Focus on Pharmacology: Phenytoin

If you work in a unit that takes care of patients with seizure disorder, stroke or any kind of neurological injury, you’re going to be giving a fair amount of antiseizure medications. One of those is phenytoin, also known as Dilantin. It’s also a medication that is common on nursing school exams and has been known to show up on the NCLEX. So, let’s dive into phenytoin using the Straight A Nursing DRRUGS acronym D – R – R – U – G – S, which is a systematic way to review the nursing implications for medications.

D: DRUG CLASS

The therapeutic drug class for phenytoin is anticonvulsant and antiarrhythmic, which may be surprising when you see it used in a patient without neurological concerns…it could be being used to shorten the action potential and decrease automaticity. But today we’re focusing on the neurological effects. Its pharmacologic class is the hydantoins. Hydantoins are typically used to treat partial and tonic-clonic seizures. If you’d like a review on seizure disorders, refer to this blog post or this podcast episode (or go big and review both!).

Hydantoins delay the influx of sodium across neuronal membranes, which dampens CNS activity. Recall that sodium is the main factor in the initiation of a neuronal action potential. So, with less sodium moving across that membrane, we have suppression of neuronal activity. It’s important to note that we aren’t blocking the sodium channels completely, that would cause all neuronal activity to cease such as with local anesthetics. We’re just making them less sensitive so that neuronal activity isn’t high enough for seizure activity to occur. 

The first hydantoin is phenytoin or Dilantin, which has been in use since 1930. It is considered a “broad-spectrum” medication, meaning it treats most types of epilepsy except for absence seizures. One of the benefits of this drug class is that hydantoins like phenytoin suppress the seizure activity without CNS depression. 

R: ROUTES

Phenytoin is available as an IV medication or PO medication. 

R: REGULAR DOSE RANGE

For an adult, a common dose of phenytoin for antiseizure effects involves a loading dose and maintenance dose. A loading dose is used when it’s necessary to get the patient’s blood levels of a drug up to therapeutic levels quickly. In the case of phenytoin, the IV loading dose for an adult is 15-20mg/kg NTE 25-50mg/min. The maintenance dose is quite a bit less at 5-6mg/kg per day given in 1-3 divided doses. 

For the PO route the adult dose range for the loading dose is also 15-20 mg/kg given in 3 divided doses every 2-4 hours. The maintenance PO dose is the same as the IV dose, 5-6 mg/kg per day in 1-3 divided doses.

Phenytoin is a great example of a dosage calculation question that is dosed by weight, with a little trickiness thrown in with the divided doses. Let’s test your dosage skills with a quick sample problem. 

The MD has ordered a loading dose of phenytoin PO for your patient who has been having seizures. The dose is 15mg/kg and phenytoin comes in an oral suspension of 125mg per 5 ml. Your patient weighs 105 pounds. How many ml will she take with each dose?

Take a moment to see if you can figure this one out, rounded to the nearest whole number (answer at the end of this post…no peeking!). 

U: USES

We covered this when we talked about the drug class, but it’s listed here just to make sure you know why YOUR patient is receiving it. Recall, drugs often have multiple therapeutic effects. In this case, our patient is taking the medication to prevent seizure activity. 

G: GUIDELINES

When giving the IV phenytoin, make sure you do not give it faster than 25-50 mg/minute. In fact, this is another perfect opportunity to practice our medication math. Ready?

Your patient is to receive a loading dose of IV phenytoin of 15mg/kg. He weighs 123 pounds and the medication comes in a concentration of 50mg/ml. How many minutes will you take to infuse this medication if the rate of infusion is 25mg/minute, rounded to the nearest whole number? (ANSWER AT THE END OF THIS POST!)

