Baclofen Toxicity

Etiology

Baclofen is a synthetic derivative of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) and acts primarily within the central nervous system (CNS). Therapeutic effects occur at a plasma concentration of 0.1 to 0.4 mg/L.[2] Due to its water solubility, baclofen does not readily cross the blood-brain barrier (BBB) when administered orally, necessitating higher oral doses compared with intrathecal delivery to achieve a therapeutic effect. The recommended starting oral dose in adults is 5 mg 3 times daily, with a usual maximum dose of 80 mg per day.[3] Serious toxicity may occur with oral doses greater than 200 mg.[4] Impaired kidney function increases the risk of oral baclofen poisoning due to primary renal elimination.

Intrathecal baclofen is administered at substantially lower doses because direct delivery into CSF bypasses the BBB. A screening test is used to determine the appropriate starting intrathecal dose. An initial bolus of 50 mcg is administered, followed by titration until a positive response is observed, defined as a decrease in muscle tone, reduction in spasms, or improvement in functional ability.[5] Sedation is less common with intrathecal administration due to lower systemic exposure.[6] Despite this safety advantage, intrathecal pump systems carry a risk of severe toxicity if a pump malfunction or dosing error results in supratherapeutic delivery.

Epidemiology

The incidence of adverse effects with therapeutic baclofen dosing ranges from 10% to 75%.[7] The most commonly reported reactions include drowsiness and dizziness. Complications associated with intrathecal pumps are frequent, with 30% to 50% of pump recipients experiencing drug-related events, such as infection, overdose, and withdrawal.[8] Expanded off-label use has increased the risk of both intentional and accidental baclofen toxicity. A US study of baclofen exposures from 2014 to 2017 reported 15,397 total cases, with an overall increase of 36.2% during the study period.[9] Additionally, an Australian study evaluating baclofen-related mortality from 2000 to 2022 identified 102 exposure-related deaths. Most fatal cases were intentional (54.9%) and involved a history of substance use disorders.[10]

Pathophysiology

Baclofen is an agonist of G protein–mediated GABA_B receptors located in the brain and spinal cord, present on both presynaptic and postsynaptic neurons. Drug activity predominates in the spinal cord at therapeutic concentrations. Presynaptic GABA_B receptor binding decreases calcium influx and reduces neurotransmitter release.[11] Postsynaptic GABA_B receptor activation increases potassium efflux and induces neuronal hyperpolarization, producing inhibitory effects.[12] These mechanisms result in reduced spinal spasticity. Baclofen loses receptor specificity within the spinal cord and exerts greater effects within the CNS at supratherapeutic concentrations, leading to profound sedation. CNS depression may be severe enough to mimic brain death.[13] Additional manifestations of significant baclofen toxicity include respiratory depression, flaccid paralysis, autonomic instability, and seizures.

Toxicokinetics

Oral baclofen is rapidly absorbed, with bioavailability ranging from 70% to 85%. Absorption is dose-dependent and increases with higher doses. Peak therapeutic plasma concentrations occur within 2 to 3 hours of ingestion.[14] Baclofen has a relatively short therapeutic half-life of 2 to 6 hours, although marked prolongation may occur in overdose. A case report described a 36-hour half-life following a 450-mg baclofen overdose.[15] The drug has a small volume of distribution (0.7 L/kg) and low protein binding (30%), making it amenable to removal via hemodialysis to enhance elimination.

History and Physical

Initial evaluation of baclofen toxicity should prioritize airway, breathing, and circulation, with supportive care provided as needed to stabilize the patient. A detailed history should be obtained after stabilization. History should determine whether baclofen exposure resulted from intentional ingestion, necessitating suicidality assessment, or an inadvertent medication error. Further inquiry should also be directed toward symptom characterization, onset, baclofen dose, administration route, timing of last dose, duration of therapy, recent dose adjustments, and urine output. Relevant past medical history may reveal underlying renal disease and recent initiation of medications affecting renal function. In patients with intrathecal pumps, important information includes the date of pump implantation, the presence of pump alarms, the last battery change, and the most recent drug refill.[16]

Intrathecal pump sites should be examined for signs of infection or fluid collection, and any pump alarms should be addressed. Neurologic examination should include assessment of mental status, pupillary response, reflexes, muscle tone, and clonus. Hypotonia and flaccid paralysis are suggestive of baclofen toxicity.

