Many
antidepressants inhibit
serotonin or
norepinephrine reuptake or both to achieve their clinical effect. The
selective serotonin reuptake inhibitor class of
antidepressants (
SSRIs) includes
citalopram,
escitalopram (active enantiomer of
citalopram),
fluoxetine,
fluvoxamine,
paroxetine, and
sertraline. The
SSRIs are as effective as
tricyclic antidepressants in treatment of major depression with less significant side effects. As a result, they have become the largest class of medications prescribed to humans for depression. They are also used to treat
obsessive-compulsive disorder,
panic disorders,
alcoholism,
obesity,
migraines, and
chronic pain. An SSRI (
fluoxetine) has been approved for veterinary use in treatment of canine
separation anxiety.
SSRIs act specifically on synaptic
serotonin concentrations by blocking its reuptake in the presynapse and increasing levels in the presynaptic membrane. Clinical signs of SSRI overdose result from excessive amounts of
serotonin in the central nervous system. These signs include
nausea,
vomiting,
mydriasis,
hypersalivation, and
hyperthermia. Clinical signs are dose dependent and higher dosages may result in the
serotonin syndrome that manifests itself as
ataxia,
tremors,
muscle rigidity,
hyperthermia,
diarrhea, and
seizures. Current studies reveal no increase in appearance of any specific clinical signs of
serotonin toxicity with regard to any SSRI medication. In people,
citalopram has been reported to have an increased risk of electrocardiographic abnormalities. Diagnosis of SSRI
poisoning is based on history, clinical signs, and response to
therapy. No single clinical test is currently available to confirm SSRI toxicosis. The goals of treatment in this intoxication are to support the animal, prevent further absorption of the
drug, support the central nervous system, control
hyperthermia, and halt any seizure activity. The relative safety of the
SSRIs in overdose despite the occurrence of
serotonin syndrome makes them more desirable than other
antidepressants. The prognosis in animals that receive treatment is excellent. In one retrospective study, there were no deaths in 313 SSRI-poisoned dogs. No characteristic or classic histopathologic lesions result from SSRI toxicosis. Differential diagnoses for SSRI overdose must include ingestions of other serotonergic medications such as phenylpiperidine
opioids (
fentanyl and
tramadol),
mirtazapine,
buspirone,
amitraz, and
chlorpheniramine.