Other guidelines to know include:

• This medication will adversely interact with a LOT of other medications and alcohol causing the patient to have elevated serum phenytoin levels. There are a handful of other medications that will decrease the serum phenytoin levels, so it’s always good to know what else your patient is taking.
• Phenytoin can also decrease the effectiveness of many drugs, including amiodarone, warfarin and oral contraceptives.
• Phenytoin may decrease the absorption of folic acid.
• Concurrent administration of PO phenytoin with enteral feeding may decrease its absorption as well.
• KEY POINT: Phenytoin has a narrow therapeutic window. This means there’s a small range where it’s effective. If it’s too low, the patient may still have seizures, and if it’s too high then the medication becomes toxic. Patients taking phenytoin will need regular and close monitoring of their serum phenytoin levels. 
• Signs of phenytoin toxicity are neurological in nature: ataxia, confusion, slurred speech, nystagmus, nausea and confusion
• Phenytoin is very toxic to the tissues, so avoid IV extravasation. Keep a close eye on those IV sites!
• IV phenytoin has a slight yellow color to it. This is normal. 
• If you pull phenytoin from the refrigerator there could be a precipitate. This will go away once the medication reaches room temperature. 
• Teach the patient to avoid drinking alcohol while taking this medication.
• KEY POINT: Teach patient the importance of good dental hygiene and regular cleanings (we’ll talk about why this is important in a bit). 
• Antacids can decrease the absorption of PO phenytoin, so instruct patients not to take phenytoin within three hours of taking an antacid.

S: SIDE EFFECTS

Like any medication that affects the CNS, phenytoin has a lot of neurological side effects. However, it has some other, seemingly unrelated side effects as well. Some of the most common are:

• Gingival hyperplasia (DING DING DING!) This WILL be an exam question! This is an overgrowth of the gums which leads to gum disease and tooth loss.
• Nausea
• Hypertrichosis (excessive hair growth)
• Rash
• Hypotension
• Life threatening: Stevens-Johnson Syndrome, aplastic anemia and agranocytosis

So there you have it…a brief overview of the must-know information about phenytoin, a commonly prescribed medication used to prevent seizures.  Did you take a crack at the dosage calculations questions? Then scroll on down for the answers! 

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QUESTION A ANSWER: 29 ml. At first you may have thought, WOAH! That’s a lot of oral elixir to drink. And you’d’ be right. Maybe you even double-checked your math. If you did, GOOD JOB! Any time you give multiple containers of ANYTHING, always always always double check your math. In this case, the correct answer actually is 29 ml. Yes, it’s a large dose, but recall this is the LOADING dose. The maintenance dose will be much smaller.

QUESTION B ANSWER: 34 minutes. There’s a good chance this was a tricky one for you, and that’s ok! To start, you’re simply going to begin with the order, then convert kg to pounds, then factor in the weight of the patient to arrive at the patient’s total dose. From there, you simply divide by 25 to determine how many minutes it will take you to give this medication at the prescribed rate. If you were to do this as a single equation,  your final conversion would simply be 1/25mg*min (the 25 goes on the bottom because we are dividing by the 25 minutes).

Did you have trouble with these? Don’t worry! I’ve got a step-by-step course that takes you from the very basics of dosage calculations all the way up to the tricky ones. In the Dosage Calculations Bootcamp I teach you a framework for approaching dosage calculations problems conceptually. If you’re looking at them simply as math problems, you’re going to quickly discover that they’re not like math problems at all…they’re little puzzles. And yes, you can get excellent at doing them, it just takes a solid understand and some practice, both of which you can get in my Dosage Calculations Bootcamp.

Get this on audio in episode 121 of the Straight A Nursing Podcast.

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References

Epilepsy Foundation. (n.d.). Summary of antiepileptic drugs. Epilepsy Foundation. https://www.epilepsy.com/article/2014/3/summary-antiepileptic-drugs

Deglin, Judith Hopfer, and April Hazard Vallerand. Davis’s Drug Guide for Nurses, with Resource Kit CD-ROM (Davis’s Drug Guide for Nurses). Philadelphia: F A Davis Co, 2009. Print.

Holland, N., & Adams, M. P. (2007). Core Concepts in Pharmacology (2nd ed.). Pearson Prentice Hall.