Lumbar puncture should only be delayed in the presence of straight contraindications.^4,5 Cultures of CSF after parental antibiotics have been given are sterile within 2 hours in meningococcal and 6 hours after pneumococcal meningitis.⁶ Cellular and biochemical changes remain in the CSF only up to 44-68 hours after antibiotics are started.⁷ Blood culture is positive in 40-50% meningococcal meningitis and 80% in pneumococcal meningitis.⁷ The information obtained can guide subsequent management. Avoiding lumbar puncture can result in missing the exact bacterial cause of meningitis and the antibiotic susceptibility.

(2) Are There Any Contraindications for Lumbar Puncture?

In the presence of contraindications, lumbar puncture should not be performed. There are eight commentaries found in the literature concerning when to do or when not to do lumbar puncture in suspected meningitis.^4,5,8-13 Lumbar puncture should be deferred if there are signs of incipient cerebral herniation. There is some mild disagreement on what exact level of Glasgow coma score (GCS) would contraindicate lumbar puncture. Some debate exists as to whether GCS <13 or 12 is a more appropriate cut-off.^14,15 Deteriorating conscious level is more commonly agreed as a contraindication.¹³ Decerebrate posturing, may be mistaken as tonic seizures, indicates early brain herniation and definitely contraindicate lumbar puncture. Neck rigidity can be one of the signs of herniation through foramen magnum. Meningococcal infection with purpuric rash is considered by some workers to be a contraindication because of cardio-respiratory compromise and impending cerebral oedema.^16,17 Papilloedema is uncommon in meningitis as it may take at least 24-48 hours to develop in raised intracranial pressure. Its absence does not mean it is safe to do lumbar puncture. Normal CT scan brain does not mean it is safe to do lumbar puncture (details will be discussed in subsequent text). Clinical signs, not a CT scan brain, are the best indicators of when to or when not to do lumbar puncture.¹² Table 1 provides a summary of general recommended contraindications of lumbar puncture.

(3) What Are the Precautions When One Performs Lumbar Puncture?

In some ill patients, there may be a need for supplementary oxygen and parenteral fluid during the procedure. Care should be taken to avoid excessive flexion that might compromise cardiorespiratory status. A small caliber needle should be used. The smallest sample volume should be obtained whenever possible. Opening pressure provides important information.¹² Vital monitoring of patients during the procedure is crucial. The detection of an elevated opening pressure in a well-sedated patient should alert the adoption of subsequent remedial measures to reduce intracranial pressure.

(4) Is Neuroimaging Required Before Lumbar Puncture?

When one considers CT scan, we have to remember that the procedure is associated with risks of high-dose radiation. Children are more radiosensitive than adults.¹⁸ Brain tissue is far more radiosensitive than previously thought, thus the risk of long-term radiation cannot be ignored.¹⁹ CT scan brain is considered unhelpful as it is normal in 69.8% of uncomplicated cases of acute bacterial meningitis.²⁰ No clinically significant CT scan abnormalities were found that was not unsuspected on clinical assessment.²⁰ CT scan brain only provides structural information and does not measure intracranial pressure. Structural information characterising brain shift anatomically provided by CT scan brain from which pressure data must be inferred are: "tense dura, flattened gyri, narrowed sulci, effaced cisterns, compressed ventricles and displacement of brain" (Table 2).^21,22 CT scan brain is an insensitive method for exclusion of raised intracranial pressure. Talan et al reviewed 122 patients admitted for suspected acute bacterial meningitis. Interposition of CT scan brain before lumbar puncture delayed initiation of antibiotics therapy by an average of 2 hours.²³ Rennick et al found that CT scan brain was normal in 5 of the 14 children at or about time of herniation.²⁴ Several reasons have been proposed to explain why CT scan brain cannot reliably detect cerebral herniation. (i) Considerable variability in the size of normal lateral ventricles. (ii) Purulent material may prevent narrowing of the subarachnoid and ventricular spaces. (iii) Meningeal hardening from inflammation may cause decreased meningeal compliances.¹³ A prospective study involving 301 adults with suspected meningitis confirmed that clinical features could be used to identify patients who are likely to have abnormal CT scan findings. Predictors of abnormal findings included: immuno-compromised status, presence of signs that are suspicious of space-occupying lesions, or moderate-to-severe impairment of consciousness (Table 3).²⁵ The overall clinical impression made by physicians was shown to have the highest predictive value in identifying patients with CT-defined contraindications to lumbar puncture (Likelihood ration LR 18.8).²⁶ Neuroimaging is advised in diseases mimicking meningitis, e.g., posterior fossa tumour, acute hydrocephalus, cerebral abscess and intracranial hemorrhage (Table 4).²⁷ In children, failure to respond to treatment is another indication for neuro-imaging, even though there is seldom any specific intervention.²⁸ CT scan brain is considered not helpful in the management of uncomplicated meningitis.⁹ There is no substitute for a complete, accurate history and physical examination. The findings can correctly guide us in reaching a decision of whether to do or delay lumbar puncture.¹² For children indicated for neuroimaging, they should be stabilised prior to the procedure and properly escorted with adequate monitoring of vital functions when performing CT scan brain.

