4.3 Central Nervous System Infections

ABIM Internal Medicine Exam

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2 Chapter 1: Cardiovascular Disease (14% of Exam)

1.1 Ischemic Heart Disease & Acute Coronary Syndromes 1.2 Antithrombotic Therapy, Secondary Prevention & Mechanical Complications of MI 1.3 Atrial Fibrillation, Atrial Flutter & Stroke Prevention 1.4 Supraventricular Tachycardias, Ventricular Arrhythmias & Bradycardias 1.5 Heart Failure: Classification, Staging & Guideline-Directed Medical Therapy 1.6 Device Therapy, HFpEF & the Cardiomyopathies 1.7 Valvular Heart Disease & Endocarditis Prophylaxis 1.8 Pericardial Disease & Acute Aortic Syndromes 1.9 Hypertension: Classification, Diagnosis & Secondary Causes 1.10 Hypertensive Crises & Dyslipidemia 1.11 Adult Congenital Heart Disease & Coarctation of the Aorta 1.12 Syncope: Reflex, Orthostatic & Cardiac Causes 1.13 Cardiac Physical Examination, Murmurs & Antithrombotic Selection

3 Chapter 2: Endocrinology, Diabetes & Metabolism (9% of Exam)

2.1 Diabetes Mellitus: Diagnosis, Glycemic Targets & Pharmacotherapy 2.2 Insulin Therapy, Hyperglycemic Emergencies & Chronic Complications 2.3 Thyroid Function Testing & Hypothyroidism 2.4 Thyrotoxicosis, Thyroid Storm & Thyroid Nodules 2.5 Adrenal & Pituitary Disorders 2.6 Hypercalcemia, Hypocalcemia & Disorders of Mineral Metabolism 2.7 Osteoporosis & Paget Disease of Bone 2.8 Ovarian, Testicular & Reproductive Endocrine Disorders 2.9 Hypoglycemia, Endocrine Tumors, Polyglandular Syndromes & Severe Hypertriglyceridemia

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3.1 Esophageal Disease 3.2 Stomach & Duodenal Disease and Gastrointestinal Hemorrhage 3.3 Inflammatory Bowel Disease 3.4 Celiac Disease & Irritable Bowel Syndrome 3.5 Diverticular Disease & Colorectal Cancer Screening 3.6 Viral Hepatitis, Steatotic & Alcohol-Associated Liver Disease 3.7 Cirrhosis & the Complications of Portal Hypertension 3.8 Acute Pancreatitis 3.9 Chronic Pancreatitis & Biliary Tract Disease 3.10 Inherited, Autoimmune & Drug-Induced Liver Disease 3.11 Small Intestinal Disease, Ischemia & Gastrointestinal Complications of HIV

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4.1 Lower Respiratory Tract Infections & Sepsis Syndromes 4.2 Viral Respiratory Pathogens & Tuberculosis 4.3 Central Nervous System Infections 4.4 Skin, Soft Tissue, Bone & Joint Infections 4.5 Infective Endocarditis & Cardiovascular Infection 4.6 Clostridioides difficile & Catheter-Related Bloodstream Infection 4.7 HIV Diagnosis, Antiretroviral Therapy & Prevention 4.8 Opportunistic Infections & Sexually Transmitted Infections 4.9 Fever of Unknown Origin, Neutropenic Fever & Toxic Shock 4.10 Travel-Related Illness, Vector-Borne Disease & Outbreaks 4.11 Infection Prevention, Nosocomial Infection & Postexposure Prophylaxis 4.12 Rheumatic Fever, Poststreptococcal Disease, Enteric & Hepatic Infections

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9.1 Acute Kidney Injury: Staging, Etiology & Urine Microscopy 9.2 Glomerular Disorders & Nephrotic Syndrome 9.3 Nephritic Syndromes, RPGN & Indications for Renal Replacement Therapy 9.4 Chronic Kidney Disease: Staging & Progression Retardation 9.5 Complications of Chronic Kidney Disease: Anemia, CKD-MBD & Acidosis 9.6 Hyponatremia, Hypernatremia & Water Balance 9.7 Potassium Disorders & Systematic Acid-Base Analysis 9.8 Renal Tubular Acidosis & Nephrolithiasis 9.9 Prostate Disorders, Urinary Incontinence & Other Urologic Conditions 9.10 Cystic Kidney Disease, Obstructive Uropathy, Tubulointerstitial Disease & Hematuria

