What are high-yield Bloom's taxonomy level 3, 4, 5 questions on Central Nervous System (CNS) infections relevant to Master of Chirurgie (MCh) Neurosurgery exams like Indian Nationals (INI) Super Speciality (SS)?

High-Yield Bloom's Taxonomy Level 3-5 Questions on CNS Infections for MCh Neurosurgery Exams

Application Questions (Bloom's Level 3)

Question 1: CSF Analysis and Diagnostic Decision-Making

A 45-year-old post-craniotomy patient develops fever on postoperative day 5 with an external ventricular drain (EVD) in place. CSF shows: WBC 850/μL (70% neutrophils), protein 180 mg/dL, glucose 38 mg/dL (serum glucose 110 mg/dL), Gram stain negative. What is your immediate management approach?

Key Learning Points:

• CSF should be obtained from the EVD reservoir in patients with intracranial devices 1
• The CSF-to-blood glucose ratio is 0.35 (38/110), which is above the threshold of 0.23 that suggests bacterial infection, but the neutrophilic pleocytosis and elevated protein are concerning 2
• If CSF flow is obstructed, sample both the device and lumbar space 1, 2
• Gram stain sensitivity is limited; negative results do not exclude bacterial infection 1
• Remove the EVD if the patient develops stupor or meningitis signs, and culture the catheter tip 2
• Most common organisms in postoperative CNS infections are Staphylococcus aureus (50%) and Propionibacterium acnes (25%) 3

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Question 2: Imaging Selection in Suspected Brain Abscess

A 32-year-old immunocompetent patient presents with 5 days of progressive headache, fever, and new-onset focal seizures. CT head shows a ring-enhancing lesion in the right temporal lobe. What is the optimal next imaging study and why?

Key Learning Points:

• MRI with DWI/ADC and T1-weighting with and without gadolinium is the preferred method for diagnosing brain abscesses 4
• MRI is superior to CT for subtle examination of brain parenchyma and meninges 1
• DWI/ADC sequences help differentiate abscess (restricted diffusion) from other ring-enhancing lesions like tumors or demyelination 4
• Contrast-enhanced CT is an acceptable alternative when MRI is unavailable 4
• Cerebral CT is indicated for rapid diagnosis but MRI should be preferred for definitive characterization 1

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Question 3: Antibiotic Timing in Suspected Meningitis

A 28-year-old presents with fever, severe headache, photophobia, and nuchal rigidity. Fundoscopy shows papilledema. What is your immediate management sequence?

Key Learning Points:

• If bacterial meningitis is suspected and lumbar puncture is delayed for any reason (including imaging), start empirical antibiotics immediately after obtaining blood cultures 1, 2, 5
• Papilledema suggests elevated intracranial pressure; imaging is required before LP to exclude mass lesions or obstructive hydrocephalus 1
• Non-contrast CT is adequate to exclude contraindications to LP 1
• Delaying antibiotics for imaging is a critical pitfall that increases mortality, particularly with S. pneumoniae 2, 5
• Blood cultures should be obtained before antibiotics but should not delay antibiotic administration 1

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Analysis Questions (Bloom's Level 4)

Question 4: Differential Diagnosis of Post-Neurosurgical Fever

A 55-year-old underwent resection of a glioblastoma with Gliadel wafer placement 10 days ago. He now presents with fever (38.5°C), mild confusion, and CSF from the Ommaya reservoir shows: WBC 120/μL (80% lymphocytes), protein 95 mg/dL, glucose 48 mg/dL (serum 105 mg/dL). Gram stain and routine bacterial cultures are negative at 48 hours. Analyze the differential diagnosis and justify your diagnostic workup.

Key Learning Points:

• Candida infection is associated with foreign intracranial material, including Gliadel wafers 6
• Lymphocytic pleocytosis with negative bacterial cultures suggests viral, fungal, or tuberculous etiology 2, 5
• In immunocompromised patients or those with CNS instrumentation, maintain high suspicion for infection regardless of normal CSF parameters until cultures are finalized 2, 5
• For immunocompromised patients, obtain cryptococcal antigen, fungal stains and cultures, AFB smears and cultures, and PCR for HSV, CMV, JC virus 1, 2
• Candida albicans is the predominant species (73%) in post-neurosurgical CNS infections 6
• CSF glucose >35 mg/dL and CSF-to-blood ratio >0.23 make bacterial meningitis less likely but do not exclude fungal infection 2
• Larger CSF volumes (5-10 mL) increase sensitivity for mycobacterial and fungal cultures 2

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Question 5: Management of Brain Abscess with Multiple Etiologies

A 40-year-old with chronic sinusitis presents with a 2-week history of progressive headache and left-sided weakness. MRI shows a 3.5 cm ring-enhancing lesion in the right frontal lobe with surrounding edema and 8mm midline shift. Analyze the pathogenesis, likely organisms, and construct a comprehensive management plan.

