You might think the answer is obvious. The word arthritis comes from the Greek (arthro-, joint, and -itis, inflammation), so surely the pain comes from the joint. In practice it is rarely that simple. What we have learned about how pain works tells us that joint damage is often only the starting point, and that lasting relief usually depends on recognising and treating each part of the pain picture rather than the joint alone.
This post walks through the main sources of pain in arthritis: the joint itself, the nerves, the muscles, and the part that body weight can play. Treating all of them, rather than just one, is the basis of the approach I take with the dogs and cats I see.
Joint pain
Damage to the cartilage within the joint, caused by joint misalignment, injuries, and infection, produces a cascade of inflammatory molecules which increase blood supply to the affected area and promote rest and immobility…. by triggering pain sensitive nerves! In the case of a short term injury, such as a sprain, this is exactly what the body needs (as uncomfortable as it may be). However, when continuing inflammation in turn damages cartilage and becomes a vicious cycle, pain becomes a significant source of distress and reduced quality of life for affected people and animals. Many effective treatments for pain and reduced mobility belong to the class of ‘non-steroidal anti-inflammatory drugs’, including aspirin and ibuprofen for humans, and drugs such as meloxicam and carprofen for animals. These drugs decrease the production of inflammatory molecules, so can reduce the pain and joint damage which would otherwise have occurred. Newer treatments include antibodies against some of the pain promoting molecules, and joint injections, which we consider in our joint dysplasia blog.
Nerve Pain
In some people and animals it appears that ongoing pain signals from inflamed joints change the way that the nerves in the spinal cord (which transmit information to the brain) behave.
Rather than only being active when a pain signal is perceived, these nerves become active in response to normal sensations, such as heat or cold, light touch of the area, or the unconscious sensations associated with normal joint movement.
Many of you will be familiar with this phenomenon through having experienced sunburn – warm water or scratchy fabrics touching a sunburned area can be very painful, although in normal skin they would not cause pain. Luckily sunburn tends to improve with time, but with continuous inflammation in the joint the nerves can end up in a constant state of hyperexcitability.
Unfortunately the usual drugs which are effective in arthritis are less effective at treating nerve pain, so we have to assess the benefits and risks of prescribing unlicensed (usually human) drugs which do show some activity for this type of pain.
Muscle Pain
Recently we have begun to appreciate that by shifting their weight to avoid pressure on painful joints, arthritis sufferers may put extra strain on their muscles in one area of the body. Many of us know how painful muscle knots can be. What we are also realising is that these painful muscle knots can act in the same way as a painful joint, to excite the nerves in the spine and increase the amount of pain signals transferred to the brain, so muscle pain could make joint pain worse. Treating muscle pain is most often achieved through the use of physical therapy such as massage, TENS, or acupuncture. I have written more about how acupuncture can help with chronic pain, and which patients it tends to suit, in getting to the point: acupuncture in dogs and cats.
Pain from Fat?
We are always told to maintain a healthy body weight and the same is true for our pets. Recently is has been established that fat tissue not only adds extra weight for the joints to carry, but is also a potent source of inflammatory molecules which could aggravate inflammatory diseases such as osteoarthritis. The difficulty in overweight animals that have painful joints is helping them exercise without causing them discomfort. Hydrotherapy, in which the body weight is supported by water, can be useful for these patients.
Pain also shapes behaviour, and behaviour can in turn make pain worse. I look at how we can use this the other way around, supporting recovery through behavioural change, in supporting physical rehabilitation with behavioural modification.
While osteoarthritis was once thought the result of wear and tear on the joints we are now beginning to appreciate the complexities of the disease and the different ways in which it can affect us and our pets. Hopefully this understanding will stand us in better stead to treat all the aspects of the disease.
The information in this blog is general advice only, none of the treatments should be performed by unqualified persons. If you are concerned about your pet you should seek appropriate veterinary attention.
