Hyponatremia is the Most Likely Cause
The most likely cause of this patient's condition is hyponatremia (Option C). This 18-year-old presents with classic features of volume depletion from vomiting and diarrhea, leading to hypovolemic hyponatremia, which explains the progressive lethargy despite only mildly elevated blood glucose 1, 2.
Clinical Reasoning
Why Hyponatremia is the Primary Problem
Serum sodium of 131 mmol/L represents mild hyponatremia (defined as 130-134 mmol/L), which can cause malaise, weakness, confusion, nausea, and lethargy—matching this patient's presentation 1, 3, 4.
The clinical context strongly suggests hypovolemic hyponatremia: 2 days of vomiting and diarrhea with decreased oral intake leads to both sodium and water losses, with relatively greater sodium loss causing hyponatremia 5, 2.
Progressive lethargy is a neurologic manifestation of hyponatremia, not hyperglycemia at this glucose level 3, 4.
Why NOT Diabetic Ketoacidosis (DKA)
Blood glucose of 8.5 mmol/L (153 mg/dL) is too low for DKA, which typically requires glucose >250 mg/dL (13.9 mmol/L) 5, 6.
No mention of ketones, acidosis, or classic DKA symptoms (abdominal pain, fruity breath, rapid breathing) 6, 7.
Previously healthy with no diabetes history makes new-onset DKA less likely 6.
Why NOT Uncontrolled Hyperglycemia
Glucose of 8.5 mmol/L (153 mg/dL) is only mildly elevated and would not cause progressive lethargy in isolation 5, 6.
This glucose level falls within acceptable ranges during acute illness (target 140-180 mg/dL) 6, 7.
Why NOT Hyperosmolar Hyperglycemic State (HHS)
HHS requires severe hyperglycemia >600 mg/dL (33.3 mmol/L) with serum osmolality >320 mOsm/kg 8.
HHS typically occurs in type 2 diabetics, not previously healthy young adults 8.
The patient has hyponatremia, not the hypernatremia often seen with HHS 8.
Diagnostic Approach
Confirming Hypovolemic Hyponatremia
Check for at least 4 of these 7 signs of moderate-to-severe volume depletion: confusion, non-fluent speech, extremity weakness, dry mucous membranes, dry tongue, furrowed tongue, sunken eyes 5.
Additional volume status indicators: postural pulse change >30 beats/min, severe postural dizziness preventing standing, decreased skin turgor 5, 1.
Laboratory confirmation: elevated BUN/creatinine ratio, urine sodium <30 mmol/L (suggests hypovolemia), urine osmolality <100 mOsm/kg with low urine sodium confirms hypovolemic hyponatremia 1, 2.
Immediate Management
Fluid Resuscitation
Administer isotonic (0.9%) saline for volume repletion in hypovolemic hyponatremia 5, 1, 2.
Correction rate must not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 2, 3.
Monitor serum sodium every 4-6 hours initially during active correction 1, 7.
Addressing Concurrent Issues
If the patient is diabetic, never discontinue insulin even with poor oral intake—reduce dose but maintain basal coverage 5, 6, 7.
Provide sodium-containing replacement fluids (broth, sports drinks) once oral intake tolerated 6, 7.
Target 150-200g carbohydrate daily via liquid/soft foods to prevent starvation ketosis if diabetic 6, 7.
Critical Pitfalls to Avoid
Do not use hypotonic fluids initially—isotonic saline is required for hypovolemic hyponatremia despite the low sodium 5, 1.
Do not correct sodium too rapidly (>8 mmol/L/24h)—this risks osmotic demyelination syndrome, especially with chronic hyponatremia 1, 2, 3.
Do not ignore mild hyponatremia (130-135 mmol/L)—even this level increases fall risk 21% vs 5% and is associated with 60-fold increased mortality when <130 mmol/L 1, 3.
Do not assume hyperglycemia is the primary problem without checking for DKA criteria (glucose >250 mg/dL, ketones, acidosis) 5, 6.