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The pain experienced by horses with laminitis is not due simply to changes within the foot. Work carried out at Edinburgh Vet School suggests that neuropathic pain may be involved.
Pain usually results from inflammation in the affected tissue. Specialised pain receptors, known as nociceptors, are present in most tissues. When they are stimulated they send impulses towards the brain and spinal cord. Two types of nerve fibre are associated with the pain receptors. Ad fibres are fast response fibres, which respond to both thermal and mechanical injury. They are responsible for transmitting sudden sharp pain, such as a needle prick. The second type of fibres (C fibres) transmit impulses more slowly. They are associated with receptors that respond to mechanical, heat and chemical damage. They are responsible for dull,
Pain usually results from inflammation in the affected tissue. Specialised pain receptors, known as nociceptors, are present in most tissues. When they are stimulated they send impulses towards the brain and spinal cord. Two types of nerve fibre are associated with the pain receptors. Ad fibres are fast response fibres, which respond to both thermal and mechanical injury. They are responsible for transmitting sudden sharp pain, such as a needle prick. The second type of fibres (C fibres) transmit impulses more slowly. They are associated with receptors that respond to mechanical, heat and chemical damage. They are responsible for dull,
Neuropathic pain in laminitis
aching pain. Stimulation causes release of inflammatory mediators and transmitter substances in the spinal cord. This leads to "central sensitisation" a process by which pain perception is further enhanced.
Unlike other types of receptors, the nociceptors do not suffer from "fatigue". That is, repeated stimulation does not lead to a reduced response. In fact continued stimulation may actually lead to increased sensitivity and a greater response.
Pain can also originate in the nerve cells themselves. This phenomenon is called neuropathic pain (literally pain due to nerve disease). It is characterised by allodynia (pain produced by a stimulus that would not normally do so), hyperalgesia (an exaggerated pain response to a painful stimulus), and spontaneous pain. Neuropathic pain is associated with characteristic changes in sensory nerve cell bodies, and is the result of peripheral nerve injury.
Miss Emma Jones and her colleagues at the Edinburgh Vet School found evidence of damage to the sensory nerve cell bodies in horses with laminitis. They looked at nerve cells in the dorsal root ganglia of cervical segment C7, which contains the cell bodies of the nerves that supply the front foot. They took samples from three laminitic horses and compared them with those from three normal horses
First they used special staining techniques to identify the different types of nerve fibres in the dorsal root ganglion. Then they looked for signs of damage in the nerves.
They found that there was more ATF3, a factor that is produced in response to nerve injury, in the nerves of horses with laminitis than in normal horses. Similarly, 70% of nerve cells in laminitic horses contained NPY, a neuropeptide that is only produced in response to nerve fibre damage. In contrast, only 5% of nerve cells from normal horses contained NPY.
These results show that nerve cell damage does occur in laminitic horses. So at least some of the pain experienced by horses with chronic laminitis may be the result of neuropathic pain.
This raises the possibility of using some of the treatments used for this problem in people, such as gabapentin or ketamine. According to Jones, infusions of ketamine do appear to produce a reduction in pain level but are not practical at the moment.
She explained that we do not know how long neuropathic pain takes to become established. Experimental work in rats suggests that it might take about 3 days.
"At the moment we can't say whether this is a general effect or if it is just related to the feet" she said. "It may be that oedema within the foot leads to damage to the nerves, which extends back to the nerve cell body in the dorsal root ganglion."
Unlike other types of receptors, the nociceptors do not suffer from "fatigue". That is, repeated stimulation does not lead to a reduced response. In fact continued stimulation may actually lead to increased sensitivity and a greater response.
Pain can also originate in the nerve cells themselves. This phenomenon is called neuropathic pain (literally pain due to nerve disease). It is characterised by allodynia (pain produced by a stimulus that would not normally do so), hyperalgesia (an exaggerated pain response to a painful stimulus), and spontaneous pain. Neuropathic pain is associated with characteristic changes in sensory nerve cell bodies, and is the result of peripheral nerve injury.
Miss Emma Jones and her colleagues at the Edinburgh Vet School found evidence of damage to the sensory nerve cell bodies in horses with laminitis. They looked at nerve cells in the dorsal root ganglia of cervical segment C7, which contains the cell bodies of the nerves that supply the front foot. They took samples from three laminitic horses and compared them with those from three normal horses
First they used special staining techniques to identify the different types of nerve fibres in the dorsal root ganglion. Then they looked for signs of damage in the nerves.
They found that there was more ATF3, a factor that is produced in response to nerve injury, in the nerves of horses with laminitis than in normal horses. Similarly, 70% of nerve cells in laminitic horses contained NPY, a neuropeptide that is only produced in response to nerve fibre damage. In contrast, only 5% of nerve cells from normal horses contained NPY.
These results show that nerve cell damage does occur in laminitic horses. So at least some of the pain experienced by horses with chronic laminitis may be the result of neuropathic pain.
This raises the possibility of using some of the treatments used for this problem in people, such as gabapentin or ketamine. According to Jones, infusions of ketamine do appear to produce a reduction in pain level but are not practical at the moment.
She explained that we do not know how long neuropathic pain takes to become established. Experimental work in rats suggests that it might take about 3 days.
"At the moment we can't say whether this is a general effect or if it is just related to the feet" she said. "It may be that oedema within the foot leads to damage to the nerves, which extends back to the nerve cell body in the dorsal root ganglion."
Report by Mark Andrews.