Key Learning Points:

• Brain abscesses from contiguous spread (sinusitis) are typically polymicrobial with streptococci, staphylococci, and anaerobes 1
• The European Society of Clinical Microbiology and Infectious Diseases strongly recommends neurosurgical aspiration or excision as early as possible 4
• Empirical therapy should include a third-generation cephalosporin combined with metronidazole for community-acquired brain abscesses 4, 7
• Metronidazole is FDA-approved for CNS infections including brain abscess caused by Bacteroides species and Clostridium species 7
• Microbiological diagnosis requires aspiration or excision with culture and molecular diagnosis 1, 4
• Corticosteroids are recommended for severe symptoms due to perifocal edema or impending herniation 4
• The 8mm midline shift indicates significant mass effect requiring urgent neurosurgical intervention 4
• Duration of therapy should be 6-8 weeks of IV antibiotics for aspirated abscesses; 4 weeks may be considered after excision 4
• Antibiotics should be withheld until aspiration/excision unless severe underlying disease and neurosurgery can be performed within 24 hours 4

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Question 6: CSF Interpretation in Partially Treated Meningitis

A 35-year-old received 2 doses of oral amoxicillin for "sinusitis" 24 hours before presenting with worsening headache, fever, and confusion. LP shows: WBC 450/μL (60% neutrophils, 40% lymphocytes), protein 150 mg/dL, glucose 32 mg/dL (serum 95 mg/dL), Gram stain negative. Analyze how prior antibiotics affect your interpretation and management.

Key Learning Points:

• Prior antibiotic treatment may modify CSF findings, though CSF WBC count may not be greatly affected 5
• CSF-to-blood glucose ratio is 0.34, which is above 0.23 but the absolute glucose <35 mg/dL strongly suggests bacterial infection 2, 5
• Mixed neutrophil-lymphocyte pattern can occur in partially treated bacterial meningitis 5
• No single CSF test is fully reliable in distinguishing bacterial from viral meningitis; interpret in clinical context 5
• CSF lactate has better diagnostic accuracy than WBC count in differentiating bacterial from other meningitis types 5
• Negative Gram stain does not exclude bacterial meningitis; sensitivity is limited 1
• Empirical antibiotic therapy should not be delayed while awaiting culture results 1, 2

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Synthesis/Evaluation Questions (Bloom's Level 5)

Question 7: Complex Case Management - Postoperative Ventriculitis

A 62-year-old diabetic underwent emergency decompressive craniectomy for traumatic brain injury with EVD placement. On postoperative day 7, he develops fever (39°C), decreased GCS from 12 to 8, and CSF from EVD shows: WBC 2,500/μL (85% neutrophils), protein 280 mg/dL, glucose 18 mg/dL (serum 180 mg/dL), Gram stain shows Gram-positive cocci in clusters. Design a comprehensive management strategy addressing all aspects of care, justify each decision, and identify potential complications.

Comprehensive Management Strategy:

Immediate Actions:

• Remove the infected EVD and culture the catheter tip 2
• Obtain blood cultures before antibiotics 1
• Start empirical antibiotics immediately without waiting for imaging or LP 1, 2
• Given Gram-positive cocci in clusters (likely S. aureus, the most common pathogen at 50%), initiate vancomycin pending susceptibilities 3
• Control hyperglycemia (serum glucose 180 mg/dL) as diabetes increases infection risk 1

Diagnostic Workup:

• CSF-to-blood glucose ratio is 0.1 (18/180), well below 0.23, confirming bacterial infection 2, 5
• WBC 2,500/μL with 85% neutrophils is typical for bacterial meningitis 2, 5
• Process CSF within 30 minutes to prevent cellular degradation 2
• Send CSF for culture, antimicrobial susceptibility testing, and consider CSF lactate 1, 5

