Polymorphic Pleocytosis: Definition and Clinical Significance
Polymorphic (or polymorphonuclear) pleocytosis refers to an abnormal increase in white blood cells in the cerebrospinal fluid (CSF) where neutrophils (polymorphonuclear leukocytes) are the predominant cell type, typically exceeding 5 cells per microliter. 1, 2
Key Characteristics
- Cell composition: Predominantly neutrophils/polymorphonuclear cells (PMNs) rather than lymphocytes 1
- Cell count range: Can vary from mild elevations (tens of cells) to several hundred or even thousands of cells per microliter, depending on the underlying cause 1, 2
- Associated findings: Often accompanied by elevated protein (100-200 mg/dL) and variable glucose levels 1
Clinical Significance and Differential Diagnosis
Infectious Causes
Bacterial meningitis is the most critical diagnosis to consider with polymorphic pleocytosis, typically presenting with:
- Very low CSF glucose (<20-30 mg/dL) 1
- Markedly elevated protein 1
- Neutrophilic predominance with high cell counts 1
Tickborne rickettsial diseases (Rocky Mountain Spotted Fever, ehrlichiosis, E. ewingii infection) characteristically produce neutrophilic pleocytosis 1:
- E. ewingii infection specifically shows neutrophilic pleocytosis 1
- RMSF can show either polymorphonuclear or lymphocytic predominance 1
Partially treated bacterial meningitis may present with polymorphic pleocytosis but modified CSF parameters 1
Non-Infectious Causes
Post-seizure pleocytosis can produce polymorphic pleocytosis 3, 4:
- 57% of seizure-related pleocytosis shows PMN predominance 3
- 86% of alcohol withdrawal seizures demonstrate PMN predominance 3
- Mean white cell count of 72/mm³ (range 3-464) 3
- Typically transient and resolves within days 3
CMV polyradiculomyelopathy presents with neutrophilic pleocytosis (usually 100-200 neutrophils/mL) accompanied by hypoglycorrhachia and elevated protein 1
Tuberculous meningitis during treatment can paradoxically develop sudden polymorphonuclear pleocytosis 4-108 days after starting chemotherapy, which resolves within a week 5
Diagnostic Approach
Critical Distinguishing Features
CSF glucose is the key discriminator 1:
- CSF glucose <20-30 mg/dL or CSF:plasma glucose ratio <0.5 strongly suggests bacterial meningitis 1
- Normal glucose with polymorphic pleocytosis suggests non-infectious causes (seizures, post-treatment inflammation) 3, 5
CSF lactate measurement provides rapid differentiation 1:
- CSF lactate <2 mmol/L effectively rules out bacterial disease 1
- Elevated lactate supports bacterial infection 1
Gram stain findings 1:
- Gram-negative diplococci indicate meningococcal meningitis 1
- Positive Gram stain with polymorphic pleocytosis mandates immediate antibiotic therapy 1
Clinical Context Matters
Timing and clinical presentation guide interpretation 1, 3:
- Acute onset with fever, altered mental status, and meningismus suggests bacterial meningitis requiring immediate empiric antibiotics 1
- Recent seizure activity with rapid clinical recovery suggests post-ictal pleocytosis 3, 4
- Patients on tuberculosis treatment developing new pleocytosis may have paradoxical inflammatory response 5
Critical Management Pitfall
When polymorphic pleocytosis is present with fever and rash, and bacterial meningitis cannot be definitively excluded, empiric treatment for both tickborne rickettsial disease AND meningococcal infection is necessary, as laboratory testing cannot reliably distinguish between these conditions. 1 This dual coverage approach prioritizes patient survival over diagnostic certainty in critically ill patients.