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In human medicine, antiepileptic drugs are widely used for management of central and peripheral neurogenic pain. The rationale for the use of antiepileptic drugs in neurogenic pain is the same as the basis for their use in epilepsy – their ability to suppress discharges in pathological neurons (Boivie, 1999). The antiepileptic drugs used in people to treat neuropathic pain include carbamazepine, oxcarbazepine and gabapentin. None of these drugs are licensed for use in cats, and their safety regarding long term treatment is unknown.
An information sheet written by veterinary neurologist, Claire Rusbridge, proposes that phenobarbitone at a dose rate of 2-3mg/kg BID can be used for management of FOPS. Intramuscular injection of phenobarbitone can be used to give faster relief (Rusbridge, Feline Orofacial Pain Syndrome: Face and tongue mutilation syndrome in Burmese cats, 2008). Serum phenobarbitone levels should be checked after 2 – 3 weeks of treatment to ensure serum levels are adequate. The information sheet states that most cats require a serum phenobarbitone concentration of 20-25mg/l (100-120μmol/l) to control episodes. Due to the episodic nature of the disease, an effort should be made after four weeks, if there is resolution of the signs, to wean the cat off the phenobarbitone (Rusbridge, Feline Orofacial Pain Syndrome: Face and tongue mutilation syndrome in Burmese cats, 2008). In some cats, lifelong therapy may be required (Rusbridge, Feline Orofacial Pain Syndrome: Face and tongue mutilation syndrome in Burmese cats, 2008).
Instead of phenobarbitone, other antiepileptic drugs (e.g. carbamazepine or gabapentin) can be used. The recommended human dose rate for carbamazepine for the treatment of trigeminal neuralgia is 200 – 1200mg/kg daily. The pain relief provided by the anti-epileptic drug carbamazepine in trigeminal neuralgia is correlated with plasma concentration (Boivie, 1999). An information sheet written by veterinary neurologist Claire Rusbridge suggests a dose rate of carbamazepine for cats with FOPS of 25 mg BID. Adverse side effects of carbamazepine in humans include haematologic abnormalities, hepatitis, hyponatraemia and induction of microsomal enzyme systems which may influence the metabolism of some drugs (Attal, Cruccu, Haanpää, & al, 2006). Should this drug be used, regular monitoring of liver enzyme, sodium levels and haematology is required (Attal, Cruccu, Haanpää, & al, 2006). Intravenous oxcarbazepine (a keto-derivative of carbamazepine) was shown in cats to suppress tooth pulp evoked potentials recorded in the trigeminal spinal tract nucleus (Kiguchi, Ichikawa, & Kojima, 2001).
There is little veterinary experience with the drug gabapentin (Neurontin®) and its adverse effects in cats are unknown, however, in people there is a very wide safety margin, and it has been used successfully in the treatment of human facial pain disorders (Schachter & Carrazana, 1997). A dose rate of 3 mg/kg PO SID has been suggested for the treatment of chronic pain in cats, and up to 10-30 mg/kg PO q 8h for refractory seizures (Plumb, 2005). Gabapentin (Neurontin®) is available in New Zealand as 100 mg capsules. As for phenobarbitone, attempts should be made to wean the cat off any alternative antiepileptic drug after 4 weeks if the clinical signs have resolved (Rusbridge, Feline Orofacial Pain Syndrome: Face and tongue mutilation syndrome in Burmese cats, 2008). Gabapentin is excreted unchanged by the kidneys, are required in renal compromise (NEURONTIN® data sheet, 2007), therefore renal parameters (serum biochemistry and urine specific gravity) should be checked prior to commencing treatment. An example of how gabapentin can be used in a treatment plan for cats with feline orofacial pain syndrome is described below in the recommendations for treatment. It is important to bear in mind that pharmacokinetic studies of gabapentin in cats is lacking, and although the safety margins are high in humans and rodents (NEURONTIN® data sheet, 2007); it is not certain whether this is the case in cats. Therefore treatment should be started using low doses, and owners should be warned of the potential for adverse effects. More information about gabapentin (Neurontin ®) is available at the Medsafe Website (see works cited).
Antidepressants such as amitriptyline have been used for the treatment of central neurogenic pain in humans. It is hypothesized that the pain relieving effect of antidepressants such as amitriptyline, clomipramine, and doxepin is mainly due to their effect on the noradrenergic systems (Boivie, 1999) and N-methyl-D-aspartate (NMDA) receptor blockade (Sindrup, Otto, Finnerup, & Jensen, 2005). It is suggested that tricyclic antidepressants such as amitriptyline may also control peripheral neuropathic pain by acting as sodium channel blockers (Landsberg, Hunthausen, & Ackerman, 2003). It may also be of use in anxious cats with FOPS. For neuropathic pain in cats a dose rate of 0.5-2 mg/kg PO once daily is suggested (Plumb, 2005). When discontinuing the amitriptyline, taper the dose over 1 – 3 weeks. The pain relieving properties of the tricyclic antidepressants will take effect relatively quickly compared to their antidepressant/psychological properties which may take several weeks.
Steroids could be used as an adjunctive treatment if inflammation is present in the oral cavity. A dose of 1 – 2 mg/kg Predisolone PO once daily could be used when necessary. Bear in mind that concurrent treatment with phenobarbitone may increase the metabolism of glucocorticoids (Plumb, 2005).
Although NSAIDS and opiods have been used in the treatment of mild cases and severely affected hospitalized cases of FOPS respectively (Rusbridge, Feline Orofacial Pain Syndrome: Face and tongue mutilation syndrome in Burmese cats, 2008), it is controversial whether neurogenic pain responds well to analgesics (Boivie, 1999). Many medical doctors believe that neurogenic pain does not respond as well to analgesics than nociceptive pain, and perhaps it does not even respond at all to opiods (Boivie, 1999).