A client's BMP shows sodium 118 mEq/L (normal 135-145). They are confused and reporting a headache. What is the PRIORITY nursing concern?

SEVERE HYPONATRAEMIA (sodium below 120-125 mEq/L with symptoms) is a neurological emergency. PATHOPHYSIOLOGY: When serum sodium falls rapidly, the extracellular fluid becomes hypotonic; water moves by osmosis into brain cells; brain cells swell (cerebral oedema); the skull cannot accommodate the swelling; this causes increased intracranial pressure; SIGNS BY SEVERITY: Mild (130-135): nausea, headache, fatigue; Moderate (125-130): confusion, disorientation; Severe (below 120-125): stupor, seizures, coma, herniation, respiratory arrest. NURSING PRIORITIES: NEUROLOGICAL ASSESSMENT — level of consciousness, orientation, seizure precautions (padded side rails, suction at bedside); NOTIFY PROVIDER IMMEDIATELY; STRICT I&O — fluid restriction (often the first treatment for dilutional hyponatraemia); SEIZURE PRECAUTIONS; TREATMENT DEPENDS ON CAUSE AND ACUITY: SIADH (most common cause): fluid restriction; slow correction with 0.9% NS or 3% hypertonic saline if severe/symptomatic; CRITICAL SAFETY POINT — CORRECTION RATE: Sodium must be corrected slowly — no faster than 8-12 mEq/L per 24 hours; rapid correction can cause OSMOTIC DEMYELINATION SYNDROME (formerly central pontine myelinolysis) — permanent, devastating neurological damage from myelin destruction; CAUSES OF HYPONATRAEMIA: SIADH (most common inpatient cause); excessive hypotonic fluid administration; heart failure; cirrhosis; hypothyroidism; psychogenic polydipsia.