What is the next best step in managing a patient with anorexia nervosa, who has received thiamine supplementation and has normal coagulation studies, electrolytes, and cardiac workup, but presents with headache and left lower extremity weakness?

Neurological Evaluation for Focal Deficits in Anorexia Nervosa Post-Thiamine

Given the new-onset headache and left lower extremity weakness in a patient with anorexia nervosa who has already received adequate thiamine supplementation (200 mg IV x 3 days plus ongoing oral), and with normal electrolytes, coagulation studies, and cardiac workup, the next best step is urgent neuroimaging (MRI brain with and without contrast) to evaluate for structural lesions, followed by consideration of lumbar puncture if imaging is unrevealing. 1

Critical Reasoning: Why This Is Not Refeeding Syndrome

• Refeeding syndrome presents with diffuse neurological symptoms (delirium, confusion, encephalopathy) rather than focal deficits like unilateral lower extremity weakness 1
• The patient's electrolytes are normal, which essentially rules out active refeeding syndrome, as hypophosphatemia, hypokalemia, and hypomagnesemia are the hallmark laboratory findings 1
• Adequate thiamine was already administered (200 mg IV x 3 days meets guideline recommendations of 200-300 mg IV daily for at least 3 days), making Wernicke's encephalopathy from thiamine deficiency unlikely 1, 2
• Focal neurological deficits demand structural evaluation, not metabolic workup 1

Differential Diagnosis for Focal Weakness + Headache

High-Priority Structural Lesions

• Cerebrovascular accident (stroke or hemorrhage): Patients with severe malnutrition can have coagulopathy despite normal initial studies, and nutritional deficiencies can predispose to thrombotic events 1
• Intracranial mass or abscess: Immunocompromised state from malnutrition increases infection risk 1
• Posterior reversible encephalopathy syndrome (PRES): Can occur during refeeding with rapid fluid shifts and electrolyte corrections 1
• Subdural hematoma: Risk increased with brain atrophy from malnutrition 1

Metabolic Considerations (Less Likely Given Normal Labs)

• Hypophosphatemia-related weakness: But this would be diffuse, not focal, and phosphate is normal 1
• Hypomagnesemia: Can cause neurological symptoms but typically presents with seizures or diffuse encephalopathy, not focal weakness 1

Immediate Diagnostic Algorithm

Step 1: Urgent Neuroimaging

• MRI brain with and without contrast is preferred over CT for detecting early ischemia, posterior circulation strokes, and subtle lesions 1
• If MRI unavailable or patient unstable, obtain CT head without contrast immediately to rule out hemorrhage or mass effect 1

Step 2: Repeat Electrolyte Panel (Despite Normal Baseline)

• Recheck phosphate, magnesium, potassium, and calcium as these can drop precipitously during refeeding even after initial normalization 1
• Measure glucose to exclude hypoglycemia, which can present with focal deficits 1
• Check lactate as persistent elevation despite thiamine suggests ongoing metabolic derangement 1

Step 3: Consider Lumbar Puncture If Imaging Unrevealing

• Evaluate for infectious or inflammatory CNS processes (meningitis, encephalitis) given immunocompromised state 1
• Opening pressure to assess for idiopathic intracranial hypertension, which can occur with nutritional rehabilitation 1

Ongoing Refeeding Precautions

Continue Thiamine Despite Adequate Initial Dosing

• Maintain oral thiamine 50-100 mg daily until adequate oral intake is established, as reserves can still be depleted 1, 2
• Do not stop thiamine prematurely even though initial 3-day IV course was completed 2

Aggressive Electrolyte Monitoring and Replacement

• Daily electrolyte monitoring should continue for at least the first week of refeeding, extending beyond 3 days if any abnormalities develop 1
• Phosphate supplementation: 0.3-0.6 mmol/kg/day IV if levels drop below normal 1
• Potassium: 2-4 mmol/kg/day as needed 1
• Magnesium: 0.2 mmol/kg/day IV or 0.4 mmol/kg/day orally 1

Nutritional Advancement Strategy

• If refeeding was aggressive (>10-20 kcal/kg/day initially), consider temporarily reducing to 5-10 kcal/kg/day until neurological etiology is clarified 1
• Monitor fluid balance closely as sodium and water retention can cause volume overload and contribute to neurological complications 1
• Watch for hyperhydration which manifests as peripheral edema and can occur during rapid refeeding 1, 3

Common Pitfalls to Avoid

• Do not assume all neurological symptoms in refeeding are metabolic: Focal deficits require structural evaluation first 1
• Do not rely solely on initial normal electrolytes: These can change rapidly during the first week of refeeding 1
• Do not attribute focal weakness to "general deconditioning": Unilateral weakness is never normal and demands investigation 1
• Do not delay imaging for further metabolic workup: Time is critical if this is a stroke or hemorrhage 1

When to Escalate Care

• Immediate neurology consultation for any focal neurological deficit in the setting of malnutrition and refeeding 1
• ICU-level monitoring may be warranted given the combination of severe malnutrition, active refeeding, and new neurological symptoms 1
• Multidisciplinary involvement including nutrition support team, neurology, and psychiatry for eating disorder management 1