Why a cup overwhelms a small cat's system
A 4 kg cat drinking 240 ml of water at once is like a 60 kg human chugging 3.6 liters in minutes. Feline kidneys process only 1-2 tablespoons per hour normally. This sudden flood dilutes blood sodium below 120 mEq/L, causing cells to swell. Brain cells are especially vulnerable, which explains the neurological signs you're watching for: head tilting, dilated pupils, or stumbling as if drunk.
The danger peaks within 30-90 minutes post-drinking. At this volume, even if your cat seems fine initially, subclinical cerebral edema could develop. Wrap them in a towel to prevent injury if seizures start, and get to a vet clinic immediately. They'll likely administer controlled IV electrolytes and possibly mannitol to reduce brain swelling.
First actions when water intake can't be measured
If you didn't see the actual drinking but found your cat near an empty bowl they just emptied, assume the worst. Check for waterlogged litter (clumps heavier than usual) and gently palpate the belly — a sloshing sensation suggests fluid overload. Time matters more than exact volume now; note when you last saw them drink normally.
Withhold all water but offer ice cubes to lick. This slows intake while preventing dehydration. Monitor urine output: excessive clear urine in the next hour confirms the kidneys are overwhelmed. Avoid diuretics unless prescribed — correcting sodium too quickly can cause deadly myelinolysis.
Subtle signs that precede obvious collapse
Before the classic symptoms appear, watch for subtle changes. Your cat might blink asymmetrically, walk with a wider stance, or vocalize differently. Some compulsively lick surfaces or show delayed pupil reflexes when you shine a light. These indicate early hyponatremia affecting cranial nerves.
Record a video if you notice odd behavior — vets can spot nuances like nystagmus (involuntary eye movements) that owners miss. Check gum color: pale pink and slick instead of tacky suggests hemodilution. Never wait for vomiting or seizures to seek help; by then, neurological damage may be underway.
Why home monitoring isn't enough for this volume
Even if symptoms seem mild, a cup exceeds home-care limits. Unlike a few laps from a faucet, this volume requires bloodwork to assess sodium, potassium, and hematocrit levels. Vets may hospitalize your cat for 12-24 hours on fluid restriction with serial blood tests every 2 hours initially.
Recovery isn't linear — some cats deteriorate 6-8 hours later as fluid shifts continue. Your vet might recommend a follow-up MRI if balance issues persist, ruling out permanent thalamic lesions. Prevent recurrence by offering water in shot glasses or timed fountain settings that dispense under 30 ml per activation.
The step-by-step response, from the first minute to the vet visit, is in our Water Intoxication (Hyponatraemia) emergency guide.