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10.1 Acute Ischemic Stroke: Syndromes & Reperfusion 10.2 Transient Ischemic Attack, Secondary Prevention & Hemorrhagic Stroke 10.3 Headache & Facial Pain 10.4 Seizures, Epilepsy & Movement Disorders 10.5 Multiple Sclerosis & Neuromuscular Junction Disorders 10.6 Motor Neuron Disease & Acute Peripheral Neuropathy 10.7 Cranial Neuropathies & Neuro-Ophthalmology 10.8 Peripheral Neuropathies, CNS Tumors & Other Neurologic Disorders

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13 Chapter 12: Dermatology (3% of Exam)

12.1 Dermatitis, Papulosquamous & Inflammatory Dermatoses 12.2 Acne, Rosacea & Cutaneous Infections 12.3 Herpes Zoster & Autoimmune Blistering Diseases 12.4 Nodules, Tumors & Cutaneous Oncology 12.5 Dermatologic Emergencies & Severe Cutaneous Adverse Reactions 12.6 Pigment Disorders, Alopecia, Nail Disease & Photosensitivity 12.7 Vascular Dermatoses, Environmental Injury & Cutaneous Signs of Systemic Disease

14 Chapter 13: Obstetrics & Gynecology / Women's Health (3% of Exam)

13.1 Contraception, Preconception Care & Teratogenicity 13.2 Hypertensive Disorders of Pregnancy & Gestational Diabetes 13.3 Thyroid Disease & Venous Thromboembolism in Pregnancy 13.4 Menopause & Menopausal Hormone Therapy 13.5 Abnormal Uterine Bleeding, Endometrial Disease & Gynecologic Cancer Screening 13.6 Breast Disorders & Diseases of the Vulva and Vagina 13.7 Menstrual Disorders, Fibroids, Endometriosis & Early Pregnancy Complications

15 Chapter 14: Geriatric Syndromes (3% of Exam)

14.1 Comprehensive Geriatric Assessment & Frailty 14.2 Falls, Gait Assessment & Polypharmacy 14.3 Urinary Incontinence & Pressure Injuries in Older Adults 14.4 Mild Cognitive Impairment, Dementia & Its Subtypes 14.5 Delirium & the Pharmacotherapy of Cognitive Disorders 14.6 Palliative Care, Hospice & Advance Care Planning 14.7 Chronic Wounds, Nutrition, Swallowing, Constipation & Hip Fracture 14.8 Sensory Impairment, Dizziness, Elder Mistreatment & Care Settings

16 Chapter 15: Allergy & Immunology (2% of Exam)

15.1 Anaphylaxis: Recognition & Emergency Management 15.2 Urticaria & Angioedema 15.3 Drug Allergy, Desensitization & Adult Immunodeficiency 15.4 Food Allergy, Insect Venom Hypersensitivity, Serum Sickness & Contrast Reactions 15.5 Rhinitis, Conjunctivitis, Eosinophilic & Mast Cell Disorders

17 Chapter 16: Preventive Medicine, Immunization & Clinical Epidemiology

16.1 The USPSTF Framework & Evidence-Based Cancer Screening 16.2 Cardiovascular & Metabolic Screening and Primary Prevention 16.3 Adult Immunization & Vaccines in Special Populations 16.4 Clinical Epidemiology, Biostatistics & Study Design

18 Chapter 17: Miscellaneous - Ethics, Patient Safety & Quality Improvement (2% of Exam)

17.1 Principles of Bioethics & Decisional Capacity 17.2 Informed Consent, Treatment Refusal & Special Scenarios 17.3 Surrogate Decision-Making, Advance Directives & Confidentiality 17.4 Patient Safety Science, Medical Error & Diagnostic Reasoning 17.5 Quality Improvement Methods, Health Disparities & Health Equity

19 Chapter 18: Perioperative Medicine & Medical Consultation

18.1 Preoperative Cardiovascular Risk Assessment 18.2 Preoperative Pulmonary Risk & Perioperative Medication Management 18.3 Perioperative Anticoagulation, Bridging & Postoperative Complications

20 Chapter 19: Ophthalmology (1% of Exam)

19.1 The Acute Red Eye & Ophthalmic Emergencies 19.2 Painless Vision Loss & Chronic Ophthalmic Disease 19.3 Uveitis, Scleritis & Orbital Disease

21 Chapter 20: Otolaryngology & Dental Medicine (1% of Exam)

20.1 Dizziness, Vertigo & Epistaxis 20.2 Ear Disorders, Hearing Loss & Upper Respiratory Infections 20.3 Oral Cavity, Salivary Gland, Laryngeal & Taste and Smell Disorders

4.3 Central Nervous System Infections

Key Takeaways Explain This Topic AI

Meningitis, brain abscess, encephalitis, epidural abscess and iatrogenic or postoperative infections are the enumerated CNS infection topics.

