THE GREAT IMITATOR: DIAGNOSING AND TREATING IDIOPATHIC POLYARTHRITIS (IMMUNE-MEDIATED)

Veterinary neurologists often evaluate, diagnose, and treat patients that are referred to us for back pain and/or weakness. Polyarthritis is often considered the Great Imitator as patients suffering from this problem can appear very similar to patients with intervertebral disc disease, myopathies, meningitis, and even myasthenia gravis.

The most critical step required to diagnose polyarthritis is to consider it on your differential diagnosis list. As neurologists, we harp on localizing the lesion (i.e. C1-C5, T3-L3, left prosencephalon, etc.) and if you can’t localize the lesion because you’re only finding poorly localized back pain, think, “What about the joints?” In patients with polyarthritis, the facet joints of the vertebral column may be affected and/or the pressure placed on the back may cause discomfort in the hock or stifle joints. In patients without an obvious neuroanatomic localization, remember to pay special attention to the joints.

Initial clinical signs may include lethargy, inappetance, pyrexia, reluctance to walk, shifting-leg lameness, and poorly localized pain and discomfort. Many of these patients are small breed dogs and it’s very easy to mistakenly suspect intervertebral disc disease and start an anti-inflammatory medication, which makes diagnosis more challenging. Joint palpation and joint manipulation usually leads to findings that will lead to a diagnosis. The carpal joints can be very reliable in palpating effusion and eliciting discomfort with gentle flexion.

Once polyarthritis is suspected, the next step is obtaining joint radiographs and synovial fluid from at least two clinically affected joints. Synovial fluid in dogs with polyarthritis has poor viscosity and high cellularity. Most cases have increased numbers of non-degenerate neutrophils. Infections (i.e. rickettsial, bacterial) should be ruled out with infectious disease testing and cultures. It’s often advisable to collect CSF while a patient is anesthetized for synovial fluid aspirates as meningitis is another Great Imitator and some dogs with polyarthritis will have meningitis, too.

Thankfully, treatment of Immune-Mediated Polyarthritis is fairly straightforward and most dogs that have the non-erosive form (most common) do very well with treatment. Most dogs can be started on an immunosuppressive dose of prednisone (2 mg/kg/day); when the infectious disease testing is confirmed negative, add oral Azathioprine (2 mg/kg/day). An antibiotic (i.e. Doxycycline or Cephalexin) can also be used while the infectious disease testing is pending. After 2 weeks of treatment with Azathioprine, decrease to 2 mg/kg every other day and evaluate a CBC. A CBC should be monitored frequently while patients are receiving Azathioprine. The prednisone can be slowly tapered by decreasing the dose by 25% every 3-4 weeks until the patient is receiving a very small dose (i.e. 0.2 mg/kg) every other day (alternating with Azathioprine). Patients are typically treated for at least 6-12 months; however, some clinicians will wean patients off medications sooner than this.

Remember the Great Imitator and if you can’t localize the lesion, think, “What about the joints?” If in doubt and you’re considering empiric treatment with anti-inflammatories, use a NSAID. Low doses of corticosteroids can make diagnosis very elusive.