Introduction

Pain, as defined by the International Association for the Study of Pain, is "...an unpleasant sensory and emotional experience associated with actual or potential tissue damage..." In veterinary medicine we have no means of assessing the emotional experiences of our patients and so our assessment of pain is largely interpretation of clinical signs of pain. Physiological pain protects an animal from injury (response to standing on a hot surface). Inflammatory pain is a consequence of tissue damage. Neuropathic pain results from abnormal processing of pain pathways within the CNS or PNS. In many cases, manipulation of the patient is required in order to identify the presence and location of the pain. There are a number of structures within the spinal column that can be involved in causing pain.

Intervertebral Discs and Spinal Ligaments

The intervertebral disc (IVD) is composed of an outer annulus fibrosus (AF) surrounding the gelatinous nucleus pulposus (NP), separated by a transitional zone (TZ). Cranial and caudal boundaries of the disc are hyaline cartilage end plates (EP), with the dorsal and ventral longitudinal ligaments serving as the dorsal and ventral boundaries, respectively. The dorsal longitudinal ligament (DLL) is very well innervated with sensory nerve endings, compared to the sparse innervation of the outer third of the AF (inner AF and NP are not innervated), such that the pain of IVD disease results from stretching and tearing of the dorsal longitudinal ligament rather than the disc itself. This is in contrast to the discogenic pain described in man, where the AF is extensively innervated and when distorted will result in considerable pain.

Dorsal Root Ganglion and Nerve Roots

Mechanical compression or tension of the nerve root and/or ganglion results in repetitive firing of the afferent nociceptive pathways either as a result of direct compressive effects or altered blood flow to the nerve root. Neuropathic pain can be associated with trauma or as a result of entrapment or damage to nerves during surgical procedures. Interestingly, nerve ligation is a model for studying neuropathic pain.

Vertebral Body and Periosteum

Innervation of the osseous vertebral body and sensory nerve terminals within the periosteum have been described in man, and it is presumed this would also be reflected in the canine vertebra.

Meninges

Experimental studies in rats, rabbits and cats have shown evidence of lumbar dorsal horn neuronal stimulation resulting from mechanical stimulation of the lumbar dura. Meningeal disease from either inflammation or mechanical stimulation will result in the manifestation of spinal pain.

Articular Structures

Vertebral articulations are true diarthrodial joints having hyaline cartilage surfaces, a joint capsule, and lubricating joint fluid. The limited mobility of the joint is determined by the nature of the vertebral body and adjacent supporting structures. Vertebral joints are susceptible to the same inflammatory and non-inflammatory diseases identified in the appendicular skeleton, and are an important consideration in any patient with spinal pain.

Muscle

A large mass of muscle surrounds the spinal column and particularly inflammatory and traumatic myopathies can manifest as spinal pain.

Pain Sensation (Nociception)

Pain perception/nociception requires transmission from the periphery to the brain via spinal cord pathways, which appear to be more multisynaptic and bilateral in the dog than in man. In assessing pain in the dog or cat, both the presence or absence of pain perception, and the presence or absence of excessive pain perception require interpretation by the clinician. A behavioural response (vocalization, looking back at the stimulated site, attempting to bite) is required in response to an applied noxious stimulus, as distinct from a withdrawal reflex, which only requires an intact reflex arc at the segment being assessed. Superficial pain sensation is assessed by finger pinching the skin of a digit, and represents a sharp fast pain fibre response. Deep pain sensation requires stimulation of the periosteum of a digit with a forceps and represents a slow dull pain fibre that is small and more centrally located within the spine. Should there be a response to superficial pain testing, it can be assumed that the deep pain fibres are also intact. The importance of interpreting a behavioural response in assessing nociception is that the absence of deep pain perception has prognostic implications in spinal injuries. Hyperaesthesia refers to an abnormal or pathological increased in sensitivity, whereas paraesthesia refers to an abnormal skin sensation (burning, tingling, prickling sensation), and dysaesthesia where an ordinary stimulus results in an abnormal/painful sensation.

Spinal Pain

Most often the diagnosis of either neck or back pain is made on the basis of observation of the patient. Low head/neck carriage, reluctance to shake themselves, crying out when moving the head/neck, paraspinal muscle twitching/spasms, and often paroxysms of screaming, are variably noted with cervical spinal pain. Patients with thoracolumbar spinal pain generally walk slower and are more deliberate in rising or lying down; during activity they may vocalize. Postural changes are often appreciated including kyphosis (which could also occur with cervical pain). Proprioceptive deficits are more likely in thoracolumbar than cervical disease, as considerable compressive cervical disease can be present without neurological deficits. Reluctance to jump or use stairs is often a complaint. Patients with lumbosacral pain, tend to walk with their tails tucked or held low, are slow to rise and lay down, and often reluctant or slow in ascending as opposed to descending stairs. In addition, they often shuffle forwards while defecating.

