What can cause a slight creatine kinase (CK) total increase in a psychiatric patient on Risperidone (risperidone) with leukocytosis (White Blood Cell (WBC) count of 14), normal Salmonella Immunoglobulin G (IgG) and Immunoglobulin M (IgM) and Dengue Duo results, presenting with fever (hyperthermia), tremors, chills, and muscle pain, with a history of chest pain and abdominal pain, and potentially unreliable reporting?

Differential Diagnosis: Elevated CK in Psychiatric Patient on Risperidone with Fever

The most critical concern in this patient is Neuroleptic Malignant Syndrome (NMS), which must be ruled out immediately given the combination of risperidone use, fever, tremors, elevated CK, and leukocytosis—even with "slight" CK elevation and evolving symptoms. 1

Primary Differential Diagnoses

1. Neuroleptic Malignant Syndrome (NMS) - HIGHEST PRIORITY

NMS is a potentially fatal complication of antipsychotic drugs including risperidone that presents with hyperpyrexia, muscle rigidity, altered mental status, and autonomic instability. 1

Key diagnostic features in this case:

• Fever >37.8°C (hyperpyrexia is characteristic) 1
• Tremors and muscle pain (early manifestations of rigidity) 1, 2
• Elevated CK (additional sign includes elevated creatine phosphokinase) 1
• Leukocytosis (WBC 14) 3
• Autonomic instability suggested by chills 1

Critical clinical pearls about NMS presentation:

• Mild muscle rigidity can occur BEFORE fever onset in 71% of cases 2
• Fever may be delayed—one case showed fever onset only on hospital day 2 despite other NMS symptoms 3
• CK typically peaks on day 2 after fever onset and normalizes by day 12 2
• Muscle rigidity gradually worsens until day 4 after fever onset 2
• CK elevation in NMS commonly ranges from 2,000-15,000 IU/L, but can exceed 250-fold normal in risperidone-induced NMS 4
• Normal CK does NOT exclude NMS—cases have been reported with normal CK levels 5

Immediate management required:

• Discontinue risperidone immediately 1
• Intensive symptomatic treatment and medical monitoring 1
• Consider benzodiazepines, bromocriptine, and dantrolene 3
• Aggressive IV hydration for elevated CK and potential rhabdomyolysis 1
• Monitor for acute renal failure 1

2. Simple Drug-Induced Fever (Hypersensitivity Reaction)

Drug-induced fever is diagnosed by temporal relationship between starting risperidone and fever onset, with average lag time of 21 days (median 8 days). 6, 7

Distinguishing features:

• Fever typically resolves 1-3 days after discontinuation (up to 7 days possible) 6, 7
• Rash and eosinophilia are UNCOMMON 6
• Should NOT have significant muscle rigidity or altered mental status 7
• CK elevation would be minimal or absent 6

Management:

• Discontinue risperidone 7
• Supportive care with antipyretics and hydration 7

3. Infectious Etiology

Despite normal Salmonella and Dengue serology, infection remains a critical differential given fever, leukocytosis, and chest/abdominal pain history. 6

Essential infectious workup:

• Blood cultures (mandatory before antibiotics) 6, 8
• Urinalysis and urine culture 6
• Chest imaging if respiratory symptoms 6
• Stool cultures and C. difficile toxin testing 8
• Consider viral infections including influenza 6

Clinical context:

• Leukocytosis (WBC 14) supports infection but is also seen in NMS 3
• Fever with chest pain could indicate pneumonia or myocarditis 6
• Abdominal pain with fever warrants evaluation for intra-abdominal infection 8, 9

4. Inflammatory Myopathy

Idiopathic inflammatory myopathies present with proximal muscle weakness, elevated CK, and can have systemic symptoms. 10

Key features that would support this:

• Symmetric proximal muscle weakness developing over weeks to months 10
• CK elevation often >10 times upper limit of normal 10
• May have fever and systemic symptoms 10

However, this is LESS likely given:

• Acute presentation rather than subacute/chronic 10
• Patient on risperidone (drug effect more likely) 1
• Presence of tremors (not typical of myositis) 10

Diagnostic Algorithm

Step 1: Assess for NMS immediately

• Examine for muscle rigidity (may be subtle initially) 1, 2
• Check vital signs including pulse, blood pressure for autonomic instability 1
• Assess mental status carefully 1
• Obtain CK level urgently (repeat if initially normal) 2
• Check renal function, myoglobin 1

Step 2: If NMS suspected, discontinue risperidone IMMEDIATELY and initiate intensive supportive care 1

Step 3: Rule out infection concurrently

• Blood cultures, urinalysis, chest X-ray 6
• Consider empiric antibiotics if sepsis suspected (do NOT delay for NMS concern) 6

Step 4: Monitor CK trajectory

• If NMS: CK should peak day 2, normalize by day 12 2
• If drug fever: CK should be minimal/absent 6
• If infection: CK may be mildly elevated from muscle inflammation 6

Critical Pitfalls to Avoid

DO NOT:

• Assume normal or "slightly elevated" CK excludes NMS—it can be normal or delayed 5
• Wait for classic "lead-pipe rigidity"—mild rigidity or tremors may be the only early sign 2
• Dismiss fever <39°C as excluding NMS—maximum temperature may only reach 38.6°C 3
• Continue risperidone while "monitoring"—discontinuation is mandatory if NMS suspected 1
• Attribute all symptoms to psychiatric illness in an "unreliable" patient—this delays life-saving diagnosis 1
• Rechallenge with risperidone if NMS confirmed—recurrences have been reported 1

DO:

• Treat this as NMS until proven otherwise given the potentially fatal outcome 1
• Obtain serial CK measurements over 48-72 hours 2
• Pursue infectious workup simultaneously 6
• Monitor for rhabdomyolysis and acute kidney injury 1