Neurology

PCP submission

65 year old female with PMH hypothyroidism, hyperparathyroidism, vit D def, obesity, IDA, hemiplegia affecting left side, seizure disorder, HTN, hereditary hemorrhagic telangiectasia, RA, osteoporosis

She reports last week she felt “wobbly” She reports that she felt a little unsteady on her feet. She does use a cane or a walker, already. She states in the past this has happened when her dilantin level is elevated. She currently takes Dilantin ER (100 MG) x3 capsules at bedtime. I checked her phenytoin level, range per our lab is 10.0-20.0 mg/L. Her level is 25.7. I saw her in office yesterday and she reported that her symptoms were improving and that she felt better yesterday than she did last week.

1. Would you recommend adjusting her current regimen or repeat a level to confirm?
2. What is an “acceptable” high level before toxic? In my research, I see a level above 30, however if she was in fact feeling wobbly or unsteady last week could it have been related to her dilantin level in case that it was in fact higher than what it is now?

Specialist response

I would recommend both checking her level again and very likely adjusting her dose. First, it is important to know that phenytoin is an extremely erratic medication due to its high rate of protein binding and its zero-order kinetics. Because of this, the hourly fluctuations in phenytoin free levels can be quite variable. It is also important to have the levels checked at a consistent time (usually a ‘trough’) in order to understand the true effects of a given dose. Second, there is no set ‘acceptable’ level for phenytoin. A patient can have side effects from phenytoin at levels in the low 10s, so a random level of 25 in a 65 year old is most likely causing her gait instability. It would be important to ensure that something else isn’t occurring, but phenytoin toxicity would certainly be high on the differential. Finally, it would also be important to better understand this patient’s epilepsy diagnosis and whether she would be better served on a different anti-seizure medication. Phenytoin is less desirable for many reasons, in addition to what I’ve described above. It is highly associated with osteoporosis, and unless otherwise needed absolutely, should be avoided in patients with known osteoporosis. Other options (if needed at all) might include lamotrigine or levetiracetam.