DESCRIPTION

Gabapentin (Neurontin) is an oral anticonvulsant.

FORMS AND USES

• Gabapentin is used as adjunctive therapy in the treatment of partial complex and secondarily generalized seizure disorders.
• Typical adult dose is 300 to 600 mg orally three times daily.

TOXIC DOSE

Minimal effects have been noted with ingestions of 20 times the recommended dose.

PATHOPHYSIOLOGY

• Gabapentin is a pentamer of gamma-aminobutyric acid (GABA), an inhibitory amino acid in the CNS.
• Gabapentin primarily causes CNS depression, which resolves without sequelae unless hypoxic injury intercedes.

EPIDEMIOLOGY

Poisoning is uncommon.

CAUSES

• Toxicity typically results from therapeutic misadventure.
• Child neglect or abuse should be considered if the patient is less than 1 year of age, suicide attempt if the patient is over 6 years of age.

DRUG AND DISEASE INTERACTIONS

Toxicity of gabapentin may be increased by other CNS depressants.

PREGNANCY AND LACTATION

US FDA Pregnancy Category C. The drug exerts animal teratogenic or embryocidal effects, but there are no controlled studies in women, or no studies are available in animals or women.

Section Outline:

• DESCRIPTION
• FORMS AND USES
• TOXIC DOSE
• PATHOPHYSIOLOGY
• EPIDEMIOLOGY
• CAUSES
• DRUG AND DISEASE INTERACTIONS
• PREGNANCY AND LACTATION

DIFFERENTIAL DIAGNOSIS

Toxic causes of CNS depression include opioids, sedative-hypnotics, other anticonvulsants and many other drugs.

SIGNS AND SYMPTOMS

Vital Signs

Hypertension may develop in severe cases.

HEENT

Diplopia may develop after overdose.

Gastrointestinal

Abdominal pain and diarrhea are common.

Hematologic

Leukopenia has developed during chronic therapy.

Neurologic

• Somnolence, dizziness, ataxia, and slurred speech can occur.
• Deep coma has not been reported to date.

Genitourinary

Impotence has been reported with therapeutic doses.

PROCEDURES AND LABORATORY TESTS

Essential Tests

No tests are usually needed in asymptomatic patients.

Recommended Tests

• Blood glucose and pulse oximetry should be obtained for altered mental status.
• Complete blood count is used to monitor for leukopenia.
• Further studies should be ordered as indicated for altered mental status.
• ECG, serum acetaminophen and aspirin levels in overdose setting to detect occult ingestion.

Not Recommended Tests

Gabapentin levels are not available.

Section Outline:

• DIFFERENTIAL DIAGNOSIS
• SIGNS AND SYMPTOMS
  • Vital Signs
  • HEENT
  • Gastrointestinal
  • Hematologic
  • Neurologic
  • Genitourinary
• PROCEDURES AND LABORATORY TESTS
  • Essential Tests
  • Recommended Tests
  • Not Recommended Tests

Treatment should focus on supportive care with appropriate airway management.

DIRECTING PATIENT COURSE

The health-care professional should call the poison control center when:

• Severe or persistent effects develop.
• Coingestant, drug interaction, or underlying disease presents an unusual problem.

The patient should be referred to a health-care facility when:

• Suicide or homicide attempt is possible.
• Toxic effects develop.
• Coingestant, drug interaction, or underlying disease presents an unusual problem.

Admission Considerations

Admit patients who have persistent CNS depression following a 6-hour period.

DECONTAMINATION

Out of Hospital

Emesis should not be induced; coma or seizure may develop.

In Hospital

• Gastric lavage should be performed in pediatric (tube size 24-32 French) or adult (tube size 36-42 French) patients presenting within 1 hour of a large ingestion or if serious effects are present.
• One dose of activated charcoal (1-2 g/kg) should be administered without a cathartic if a substantial ingestion has occurred within the previous few hours.

ANTIDOTES

There is no specific antidote for gabapentin poisoning.

Section Outline:

• DIRECTING PATIENT COURSE
  • Admission Considerations
• DECONTAMINATION
  • Out of Hospital
  • In Hospital
• ANTIDOTES

PATIENT MONITORING

Cardiac and respiratory function should be monitored continuously in symptomatic patients.

EXPECTED COURSE AND PROGNOSIS

• Toxic effects typically resolve within 12 hours.
• In one case, diplopia resolved over 2 days.

DISCHARGE CRITERIA AND INSTRUCTIONS

• Patients may be discharged from the emergency department or hospital when toxic effects resolve or stabilize and after psychiatric evaluation, if needed.
• Asymptomatic patients may be discharged after decontamination and observation of 4 to 6 hours and, if needed, a psychiatric evaluation.

Section Outline:

• PATIENT MONITORING
• EXPECTED COURSE AND PROGNOSIS
• DISCHARGE CRITERIA AND INSTRUCTIONS

• Coingestion of other CNS depressants would be expected to increase the toxicity of gabapentin.
• It is important to evaluate other serious causes of altered mental status.

Poisoning by other central nervous system depressants and anesthetics.

RECOMMENDED READING

Fischer JH, Barr AN, Rogers SL, et al. Lack of serious toxicity following gabapentin overdose. Neurology 1994;44:982-983.

Garofalo E, Koto E, Feuerstein T. Experience with gabapentin overdose: five case studies. Epilepsia 1993;34(Suppl 2):157.

Author: Kennon Heard

Reviewer: Richard C. Dart