Neuropathic pain (or: nerve pain) is the result of damaged and therefore malfunctioning nerve cells. The damage creates a short circuit, so to speak. This triggers spontaneous stimuli in the nerve. These stimuli are transmitted to the brain via the spinal cord, where the pain is consciously experienced.
So, this is different from nociceptive pain; a type of pain that occurs when there is existing or imminent tissue damage. It is therefore also called tissue pain. Special pain sensors, called nociceptors, perceive stimuli that could have a harmful effect on the organism. The nociceptors send pain signals to the brain as well, where they induce a pain sensation. The difference with neuropathic pain stimuli is that this is the purpose. In other words, nociceptive pain is an essential physiological process that serves as a signal to prevent further tissue damage, whereas neuropathic pain does not and is a pathological disease in and of itself.
Neuropathic pain in laminitis
There is vasoconstriction and inflammation in the hoof during the acute phase of laminitis. It is known that each of these issues can lead to nerve damage [1]. Chronically increased blood sugar levels (hyperglycaemia) can potentially damage small nerve fibres too. Horses with EMS/insulin dysregulation (about 90% of horses with laminitis) may therefore experience this type of pain more frequently. Pain also stimulates the production of certain hormones (catecholamines) that raise blood sugar levels. A vicious circle, in other words.
The origin of neuropathic pain in chronic laminitis must be sought in physical damage to sensory nerves caused by the pulling force of the rotating or sinking coffin bone, as well as oedema [2].
Weaving
We see horses with laminitis start to weave (shift weight) or alternately raise their hooves early in the disease process. They show a clear pain response when examined with hoof testers. Aside from apparent causes (inflamed and damaged hoof tissue, coffin bone pressure on the sole, restricted blood flow and oedema), this could be due to stimulation of damaged sensory nerves.
Pain control
There are arguments both for and against the use of painkillers. There is no one-size-fits-all answer regarding whether or not to employ pain medication. The fact is that pain alleviation can be a crucial aspect of treating laminitis. The major reason horse owners choose euthanasia is persistent, severe pain with no prospect of improvement [3].
Medications
The longer laminitis persists, the greater the proportion that neuropathic pain accounts for in the overall pain sensation. It is therefore increasingly important for the vet to determine how different analgesics can be used to relieve pain. That is because neuropathic pain does not respond well to common analgesics that do help with nociceptive pain. Nerve blockers and drugs that alter the processing of pain stimuli in the brain do. Gabapentin and pregabalin, both anti-epileptic drugs, are examples of this type of treatment used in horses. The narcotics ketamine and lidocaine can also have a positive effect, as can the opiates fentanyl [4] and tramadol [5].
Acetyl-L-carnitine has been used successfully in human medicine to prevent and treat neuropathic pain [6]. It is yet to be proven whether the same is true with horses. In anticipation of evidence, veterinarians are already employing it and seeing positive effects.
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