Neurosurgical Considerations:

• Evaluate need for new EVD placement versus alternative CSF diversion 1
• If new EVD required and CSF flow obstructed, sample both device and lumbar space 1, 2
• Monitor for hydrocephalus development 1

Antibiotic Management:

• Most postoperative CNS infections are caused by S. aureus (50%) or P. acnes (25%) 3
• Adjust antibiotics based on culture and susceptibility results 1
• Plan for prolonged IV therapy (minimum 2-3 weeks for ventriculitis) 4

Complications to Monitor:

• Cerebral edema and herniation risk (GCS dropped from 12 to 8) 4
• Seizures (consider prophylaxis in this context, though not routinely recommended for abscesses) 4
• Hydrocephalus requiring permanent shunt 1
• Stroke from vasculitis/thrombosis 1
• Mortality (27% in Candida CNS infections, likely similar for bacterial ventriculitis) 6

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Question 8: Diagnostic Algorithm for Fever in Neurocritical Care

Design an evidence-based diagnostic algorithm for evaluating new-onset fever in a neurocritical care patient with an intracranial pressure monitor. Include decision points, contraindications, and justify each step with specific evidence.

Evidence-Based Diagnostic Algorithm:

Step 1: Initial Assessment

• Fever occurs in ~25% of neurocritical care patients; almost half are non-infectious 1
• Perform focused neurological examination for new focal deficits or altered consciousness 1
• Assess for contraindications to LP: focal neurologic signs suggesting supratentorial mass, signs of herniation, coagulopathy 1

Step 2: Imaging Decision

• If altered consciousness or focal neurologic signs are unexplained, consider LP unless contraindicated 1
• For new fever with new focal findings suggesting disease above foramen magnum, obtain imaging before LP 1
• Non-contrast CT is adequate to exclude mass lesions or obstructive hydrocephalus 1
• MRI is preferred for subtle examination of brain parenchyma and meninges 1

Step 3: CSF Acquisition

• In patients with intracranial devices, obtain CSF from the device reservoir 1, 2
• If CSF flow is obstructed, sample both device and lumbar space 1, 2
• Collect minimum 5 mL for standard testing, 8-10 mL for specialized panels 2
• First tube has highest contamination risk; do not send for microbiology 2
• Process within 30 minutes to prevent cellular degradation 2

Step 4: CSF Analysis

• Core panel: cell count with differential, glucose, protein, Gram stain, bacterial culture 2
• In immunologically normal hosts, normal opening pressure, <5 WBC/μL, and normal protein essentially excludes meningitis 5
• Bacterial meningitis typically shows ≥2,000 WBC/μL or ≥1,180 neutrophils/μL 2
• CSF glucose <35 mg/dL or CSF-to-blood ratio <0.23 strongly suggests bacterial infection 2, 5
• CSF lactate has better diagnostic accuracy than WBC count 5

Step 5: Specialized Testing Based on Clinical Context

• For immunocompromised: cryptococcal antigen, fungal cultures, AFB, PCR for HSV/CMV/JC virus 1, 2
• For suspected viral encephalitis: HSV NAAT (>95% sensitivity), enterovirus NAAT 1
• For device-associated infection: larger volumes (5-10 mL) for fungal/mycobacterial cultures 2, 6

Step 6: Empirical Therapy Decision

• If bacterial meningitis suspected and LP delayed, start antibiotics after blood cultures 1, 2, 5
• For postoperative infections, cover S. aureus and P. acnes 3
• For brain abscess, use third-generation cephalosporin plus metronidazole 4, 7
• Withhold antibiotics until aspiration/excision unless severe disease and surgery within 24 hours 4

Critical Pitfalls:

• Delaying antibiotics for imaging increases mortality 2, 5
• Insufficient CSF volume is a leading cause of false-negative results 2
• In immunocompromised patients, normal CSF does not rule out infection 2, 5
• Hemorrhagic contamination interferes with interpretation 2

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Question 9: Comparative Analysis - Brain Abscess vs. Subdural Empyema

Compare and contrast the clinical presentation, imaging characteristics, microbiological etiology, and surgical management of brain abscess versus subdural empyema. Develop a decision-making framework for distinguishing these entities and justify different management approaches.

