Neuropathic pain is an abnormal painful response. It is caused by injury to or disease of the nerves, spinal cord or parts of the brain that are involved in receiving, conducting and interpreting painful stimuli.
Both physical and chemical changes within the spinal nerves that convey pain contribute to neuropathic pain. Such changes include: an increase in the chemicals and receptors that transmit signals between nerves; an imbalance between pain signals coming into the brain and the brain suppressing those signals; and a change that makes nerves that don’t normally relay pain signals into ones that do.
There are different types of neuropathic pain. An exaggerated response to a painful stimulus, such as a pin prick, is called hyperalgesia. Pain from a normal stimulus that shouldn’t be painful at all, such as a light touch, is called allodynia. Hyperpathia is increasing pain after repetitive stimulation, which includes continued pain after the stimulus is gone, and pain that radiates to adjacent areas.
People who experience neuropathic pain describe it as “a pins and needles sensation,” tingling, burning, itching, numbness or cold, and sometimes feeling as if they had received a small electric shock. Grades of neuropathic pain that have been established in human medicine are definite, probable and possible.
Pets cannot describe their pain to us but we believe it’s likely to be like what people have. Neuroma formation in horses is known to cause definite neuropathic pain. A neuroma consists of an abnormal sprouting of nerve endings from a severed nerve, and affected horses develop lameness or an abnormal gait.
An example of probable neuropathic pain in dogs is the abnormal scratching behavior that is seen in King Charles Cavalier Spaniels with a spinal cord abnormality called syringomyelia. Cats with hyperesthesia syndrome incessantly lick and attack their back, and become extremely painful when petted. Neuropathic pain may be one of the causes for hyperesthesia syndrome.
Diagnosing neuropathic pain in animals involves ruling out other painful conditions, such as osteoarthritis and bone cancer, as well a thorough neurological workup. A neurological examination evaluates the different parts of the nervous system (brain, spinal cord, nerves); if those parts are not normal, further diagnostic tests are indicated. Laboratory tests, radiographs, CT or MRI imaging, and in some cases electrophysiologic testing may be recommended to look for diseases that are known to cause neurological injury and secondary pain. Examples of such diseases include: diabetes mellitus, intervertebral disk herniation, spinal cord or nerve sheath tumors, cervical malformation, and syringomyelia.
In some cases treatment is started early in conditions that are normally expected to be painful to try to prevent neuropathic pain from developing. In other cases, neuropathic pain may be the first sign.
Combining drugs that reduce pain in different ways has proven to work better than using a single drug alone. Depending upon the species, one or more drugs may be prescribed by a veterinarian.
Gabapentin, pregabalin, dextromethorphan, and amantadine are drugs that act on the relay centers for pain within the spinal cord. Amitriptyline acts both on the brain and within the spinal cord. The antibiotic minocycline has shown potential for modifying neuropathic pain.
These agents may be combined with one or more of the traditional analgesic drugs: tramadol and codeine, narcotic drugs that modulate the brain’s perception of pain; non-steroidal antiinflammatory drugs (NSAIDs) such as carprofen, deracoxib, and firocoxib; local anesthetics such as lidocaine and alpha-2 agonists; drugs such as medetomidine that modify pain, heart rate and blood pressure.
Some veterinarians have found success using combinations of herbs, diet, exercise, and acupuncture.
It is important to work closely with your veterinarian to manage neuropathic pain. Balancing the benefit of multiple medications against side effects can be challenging. Discussing the perceived effects of medications with your veterinarian is essential for the best outcome for your pet.