Phenobarbital therapeutic drug monitoring in dogs with ep...

Ah, a 4-year-old German Shepherd Dog still experiencing monthly seizures on phenobarbital. This is a common and challenging scenario for many general practitioners, and it highlights the need for a systematic approach to therapeutic drug monitoring and polypharmacy. Let's walk through this methodically.

First, commend yourself for initiating phenobarbital, which remains a highly effective first-line antiepileptic drug (AED) in dogs. The goal now is to optimize its use or to consider adding a second agent.

Phenobarbital Therapeutic Drug Monitoring

When to Check Levels:

Phenobarbital has a half-life of roughly 24–72 hours in dogs. It takes approximately five half-lives to reach a steady-state concentration in the bloodstream. Therefore, you should wait at least 10-14 days (2 weeks) after starting phenobarbital or after any dose adjustment before checking a trough level. A trough level is drawn just before the next scheduled dose.
After the initial therapeutic level is confirmed, recheck levels at 6 weeks, and then every 6 months or whenever seizure frequency changes significantly, or new side effects develop.

Target Therapeutic Range:

For canine epilepsy, the generally accepted therapeutic trough range for phenobarbital is 20–35 μg/mL.

Interpreting Your GSD's Situation:

Given your GSD is still having monthly seizures on 3 mg/kg BID, the first step is to obtain a phenobarbital trough level.
* If the level is <20 μg/mL: The dose is likely sub-therapeutic. You can increase the phenobarbital dose by 10-20% and recheck the level in another 2 weeks. Aim for the mid-to-upper end of the therapeutic range (e.g., 28-32 μg/mL) before considering it optimized.
* If the level is within 20–35 μg/mL (especially the upper end): This indicates that the epilepsy is refractory to phenobarbital monotherapy. Monthly seizures, even on therapeutic levels, warrant consideration of a second AED.
* If the level is >35 μg/mL: This is supra-therapeutic and may be causing adverse effects (excessive sedation, ataxia). You might consider a slight dose reduction, but if seizures persist, you'll definitely need to add a second AED, rather than push phenobarbital higher.

When to Add a Second AED

The decision to add a second AED is typically made when:
* Seizures persist despite phenobarbital being at the upper end of its therapeutic range (e.g., 30-35 μg/mL).
* The patient experiences unacceptable side effects from phenobarbital when trying to achieve adequate seizure control.
* The seizure frequency or severity is significantly impacting the patient's or owner's quality of life.
* The patient has experienced cluster seizures or status epilepticus, even if phenobarbital levels are sub-optimal (these are high-risk situations).

Choosing a Second AED: Levetiracetam vs. Potassium Bromide

Both levetiracetam and potassium bromide are excellent choices as add-on therapies, each with distinct advantages and considerations.

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Levetiracetam (Keppra)

Levetiracetam is often my first choice as an add-on due to its excellent safety profile, rapid onset, and minimal drug interactions.

* Dose (from VERIFIED DRUG FORMULARY):
* Standard release: 20-30 mg/kg PO q8h
* Extended release (Keppra XR): 30-60 mg/kg PO q12h (Do NOT split or crush XR tablets).
* Monitoring: No routine therapeutic drug monitoring is required for levetiracetam. You monitor clinical efficacy (seizure frequency) and for rare side effects (mild sedation, inappetence).
* Considerations: There can be a "honeymoon effect" where efficacy wanes after 4-8 months. Be prepared to potentially increase the dose or add a third agent if this occurs. It's safe to use in patients with liver disease.

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Potassium Bromide (KBr)

Potassium bromide is another highly effective second-line AED, particularly good for long-term control.

* Dose (from VERIFIED DRUG FORMULARY):
* Without loading dose: 30-40 mg/kg PO q24h. Be aware that steady state will take 3-4 months due to its extremely long half-life.
* With loading dose: 400-600 mg/kg divided over 5 days. This will achieve therapeutic levels in about a week, but often causes significant GI upset and transient sedation/ataxia.
* Monitoring:
* If a loading dose is used, check a trough level at 1 month.
* If no loading dose, check at 3 months.
* Then recheck every 6 months. The target therapeutic range when combined with phenobarbital is typically 0.8–2.0 mg/mL.
* Monitor renal function (BUN, creatinine) as it is renally excreted.
* Considerations:
* ABSOLUTELY DO NOT USE IN CATS due to risk of fatal eosinophilic pneumonitis.
* Side effects include PU/PD, polyphagia, sedation, and hindlimb ataxia (especially at higher levels). Pancreatitis is a rare but reported side effect.
* Elimination is chloride-dependent. Consistent diet is crucial; changes in dietary chloride can significantly alter KBr levels. Always administer with food to reduce GI irritation.

The Neurology Workflow Reminder

Before settling on refractory epilepsy, ensure you've revisited the basics:

Compliance: Is the owner administering the medication consistently and correctly?

Seizure Diary: Is the seizure diary accurate, detailing frequency, duration, and severity?

Reactive Seizures: Have you ruled out extracranial causes for seizures (e.g., hypoglycemia, hepatic encephalopathy, electrolyte imbalances) with recent bloodwork?

Structural Epilepsy: If an MRI hasn't been performed, especially given the GSD breed predisposition to some conditions, and if the seizures are truly refractory, it's always worth considering advanced imaging to rule out a structural lesion (e.g., brain tumor, GME) that might require specific treatment.

By systematically addressing these points, you can optimize your GSD's seizure control and improve their quality of life. Keep pushing forward; managing epilepsy is a marathon, not a sprint, and often requires adjustments.