Antibiotics and dexamethasone must not be delayed for computed tomography when bacterial meningitis is suspected; cultures are drawn first and the lumbar puncture follows imaging.

Dexamethasone reduces mortality and neurologic sequelae in pneumococcal meningitis and should be given with or before the first antibiotic dose.

Ampicillin is added for Listeria coverage in patients over 50, in pregnancy and in immunocompromised or alcohol-using patients.

Herpes simplex encephalitis produces temporal lobe involvement with cerebrospinal fluid lymphocytosis and red blood cells, and empiric acyclovir is started before polymerase chain reaction results return.

Last updated: August 2026

Acute CNS infections, rapidly spreading soft-tissue emergencies, and joint/bone infections require immediate diagnostic precision and rapid initiation of targeted antimicrobial therapy. This section details age-based meningitis regimens, lumbar puncture safety, necrotizing fasciitis management, and musculoskeletal infection workups.

1. Acute Bacterial Meningitis

Acute bacterial meningitis is a medical emergency characterized by bacterial replication and neutrophilic inflammation within the subarachnoid space and leptomeninges.

Etiology by Patient Age and Host Factors

Adults < 50 Years: Streptococcus pneumoniae (~50–60%), Neisseria meningitidis (~20–30%), Haemophilus influenzae (<5%).

Adults >= 50 Years or Immunocompromised (Cirrhosis, Alcoholism, Malignancy, Corticosteroid Therapy): Streptococcus pneumoniae , Neisseria meningitidis , PLUS Listeria monocytogenes (~15–20%) and aerobic Gram-negative bacilli ( E. coli , Klebsiella spp.).

Post-Neurosurgery, Penetrating Head Trauma, or CSF Shunts: Staphylococcus aureus (including MRSA), Staphylococcus epidermidis , Pseudomonas aeruginosa , and enteric Gram-negative bacilli.

Clinical Presentation

Classic Triad: Fever, altered mental status, and nuchal rigidity (present in >95% of patients with at least 2 of 4 symptoms: headache, fever, neck stiffness, altered sensorium).

Physical Signs: Kernig sign (resistance or pain upon passive knee extension with hip flexed 90°) and Brudzinski sign (involuntary flexion of hips/knees upon passive neck flexion). Both have high specificity (>95%) but low sensitivity (<15%).

Petechial/Purpuric Rash: Strongly suggests Neisseria meningitidis bacteremia with meningococcemia and disseminated intravascular coagulation (Waterhouse-Friderichsen syndrome).

CSF Analysis Profiles Across CNS Infections

Infection Type Opening Pressure WBC Count (/mcL) & Differential Protein (mg/dL) Glucose (mg/dL) & CSF/Serum Ratio Diagnostic Tests

Normal Reference 80–200 mm H2O <5 (100% Lymphocytes/Monocytes) 15–45 45–80 (CSF/Serum >=0.6) Negative

Acute Bacterial Meningitis Markedly Elevated (>200–350 mm H2O) Markedly Elevated (1,000–10,000+), >80–90% Neutrophils (PMNs) Markedly Elevated (>100–500+) Markedly Decreased (<40 mg/dL or CSF/Serum <0.4 ) Gram stain positive (60–80%), Bacterial culture, BioFire FilmArray PCR

Viral (Aseptic) Meningitis Normal to Mildly Elevated (100–250 mm H2O) Mildly Elevated (10–500), Lymphocytic predominance (PMNs in first 24h) Mildly Elevated (50–100) Normal (>45 mg/dL or CSF/Serum >0.6) Viral PCR (Enterovirus, HSV-2, Arbovirus)

Fungal (Cryptococcal) Meningitis Markedly Elevated (>200–400+ mm H2O) Moderately Elevated (20–500), Lymphocytic predominance Elevated (100–500) Low to Normal (<40 mg/dL) Cryptococcal Antigen (CrAg) positive (>98%), India ink stain (budding yeast)

Tuberculous Meningitis Elevated (>200–300 mm H2O) Moderately Elevated (100–500), Lymphocytic predominance Markedly Elevated (>150–500+) Markedly Decreased (<30 mg/dL or CSF/Serum <0.3) Acid-fast bacilli (AFB) smear, GeneXpert MTB/RIF, CSF AFB culture

Safe Approach to Lumbar Puncture & Indications for Prior Head CT

Critical Rule: If bacterial meningitis is suspected, draw blood cultures and immediately administer empiric IV antibiotics and dexamethasone . Never delay antimicrobial administration to perform neuroimaging or lumbar puncture.

