What are the differential diagnoses for a 58‑year‑old man with subacute balance impairment, low‑grade fever, descending weakness, headache, neck stiffness, speech and memory changes over two weeks, normal CT/MRI, negative initial lumbar puncture, marked leukocytosis and elevated C‑reactive protein, iron‑deficiency anemia, and partial improvement with intravenous antibiotics that recurs after discontinuation?

Differential Diagnosis: Subacute CNS Infection with Systemic Inflammation

This clinical presentation most strongly suggests a partially treated bacterial CNS infection—specifically bacterial meningitis, brain abscess, or spinal epidural abscess—given the response to and relapse after discontinuation of IV antibiotics. 1

Primary Diagnostic Considerations

1. Partially Treated Bacterial Meningitis/Meningoencephalitis

This is your leading diagnosis. The key features pointing here include:

• Classic meningitis symptoms present: fever, headache, neck stiffness, altered mental status (speech/memory changes) 1
• Critical caveat: The "negative" LP may be falsely reassuring if done after antibiotic exposure or if specific pathogens weren't tested
• Marked leukocytosis and elevated CRP indicate ongoing bacterial infection
• Symptom improvement with IV antibiotics followed by relapse is pathognomonic for inadequately treated bacterial infection

Important pitfall: The ESCMID guidelines emphasize that classic symptoms can be absent in 50-60% of bacterial meningitis cases, but this patient has the full constellation 1. A negative initial LP does not exclude meningitis—CSF culture sensitivity drops significantly with prior antibiotics, and some organisms (Listeria, TB) require special culture techniques.

2. Listeria Monocytogenes Meningoencephalitis/Brain Abscess

This deserves special consideration in this 58-year-old patient:

• Listeria causes rhombencephalitis (brainstem involvement) presenting with ataxia, cranial nerve deficits, and descending weakness 2
• Can present with normal initial imaging or develop abscesses over days to weeks 2
• Often misses standard empiric coverage if not specifically targeted
• Associated with iron deficiency states and immunocompromise
• The case report of a 70-year-old with similar presentation (fever, headache, neurologic symptoms, initially negative workup) who had multiple brain abscesses from Listeria is directly relevant 2

3. Spinal Epidural Abscess

The descending weakness pattern is highly suspicious:

• Descending weakness is the hallmark of spinal cord compression 3
• Can present with meningismus due to associated meningitis
• MRI of spine is mandatory—CT/MRI brain would be normal
• The case of a 56-year-old with fever, back pain, and progressive tetraparesis from spinal epidural abscess with Staph aureus bacteremia mirrors this presentation 3

4. Tuberculous Meningitis

Consider given the subacute (2-week) timeline:

• Subacute presentation over weeks is classic
• Initial LP can be negative without specific AFB staining and prolonged cultures
• Basilar meningitis causes cranial nerve deficits and can affect balance/gait
• Requires specific testing (AFB stain, TB PCR, adenosine deaminase)

Secondary Considerations

5. Autoimmune/Paraneoplastic Encephalitis

Less likely given antibiotic response, but consider:

• Acute disseminated encephalomyelitis (ADEM) can follow infections 4
• However, ADEM would not improve with antibiotics and would show characteristic MRI changes
• Autoimmune encephalitis typically doesn't cause fever and systemic inflammation to this degree

6. Occult Malignancy with CNS Involvement

The iron-deficiency anemia raises this concern:

• Lymphomatous meningitis can present similarly
• Carcinomatous meningitis from GI malignancy (explaining Fe-deficiency)
• Would require cytology on LP and systemic workup

Critical Next Steps

Immediate actions required:

1. Restart broad-spectrum antibiotics immediately including:

   • Vancomycin + ceftriaxone (or cefotaxime) for bacterial meningitis
   • ADD ampicillin specifically for Listeria coverage (this is likely missing from current regimen) 4, 2
   • Consider adding metronidazole if abscess suspected

2. Repeat lumbar puncture with expanded testing:

   • Standard CSF studies (cell count, protein, glucose, Gram stain, culture)
   • Listeria culture specifically requested
   • TB studies (AFB, TB PCR, adenosine deaminase)
   • Fungal studies
   • Cytology for malignancy
   • Autoimmune/paraneoplastic panel if above negative

3. Obtain MRI of entire spine with contrast:

   • Rule out epidural abscess
   • Look for leptomeningeal enhancement

4. Repeat brain MRI with contrast:

   • Initial imaging may have been too early
   • Look specifically for brainstem lesions (Listeria rhombencephalitis)
   • Look for ring-enhancing lesions (abscesses)

5. Blood cultures (if not already done)

6. Investigate iron-deficiency anemia source:

   • GI malignancy workup (colonoscopy, upper endoscopy)
   • This may reveal primary source of infection or occult malignancy

Key Clinical Pearls

• Never discontinue antibiotics in suspected CNS infection without definitive alternative diagnosis
• Listeria is frequently missed because it requires ampicillin; standard cephalosporin regimens don't cover it
• Normal imaging does not exclude CNS infection—abscesses and epidural collections can develop over days
• The combination of descending weakness + meningismus mandates spinal imaging
• A single negative LP in the setting of high clinical suspicion requires repeat LP with expanded testing

The iron-deficiency anemia is likely a red herring representing chronic GI blood loss from unrelated pathology, but warrants investigation once the acute CNS process is controlled.