Clinical Presentation:

Brain Abscess:

• Progressive headache over days to weeks 4
• Focal neurological deficits based on location 4
• Seizures (focal or generalized) 4
• Fever may be absent in chronic cases 8
• Symptoms of increased ICP (nausea, vomiting, papilledema) 4

Subdural Empyema:

• Neurosurgical emergency with rapid progression 1
• More acute presentation (hours to days) 1
• Severe headache, fever, altered mental status 1
• Seizures more common than in abscess 1
• Rapid neurological deterioration 1

Imaging Characteristics:

Brain Abscess:

• MRI with DWI/ADC shows restricted diffusion (bright on DWI, dark on ADC) 4
• Ring enhancement on T1 post-gadolinium 4
• Surrounding vasogenic edema 4
• Mass effect proportional to size 4
• Smooth, well-defined capsule in mature abscess 4

Subdural Empyema:

• Crescent-shaped extra-axial collection 1
• Does not cross midline (limited by falx) 1
• May show restricted diffusion but less pronounced than abscess 1
• Meningeal enhancement common 1
• Associated cerebral edema and mass effect 1

Microbiological Etiology:

Brain Abscess:

• Contiguous spread: streptococci, staphylococci, anaerobes (Bacteroides, Peptostreptococcus) 1, 4
• Hematogenous spread: depends on source (endocarditis, pulmonary, dental) 1
• Post-neurosurgical: S. aureus (50%), P. acnes (25%) 3
• Immunocompromised: Candida, Aspergillus, Toxoplasma, Nocardia 1, 6

Subdural Empyema:

• Streptococci, staphylococci, aerobic Gram-negative bacilli 1
• Often polymicrobial from sinusitis or otitis 1
• S. aureus in post-traumatic or post-surgical cases 1

Surgical Management:

Brain Abscess:

• Neurosurgical aspiration or excision strongly recommended as early as possible 4
• Aspiration preferred for deep/eloquent locations 4
• Excision for large abscesses (>2.5 cm), multiloculated, or failed aspiration 4
• Antibiotics withheld until aspiration unless severe disease and surgery within 24 hours 4
• Duration: 6-8 weeks IV for aspirated; 4 weeks after excision 4

Subdural Empyema:

• Neurosurgical emergency requiring immediate drainage 1
• Craniotomy preferred over burr holes for adequate drainage 1
• Cannot delay for culture results given rapid progression 1
• Start empirical antibiotics immediately 1
• Duration: minimum 3-4 weeks IV antibiotics 1

Decision-Making Framework:

1. Imaging is definitive: MRI distinguishes intra-axial (abscess) from extra-axial (empyema) 1, 4
2. Urgency differs: Empyema requires immediate surgery; abscess allows time for optimization 1, 4
3. Antibiotic timing: Empyema - start immediately; Abscess - can wait for aspiration if surgery within 24 hours 1, 4
4. Prognosis: Empyema has higher mortality due to rapid progression and cortical vein thrombosis 1

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Question 10: Evidence-Based Protocol Development

You are tasked with developing an institutional protocol for managing suspected CNS shunt infections in pediatric patients. Synthesize the evidence to create a comprehensive protocol including diagnostic criteria, CSF sampling technique, empirical antibiotic selection, and criteria for shunt removal versus salvage. Justify each recommendation with specific evidence and address controversies.

Comprehensive CNS Shunt Infection Protocol:

I. Diagnostic Criteria and Clinical Suspicion

Clinical Presentation:

• 5-15% of shunts become infected 1
• Fever, headache, nausea/vomiting, altered mental status 1
• Signs of shunt malfunction (increased ICP) 1
• Abdominal pain (if VP shunt) 1
• Routes of infection: contamination at placement, retrograde from distal end, skin breakdown, hematogenous seeding 1

Risk Factors:

• Recent shunt placement or revision 1
• Skin breakdown over shunt hardware 1
• Immunosuppression 1
• Previous shunt infection 1

II. CSF Sampling Technique

Sampling Location:

• Obtain CSF from the CSF reservoir (Ommaya or shunt tap) 1, 2
• If CSF flow to subarachnoid space is obstructed, sample both reservoir and lumbar space 1, 2
• Avoid lumbar puncture alone as it may miss ventriculitis 1

Sample Collection:

