Phenytoin: what to check before you give it

What it does and why it is prescribed

Phenytoin stabilises neuronal membranes by blocking voltage-gated sodium channels, which slows the recovery of these channels after firing and limits the spread of seizure activity. It doesn't suppress a single seizure focus so much as prevent that activity from propagating across the brain.

It's prescribed for tonic-clonic and complex partial seizures, and it remains a first-line option for status epilepticus once benzodiazepines have been given, usually as fosphenytoin in the acute setting to avoid the infusion complications of phenytoin itself. It is also occasionally used for seizure prophylaxis after neurosurgery or traumatic brain injury, though that use has narrowed as guidelines have shifted toward other agents with fewer interactions.

Nursing considerations before giving it

Confirm the route and formulation before administering. Oral phenytoin has erratic absorption and interacts with enteral feeds, so tube feeds are typically held for one to two hours before and after a dose. IV phenytoin must be given slowly, no faster than 50 mg per minute in adults, through a large vein with a filter, because rapid infusion risks cardiac arrhythmias and hypotension.

Check the line and the fluid it's running with. Phenytoin is only compatible with normal saline and will precipitate if mixed with dextrose-containing solutions, forming crystals that can occlude the line or embolise. Flush the IV line with saline before and after the dose, and never piggyback it into a dextrose infusion.

Pull the most recent trough level and correlate it with the patient's presentation, since dose adjustments are guided as much by clinical signs as by the number itself, particularly in patients with low albumin where total phenytoin levels can be misleading.

What to monitor

The therapeutic range is 10 to 20 mcg/mL. Levels drawn as troughs, just before the next dose, give the most reliable picture. In patients with hypoalbuminemia or renal failure, a normal-looking total level can still reflect toxic free drug, so correct for albumin or request a free phenytoin level when the clinical picture doesn't match the number.

Monitor injection sites for purple glove syndrome, a serious local reaction of oedema, discolouration, and pain that can progress to compartment syndrome, particularly with peripheral IV administration. Watch gums at every admission assessment for gingival hyperplasia, the classic long-term marker of phenytoin therapy, and reinforce oral hygiene as prevention rather than waiting for it to appear.

Side effects versus adverse effects

Expected side effects include gingival hyperplasia, mild drowsiness, and hirsutism, effects that are unpleasant but don't necessarily require stopping the drug and are managed with dose review and dental care. Nystagmus at the extremes of lateral gaze is an early, dose-related sign that often appears before other toxicity symptoms and is worth checking for at each assessment.

Adverse effects that signal toxicity include ataxia, slurred speech, confusion, and diplopia, all pointing to levels above the therapeutic range. Beyond that, watch for signs that go past dose-related toxicity altogether: Stevens-Johnson syndrome or toxic epidermal necrolysis, agranulocytosis, and hepatotoxicity are rare but serious idiosyncratic reactions that require immediate discontinuation rather than dose adjustment.

What to hold for and when to call

Hold the dose and call the prescriber for new nystagmus, ataxia, or confusion, since these suggest the level is at or above the toxic threshold. A heart rate or rhythm change during an IV infusion is also a reason to stop the infusion immediately and notify the team, given phenytoin's cardiac depressant effects at rapid infusion rates.

Call promptly for any new rash, especially one with mucosal involvement, fever, or blistering, since this can be the early presentation of a hypersensitivity reaction that progresses quickly. Any bruising, unusual bleeding, or signs of infection in a patient on long-term therapy warrants a check of full blood count, given the small risk of bone marrow suppression.

Patient teaching

Teach patients to never stop phenytoin abruptly, even if they feel well, since sudden withdrawal can precipitate status epilepticus. Any change in medication, including over-the-counter products, should be checked against phenytoin first because of its extensive interaction profile.

Emphasise dental hygiene and regular dental review to manage gingival hyperplasia, and explain that this is a known drug effect, not a sign they're doing something wrong. Advise women on hormonal contraception that phenytoin can reduce its effectiveness, and that additional contraception should be discussed with their prescriber. Tell patients to report a rash immediately rather than waiting to see if it settles, and to carry identification noting they take an anticonvulsant.