Summary of "Meningitis - causes, symptoms, diagnosis, treatment, pathology"

Main Concepts and Ideas

Definition and Anatomy
- Meningitis is inflammation of the meninges, specifically the two inner layers called the Leptomeninges (arachnoid mater and pia mater).
- The meninges consist of three layers: dura mater (outer), arachnoid mater (middle), and pia mater (inner).
- The subarachnoid space between the arachnoid and pia mater contains Cerebrospinal Fluid (CSF), which cushions the brain and spinal cord and provides nutrients.
Cerebrospinal Fluid (CSF) Characteristics
- Normal CSF contains up to 5 white blood cells per microliter, mostly lymphocytes and monocytes, with few polymorphonuclear cells (PMNs).
- CSF contains proteins (15-50 mg/dL) and glucose (45-100 mg/dL), roughly two-thirds of blood glucose concentration.
- CSF volume is about 150 mL, with 500 mL produced daily and excess absorbed into the bloodstream.
- The blood-brain barrier tightly regulates substances entering/leaving the CSF and brain.
Pathophysiology of Meningitis
- Meningitis refers to inflammation of the Leptomeninges, distinct from encephalitis (brain inflammation). Both can co-occur as meningoencephalitis.
- Triggers for Meningitis include autoimmune diseases, adverse medication reactions (e.g., intrathecal therapy), but infection is the most common cause.
- Infection reaches the CSF by:
  1. Direct spread — via anatomical defects (e.g., spina bifida, skull fractures) or through skin/nasal passages.
  2. Hematogenous spread — pathogens enter bloodstream and cross the blood-brain barrier by binding endothelial receptors or exploiting vulnerable sites (e.g., choroid plexus).
Immune Response in Meningitis
- Once pathogens enter CSF, resident white blood cells release cytokines to recruit more immune cells.
- White blood cell counts in CSF rise above normal (>5/μL), defining Meningitis.
- Typical CSF white blood cell profiles vary by cause:
  - Bacterial: >100 WBC/μL, >90% PMNs
  - Viral: 10-1000 WBC/μL, >50% lymphocytes, <20% PMNs
  - Fungal: 10-500 WBC/μL, >50% lymphocytes
  - Tuberculous: 50-500 WBC/μL, >80% lymphocytes
- Inflammation increases CSF pressure (>200 mm H2O), lowers glucose concentration (<2/3 blood glucose), and raises protein levels (>50 mg/dL).
Causes of Meningitis
- Bacterial:
  - Newborns: Group B streptococci, E. coli, Listeria monocytogenes
  - Children/teens: Neisseria meningitidis, Streptococcus pneumoniae
  - Adults/elderly: Streptococcus pneumoniae, Listeria monocytogenes
  - Tick-borne: Borrelia burgdorferi (Lyme disease)
- Viral:
  - Enteroviruses (especially coxsackie virus), herpes simplex virus, HIV, mumps virus, varicella zoster virus, lymphocytic choriomeningitis virus
- Fungal: Cryptococcus, Coccidioides (mostly immunocompromised)
- Tuberculous: Mycobacterium tuberculosis
- Parasitic: Plasmodium falciparum (malaria)
Symptoms of Meningitis
- Classic triad: headache, fever, nuchal rigidity (neck stiffness)
- Additional: photophobia (light sensitivity), phonophobia (sound sensitivity)
- Meningoencephalitis may cause altered mental status and seizures.
Diagnosis of Meningitis
- Physical exam includes:
  - Kernig’s sign: pain on straightening a flexed leg while lying supine
  - Brudzinski’s sign: involuntary knee/hip flexion upon neck flexion
- Lumbar puncture (spinal tap) to collect CSF between lumbar vertebrae (e.g., L3-L4):
  - Measure opening pressure
  - Analyze CSF for white blood cells, protein, glucose
  - Use PCR to detect specific pathogens (HIV, HSV, tuberculosis, enteroviruses)
  - Other tests as indicated (e.g., Western blot)