• Collect minimum 5 mL for standard testing, 8-10 mL for specialized panels 2
• First tube has highest contamination risk; do not send for microbiology 2
• Process within 30 minutes to prevent cellular degradation 2
• Larger volumes (5-10 mL) increase sensitivity for fungal cultures 2, 6

Laboratory Analysis:

• Core panel: cell count with differential, glucose, protein, Gram stain, bacterial culture 2
• CSF lactate for improved diagnostic accuracy 5
• For immunocompromised or those on TPN/steroids/broad-spectrum antibiotics: fungal stains and cultures 1, 6
• Blood cultures if ventriculoatrial shunt 1

III. Empirical Antibiotic Selection

Standard Empirical Regimen:

• Most CNS shunt infections are bacterial 1
• Cover S. aureus (most common) and P. acnes 3
• Vancomycin PLUS either:
  • Third-generation cephalosporin (ceftriaxone or cefotaxime), OR
  • Fourth-generation cephalosporin (cefepime) for broader Gram-negative coverage 1

Special Populations:

• Immunocompromised, TPN, steroids, or broad-spectrum antibiotics: add antifungal coverage 1, 6
• Fluconazole or liposomal amphotericin B for Candida 6
• Candida albicans is predominant species (73%) 6

Antibiotic Timing:

• Start immediately after CSF and blood cultures obtained 1, 2
• Do not delay for imaging or additional testing 1, 2

IV. Criteria for Shunt Removal vs. Salvage

Indications for Shunt Removal (Preferred Approach):

• Infected devices should be removed for best cure rates 6
• Gram-positive organisms (especially S. aureus) 6, 3
• Fungal infections (mandatory removal) 6
• Clinical deterioration despite antibiotics 6
• Persistent positive CSF cultures after 48-72 hours of antibiotics 6
• Shunt malfunction requiring revision 1

Shunt Salvage Attempt (Selected Cases Only):

• Coagulase-negative staphylococci (less virulent) 3
• Early infection (<30 days post-op) with prompt treatment 3
• High surgical risk patient 6
• Rapid clinical and microbiological response to antibiotics 6

Salvage Protocol:

• Intraventricular antibiotics via reservoir (controversial, limited evidence) 6
• Daily CSF sampling to document sterilization 6
• If not sterilized within 72 hours, remove shunt 6

V. Definitive Management

Surgical Approach:

• Remove entire shunt system (proximal and distal components) 6
• Place external ventricular drain for temporary CSF diversion 6
• Culture shunt hardware 2, 6
• Delay new shunt placement until CSF sterilized (typically 7-10 days of negative cultures) 6

Antibiotic Duration:

• Minimum 10-14 days IV antibiotics after CSF sterilization 6
• Longer duration (3-4 weeks) for S. aureus or Gram-negative organisms 6
• Fungal infections: minimum 4-6 weeks after shunt removal and CSF sterilization 6

VI. Addressing Controversies

Controversy 1: Intraventricular Antibiotics

• Limited evidence for routine use 6
• Consider only in salvage attempts or difficult-to-treat organisms 6
• Risk of chemical ventriculitis 6

Controversy 2: Antibiotic Prophylaxis at Shunt Placement

• Appropriate prophylaxis (narrow spectrum, just before incision, discontinued within 24 hours) reduces infection risk 1
• Target S. aureus as most common pathogen 3

Controversy 3: Timing of Shunt Replacement

• Balance between adequate CSF sterilization and risk of EVD-associated infection 6
• Minimum 7-10 days of negative cultures recommended 6
• EVD itself carries infection risk; remove when patient develops stupor or meningitis signs 2

VII. Monitoring and Follow-up

Acute Phase:

• Daily CSF analysis until sterilization documented 6
• Monitor for complications: hydrocephalus, seizures, stroke 1, 4
• Adjust antibiotics based on culture results and susceptibilities 1

Long-term:

• Mortality rate for CNS fungal infections is 27% 6
• Monitor for neurological sequelae 9, 8
• Assess need for permanent shunt replacement 6

VIII. Quality Metrics

• Time from clinical suspicion to CSF sampling 2
• Time from CSF sampling to antibiotic administration 1, 2
• CSF sterilization rate at 72 hours 6
• Shunt salvage success rate (if attempted) 6
• 30-day mortality and morbidity 6
• Recurrent infection rate after shunt replacement 6