Indications for Head CT Prior to Lumbar Puncture:

Focal neurological deficits (e.g., hemiparesis, facial droop, visual field defect, unequal pupils).

New-onset seizure within the preceding 1 week.

Papilledema on funduscopic examination.

Severely depressed level of consciousness (Glasgow Coma Scale <10, stupor, or coma).

Immunocompromised state (HIV infection, solid organ or stem cell transplant, active immunosuppressive therapy).

Empiric Antimicrobial Regimens & Role of Dexamethasone

Age < 50 Years: Vancomycin (15–20 mg/kg IV q8–12h targeting trough 15–20 mcg/mL) PLUS Ceftriaxone (2 g IV q12h) OR Cefotaxime (2 g IV q4h).

Age >= 50 Years or Immunocompromised: Vancomycin (15–20 mg/kg IV q8–12h) PLUS Ceftriaxone (2 g IV q12h) PLUS Ampicillin (2 g IV q4h for Listeria monocytogenes coverage).

Note: Ceftriaxone and all other cephalosporins lack intrinsic activity against Listeria monocytogenes .

Post-Neurosurgical / CSF Shunt: Vancomycin (15–20 mg/kg IV q8–12h) PLUS Cefepime (2 g IV q8h) or Meropenem (2 g IV q8h).

Severe Beta-Lactam Anaphylaxis: Vancomycin + Moxifloxacin (or Levofloxacin) + Trimethoprim-sulfamethoxazole (for Listeria ).

Adjunctive Dexamethasone:

Dose: 10 mg IV every 6 hours for 4 days .

Timing: Must be administered prior to or concurrently with the first dose of antimicrobial therapy . If antibiotics have already been administered hours earlier, dexamethasone should not be initiated.

Benefit: Significantly reduces mortality and long-term sensorineural hearing loss in adults with Streptococcus pneumoniae meningitis. Discontinue if culture identifies N. meningitidis , Listeria , or other organisms.

Loading diagram... Suspected Acute Bacterial Meningitis Clinical Decision Algorithm

2. Viral Meningitis & Encephalitis

Aseptic (Viral) Meningitis: Most commonly caused by Enteroviruses (Coxsackievirus, Echovirus) ; self-limiting with headache, fever, photophobia, normal glucose, and lymphocytic pleocytosis.

Herpes Simplex Virus-1 (HSV-1) Encephalitis:

Pathophysiology: Retrograde transport of HSV-1 along olfactory or trigeminal nerves into the temporal and inferior frontal lobes, causing hemorrhagic necrotizing encephalitis.

Clinical Manifestations: Acute fever, severe headache, confusion, personality/behavioral changes, olfactory/gustatory hallucinations, and focal temporal lobe seizures (lip smacking, automatisms).

Diagnostics:

Lumbar Puncture: Lymphocytic pleocytosis, elevated protein, and characteristically elevated red blood cells (RBCs) in CSF from hemorrhagic cortical necrosis.

CSF HSV-1 PCR: Diagnostic test of choice (sensitivity >95%, specificity >98%).

MRI Brain: Asymmetric T2/FLAIR hyperintensity and restricted diffusion in the temporal lobes, insular cortex, and inferior frontal lobes .

EEG: Periodic Lateralized Epileptiform Discharges (PLEDs) over the affected temporal lobe.

Treatment: Initiate IV Acyclovir 10 mg/kg IV every 8 hours for 14–21 days immediately upon clinical suspicion. Maintain vigorous IV crystalloid hydration to avoid acyclovir crystal precipitation and acute crystalline nephropathy.

West Nile Virus Encephalitis: Transmitted by Culex mosquitoes. Manifests as fever, encephalitis, flaccid asymmetric paralysis (anterior horn cell loss resembling poliomyelitis), and coarse Parkinsonian tremors. Diagnosed via CSF West Nile Virus IgM antibody capture ELISA .