(5) What Is the Risk of Cerebral Herniation in CNS Infection and Its Relationship to Lumbar Puncture?

Brain herniation was considered to be a contributory factor in 30% of fatal cases of acute bacterial meningitis.^29,30 A retrospective review of 300 children with acute bacterial meningitis, 18 (6%) had episode of cerebral herniation diagnosed clinically and accountable for at least 20% of mortality.³⁰ Intracranial pressure is usually elevated in acute bacterial meningitis. Minns et al, found 33 (94%) of 35 children with acute bacterial meningitis had raised intracranial pressure, with a median opening pressure of 15 mmHg.³¹ Raised intracranial pressure by itself does not cause herniation. One of the advocated treatments of benign intracranial pressure is CSF drainage through repeated lumbar punctures. Regardless of intracranial pressure, lumbar puncture can only result in herniation if brain shift is present, i.e. when there are differences in pressure between different brain compartments. Herniation can occur at two sites, the cerebellar tonsils may protrude through foramen magnum or mesial temporal lobes may move downward through tentorial opening. What is the relationship between brain herniation and lumbar puncture? Van Crevel et al noted that lumbar puncture "removes the stopper from below, thus adding to the effects of the compression from above".²¹ There are at least 22 case reports of deterioration and herniation after lumbar puncture.^13,32-34 The literature provides a strong temporal association between lumbar puncture and cerebral herniation.²⁴ Rennick et al showed that the odds ratio of herniation in the first 12 hours after lumbar puncture was 32.6 (95% CI, 8.5 to 117.3, P<0.001).²⁴ Doing lumbar puncture in the setting of contraindications complete the process of brain herniation.

(6) How Can One Recognise Cerebral Herniation? If Cerebral Herniation Does Occur, What Are the Subsequent Measures?

Physicians should be able to recognise early signs of cerebral herniation after lumbar puncture. Close surveillance of possible herniation is warranted. The diagnosis of cerebral herniation is made by characteristic neurological picture of comatose state, palsy of cranial nerves and postural abnormalities (Table 5).³⁵ Ninety-eight herniation episodes were analysed, 37% of herniation occurred within 3 hours, 41% 4-12 hours of lumbar puncture.^{4,13,24,29,30,36} The overall mortality of cerebral herniation is 56% and half of the survivors had good outcome.^13,24,37 Many of the survivors had very high intracranial pressure during their treatment course. The literature suggests that herniation in CNS infection is not necessarily dismal. Prompt implementation of interventions to control intracranial pressure and neuro-critical measures is recommended.^1,38 These include intermittent administration of osmotic diuretics to maintain an intracranial pressure of <15 mmHg and an optimal cerebral perfusion pressure. Electroencephalographic monitoring is indicated in patients with seizures or fluctuating consciousness.¹

(7) If Lumbar Puncture Is Not Considered Safe, What Should Be Done?

Empirical antibiotics should be given without delay. Blood for cultures drawn before antibiotics can be positive in 70-80% of patients. Third generation cephalosporin is likely to be adequate.⁴ However, in countries where there is pneumococcal resistance to cephalosporins, vancomycin should be added.³⁹ When viral encephalitis is suspected especially for herpes encephalitis, acyclovir should be commenced. MRI brain can be helpful in differentiating the diagnoses after stabilisation of patients. In patients contraindicated for lumbar puncture, interventions to control raised intracranial pressure, attention to airway, breathing and circulation, together with appropriate antibiotics should be the priorities. Then this is followed by neuro-imaging, not a lumbar puncture, after the child has been stabilised (Figure 1). Patients with CNS infections are advised to be managed in an institution with access to intensive care unit.^1,3

Conclusion

The vast majority of patients presenting with CNS infection have no clinical evidence of herniation and routine CT scan brain is not indicated. Lumbar puncture should be done in the absence of contraindications followed by prompt initiation of appropriate antibiotics and in addition acyclovir if herpes encephalitis is suspected. As CT scan brain is not sensitive in the exclusion of raised intracranial pressure, a normal appearance of the CT scan brain does not necessarily mean it is definitely safe to do lumbar puncture. CT scan brain can be normal even in patients with incipient herniation. There is no substitute for a complete, accurate history and thorough examination, especially neurological examination. However, CT scan brain is indicated in excluding diseases that mimic bacterial meningitis and in a complicated case of meningitis that fails to respond to treatment. The best implication for an abnormal CT scan brain is the ascertainment of significant contraindication for lumbar puncture. Nevertheless, spotting clinical signs of impending herniation are the best indicators of who should not have a lumbar puncture done. To practice as a routine, measurement of opening pressure, proper stabilisation and monitoring of patients during and after lumbar puncture is important. Most cerebral herniation occurred within twelve hours of lumbar puncture, close surveillance, early recognition and prompt treatment of impending herniation is warranted. Patients with CNS infections are advised to be managed in an institution with access to intensive care unit. For children at risk of cerebral herniation, interventions to control raised intracranial pressure and neurocritical care are crucial. Appropriate therapy should be the priority, followed by urgent neuro-imaging and not a lumbar puncture.

Figure 1 Management algorithm for children with suspected CNS infection, in particular, meningitis.1,4,5,13,23

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