Internal Medicine · Year 3 · from Internal Medicine

Case 2: Severe Hyponatremia with Neurological Symptoms

Patient Demographics

  • Age: 58 years
  • Sex: Female
  • Occupation: Elementary school teacher

Chief Complaint

"My sister is confused and had a seizure at home."

History of Present Illness

The patient was brought to the emergency department by her sister after witnessing a generalized tonic-clonic seizure at home. For the past week, the patient has complained of progressive headache, nausea, and fatigue. Her sister notes that she has been increasingly confused over the past 2 days, with difficulty remembering conversations and appearing "not herself." The patient has a history of small cell lung cancer diagnosed 3 months ago, currently undergoing chemotherapy. She has no prior history of seizures.

Vital Signs

  • Temperature: 98.4 degrees F (36.9 degrees C)
  • Blood pressure: 124/78 mmHg
  • Heart rate: 82 bpm
  • Respiratory rate: 16/min
  • Oxygen saturation: 96% on room air

Physical Examination

General: Somnolent, arousable to voice, disoriented to time and place HEENT: Mucous membranes moist, no JVD Cardiovascular: Regular rate and rhythm, no murmurs Pulmonary: Clear bilaterally, no crackles or wheezes Extremities: No edema, skin turgor normal Neurologic: GCS 13 (E3V4M6), oriented to person only, no focal deficits, reflexes symmetric

Laboratory Findings

  • Sodium: 112 mEq/L (critical value)
  • Potassium: 4.2 mEq/L
  • Chloride: 78 mEq/L
  • Bicarbonate: 24 mEq/L
  • BUN: 8 mg/dL
  • Creatinine: 0.6 mg/dL
  • Glucose: 102 mg/dL
  • Serum osmolality: 232 mOsm/kg (low)
  • Urine sodium: 68 mEq/L (elevated)
  • Urine osmolality: 520 mOsm/kg (inappropriately concentrated)
  • TSH: 2.1 mIU/L (normal)
  • Morning cortisol: 18 mcg/dL (normal)

Diagnosis

Severe symptomatic hyponatremia secondary to Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) from small cell lung cancer

Diagnostic criteria for SIADH met:

  1. Serum sodium < 135 mEq/L with serum osmolality < 275 mOsm/kg (hypotonic hyponatremia)
  2. Urine osmolality > 100 mOsm/kg (inappropriately concentrated)
  3. Urine sodium > 40 mEq/L
  4. Clinical euvolemia
  5. Normal thyroid and adrenal function
  6. Known cause: small cell lung cancer (ectopic ADH production)

Management

Acute symptomatic hyponatremia protocol:

  1. 3% hypertonic saline 100 mL bolus over 10 minutes
  2. Recheck sodium in 20 minutes
  3. Repeat 3% saline bolus if symptoms persist
  4. Goal: raise sodium by 4-6 mEq/L in first 2-4 hours to reverse cerebral edema

Ongoing management:

  1. Admit to ICU for neurological monitoring
  2. Check sodium every 2 hours during active correction
  3. Correction rate: maximum 8-10 mEq/L in 24 hours to prevent osmotic demyelination syndrome
  4. Once acute symptoms resolve, transition to fluid restriction 1-1.5 L/day
  5. Oncology consultation for optimization of cancer treatment
  6. Consider tolvaptan if refractory to fluid restriction

Clinical Pearl

SIADH from small cell lung cancer represents a paraneoplastic syndrome where tumor cells ectopically produce ADH. The diagnosis requires excluding hypothyroidism and adrenal insufficiency, which can mimic SIADH. In this case, the patient's seizure and altered mental status constitute a medical emergency requiring immediate treatment with hypertonic saline. The key principle is to correct just enough to reverse acute symptoms (4-6 mEq/L) while respecting the maximum 24-hour correction rate to prevent osmotic demyelination syndrome.

Clinical Image

Image Description: Electrocardiogram demonstrating findings that may accompany severe electrolyte disturbances, emphasizing the importance of cardiac monitoring during correction of severe electrolyte abnormalities.

Attribution: Image from Wikimedia Commons. Licensed under CC BY-SA 4.0. Source: https://commons.wikimedia.org/wiki/File:ECG_Hyperkalemia.jpg

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