The inclusion of NMDA antagonists in chronic pain management has proven essential in the management of a variety of chronic pain. The first reports originated from Lascelles et al (2008) in their evaluation of amantadine in dogs with osteoarthritis (OA). Availability of amantadine can be variable, resulting in a need for an alternative in patients already stable on amantadine or those in need of an NMDA antagonist (latest update below on that).
The conclusion from the Lascelles study was that activity levels and lameness were improved when amantadine was used in conjunction with meloxicam. Every OA case reaches a stage when a second analgesic is required to combat centralised pain. The big question currently is when exactly should we add that next option - when does centralised pain develop? The use of pain scoring here is essential and a validated pain scale such as the Canine Brief Pain Inventory is useful to identify when to step in.
What do we mean by centralised pain? There are a number of mechanisms that overall contribute to a heightened pain state. Some examples are central sensitisation and dysfunction of the descending modulatory system. In our clinical patients it is likely that several of these mechanisms are at play (amongst others), and we can't tell clinically which one is driving the overall pain experience.
Use of memantine in chronic pain remains anecdotal with no clinical trials to support its use. Work from our colleagues in behavioural medicine informs us of doses useful for treatment of compulsive disorders and we extrapolate from this (Schneider et al, 2009) for pain management.
Dose ranges from 0.3-1.0 mg/kg SID or BID for pain management. Memantine is available as a liquid solution and 10mg and 20mg tablets. One attractive feature is that the cost is only a fraction of amantadine, although the liquid is more costly than the tablets. As an off-licence medication, clients should be made aware that this product does not carry a veterinary authorisation. Adverse effects with memantine are very few. Memantine is used in conjunction with NSAIDs as well as other drugs such an paracetamol and gabapentin.
In summary we find memantine a useful addition to our options available when an NMDA antagonist is required. Over several years I have used memantine as a first line in lieu of amantadine and I feel I certainly see a benefit in my cases. I explain the situation to clients and explain that if we don't feel there is an adequate response to memantine, we can switch to amantadine.
Recently we understand that the use of amantadine is restricted in the EU to human-only. This hasn't been well communicated to the vet community, and it is still possible to purchase amantadine via our regular wholesalers. The detail on the EU website is here.
The rationale for this is that amantadine is an anti-viral which may be of value in a future pandemic. If it is widely used in animals, then there is a risk that resistance could develop.
References
Lascelles, B. D. X., Gaynor, J. S., Smith, E. S., Roe, S. C., Marcellin-Little, D. J., Davidson, G., et al. (2008). Amantadine in a Multimodal Analgesic Regimen for Alleviation of Refractory Osteoarthritis Pain in Dogs. Journal of Veterinary Internal Medicine, 22(1), 53–59. http://doi.org/10.1111/j.1939-1676.2007.0014.x
Schneider BM, Dodman NH, Maranda L. (2009) Use of memantine in treatment of canine
compulsive disorders. J Vet Behav Clin Appl Res 4(3):118–26.
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