Signs and symptoms of baclofen poisoning are heterogeneous. CNS manifestations range from lethargy, somnolence, and confusion to delirium and coma. Paradoxical myoclonus and seizures may occur secondary to central disinhibition. Complete loss of brainstem reflexes may be observed, mimicking brain death. Common vital sign abnormalities include hypothermia, hypotension, and bradycardia, though hypertension and tachycardia may also occur, complicating differentiation from baclofen withdrawal. Baclofen has been shown to increase serotonergic activity in the brain and spinal cord, placing patients at increased risk of serotonin toxicity. [Source: Aemaz Ur Rehman et al. Baclofen-Induced Serotonin Syndrome. 2023] Cardiovascular toxicity may present with 1st-degree heart block, prolonged QTc interval, premature atrial contractions, or supraventricular tachycardia on electrocardiography (ECG).[17][18] Bronchospasm may occur secondary to activation of GABA_A receptors at high baclofen concentrations.[19]

Evaluation

Laboratory studies should be obtained after completion of a thorough history and physical examination. Bedside blood glucose measurement should be performed promptly in patients presenting with altered mental status. Additional laboratory evaluation should include a basic metabolic panel to assess electrolyte abnormalities and renal function. Ethanol concentration, hepatic panel, creatine kinase, and urinalysis may be considered, depending on the clinical context. Salicylate and acetaminophen concentrations should be obtained in cases of suspected intentional ingestion or suicidality to evaluate for possible coingestion. Serum baclofen concentration is not typically readily available and does not generally alter acute management, though drug levels may be obtained for confirmatory purposes when required. Toxic plasma concentrations following oral baclofen ingestion are reported at 1.1 to 3.5 mg/L. Plasma concentrations are not useful in intrathecal baclofen exposure due to limited BBB penetration.

Additional diagnostic evaluation should include ECG to assess for cardiac arrhythmias and computed tomography of the head to exclude alternative causes of altered mental status. In comatose patients, electroencephalography (EEG) may assist in identifying nonconvulsive status epilepticus. The pump should be interrogated when intrathecal delivery is involved, with prompt communication to the managing physician. Plain radiographs of the thoracic and lumbar spine may be obtained to assess the location and patency of the pump system and its tubing.

Treatment / Management

Treatment of baclofen toxicity is primarily supportive. Stabilization of airway, breathing, and circulation is the priority. Endotracheal intubation and mechanical ventilation is indicated in the presence of respiratory depression or profound CNS depression. Intravenous fluids may be used initially for hypotension, though vasopressor support may be required for refractory cases. Seizures are managed with benzodiazepine administration. Activated charcoal may be given for gastrointestinal decontamination early after ingestion, typically within 2 hours, if the patient is alert and able to protect the airway. Activated charcoal is contraindicated in the presence of CNS depression, vomiting, or seizures due to aspiration risk. Hemodialysis may be used to enhance drug elimination, particularly in patients with underlying renal impairment.[20][21] The EXtracorporeal Treatments in Poisoning (EXTRIP) workgroup recommends intermittent hemodialysis in baclofen toxicity with concomitant renal dysfunction in patients presenting with coma requiring mechanical ventilation.[22](B3)

All symptomatic and intentional baclofen overdoses warrant hospital admission. Patients experiencing hemodynamic instability or significant CNS depression should be admitted to the intensive care unit. Baclofen should be withheld until clinical improvement of toxicity is confirmed. Reinitiation of the medication is important after resolution of toxicity to prevent withdrawal, which may be severe.

Patients with intrathecal pumps should be treated at facilities with clinicians experienced in pump management. The recommended approach for a suspected pump malfunction causing overdose is to deactivate the pump and replace the reservoir with saline. CSF removal via lumbar puncture may be considered in severe cases to reduce drug concentration. Pump reactivation should occur within 48 hours of discontinuation to prevent baclofen withdrawal.[23]

Differential Diagnosis

Healthcare providers should consider baclofen toxicity when patients with known access to this medication present with altered mental status, deranged vital signs, or abnormal neuromuscular examination findings. Baclofen toxicity may mimic multiple alternative etiologies, including sedative-hypnotic overdose, opioid overdose, severe metabolic encephalopathy, and status epilepticus, which may contribute to delayed recognition. Additional differential diagnoses include sepsis, meningitis, encephalitis, stroke, intracranial hemorrhage, and hypoglycemia.

Prognosis

Baclofen toxicity generally has favorable outcomes when patients are stabilized early. Mental status improves as serum drug concentration decreases. Improvement in mental status usually occurs within 24 to 48 hours, although recovery may be prolonged in severe overdoses.[24][25][26] Massive baclofen overdoses greater than 1 to 2 grams may be fatal.[27]

Complications

Complications of baclofen toxicity are often secondary to severe CNS and respiratory depression. Anoxic brain injury, aspiration pneumonia, pressure ulcers, rhabdomyolysis, and hypothermia may be observed after prolonged periods without medical intervention.