Manipulation of the cervical spine by moving the head with one hand while maintaining the other hand on the cervical musculature will allow for the examiner to evaluate for a pain response and/or muscle twitches/fasciculation to be palpated. Individual palpation of the transverse processes on both sides may also allow for a pain response to be appreciated, however, most dogs will respond to over-enthusiastic palpation of C6. The thoracic spine is palpated by pushing down on either side of the spinous processes with thumb and forefinger or two fingers, taking care to support the pelvis such that orthopaedic manipulation of the appendicular skeleton is minimised. This manipulates both the thoracolumbar epaxial muscles and the spinal structures. Lumbosacral pain is assessed by pressing down between the L7 spinous process and the sacrum. Excessive pressure at this site with hyperextension of the hips will invariably result in a pain response, even in normal dogs. Elevation of the tail and rectal examination with digital pressure on the sacrocaudal spine (per rectum), should not elicit a pain response in the normal dog. Since intracranial space-occupying disease can manifest apparent cervical pain, palpation of the head over the temporal muscles above the zygomatic bones may elicit a pain response. Occasionally, dogs with increased intracranial pressure or inflammatory brain disease manifest with signs akin to a "headache." These patients are invariably more reclusive; seek cool and darkened areas to rest, and often exhibit partially closed eyelids (squinting).

Conditions resulting in spinal pain can best be considered as being either inflammatory or non-inflammatory in nature.

Inflammatory Spinal Conditions

Meningitis, Vasculitis, Steroid Responsive Meningitis-Arteritis

Inflammation of the meninges may be infectious, immune-mediated, or idiopathic. A variety of inflammatory mediators produced by inflammatory cells and endothelial cells cause sensitization of nociceptors within the meninges. Infectious agents include viruses (distemper, feline corona), bacteria (Staphylococcus, Streptococcus, Pasteurella), fungal (Cryptococcus, Coccidioides, Blastomyces), tick-borne disease and parasitic migration. Steroid responsive meningitis-arteritis (SRMA) usually reveals a non-degenerate neutrophilic pleocytosis. Immune-mediated disease meningitis is often associated with pathogen-free encephalitis (meningoencephalitis of unknown aetiology/GME).

Polyarthritis

Nonerosive polyarthritis, (including tick-borne, immune-mediated, idiopathic immune-mediated, plasmatic-lymphocytic synovitis), and the erosive (rheumatoid, feline progressive) polyarthritidies result in a painful gait and reluctance to walk. Many patients walk gingerly, as though walking on eggshells/hot surface, with a shortened stride, often in all limbs. Spinal pain may be elicited if the disease process affects the articular synovia but importantly, proprioceptive and spinal reflex deficits are not appreciated. Joint pain and/or palpable joint effusion may not necessary be present, making arthrocentesis and cytology/joint fluid analysis a requirement for confirmation/exclusion. The presence or absence of pyrexia is also not a reliable indicator.

Discospondylitis, Vertebral Osteomyelitis

Infection and inflammation of the disc and adjacent vertebral bone may occur secondary to bacterial or fungal disease. Most infections are haematogenous in origin and when identified, survey of the entire spine is warranted. Therapy is directed at the aetiological agent and pain relief.

Polyradiculoneuritis

Most commonly associated with motor nerve and ventral horn dysfunction leading to an areflexic flaccid paralysis, but dorsal nerve root ganglion inflammation commonly results in hyperaesthesia. Marked pain response will occur with just touching the patient, exacerbated by not being able to withdraw from the stimulus because of the motor paralysis. Dysphonia and dysphagia may be noted because of widespread LMN involvement.

Noninflammatory Spinal Conditions

Spinal Cord Injury

Injury may be attributed to trauma, compressive disc disease, or as a result of a vascular event. Spontaneous epidural haemorrhage is occasionally identified in larger breeds as a cause of spinal pain and paresis, and although disc extrusion is suspected in these cases, evidence of disc extrusion may not been found. Fibrocartilaginous embolic myelopathy (FCE) is a peracute onset of usually asymmetrical neurological deficits that usually only manifests pain in the initial hours post-injury.

Intervertebral Disc Disease

Degeneration and protrusion/extrusion of intervertebral discs is the most common cause of spinal pain. However, there are many patients that present with acute tetraplegia or paraplegia that fail to exhibit spinal pain. In cervical disc disease, spinal pain may be the only sign exhibited.

Cervical Spondylomyelopathy

Disc associated and osseous associated cervical spondylomyelopathy (CSM) are the two forms of this condition most recognized. The disc-associated form is most commonly seen in the Doberman Pincher and is associated with ventral compression in the caudal cervical spine, although dorsal compression can be seen with ligamentum flavum hypertrophy. Osseous-associated CSM results from vertebral canal stenosis secondary to proliferation of the articular facets and/or pedicels or vertebral arch, more commonly seen in the young adult giant breeds. It is a complex disorder with little consensus as to therapy.

Lumbosacral Stenosis

Signs of cauda equine syndrome can arise from degenerative/compressive disc disease, inflammatory disease (discospondylitis, neuritis), congenital stenosis, subluxation, and vascular compromise. A poor correlation exists between severity of signs and diagnostic imaging results; however, MRI is still regarded as the most valuable modality.

Syringomyelia

Most commonly associated with Chiari-like malformation (COMS) and a common cause of cervical pain in the Cavalier King Charles Spaniel. A fluid-filled cavitation of the spinal cord is recognized on MRI, which can extend into the thoracic and lumbar spinal cord. Damage in the region of the dorsal horn cells is thought to result in the persistent neck scratching, often without making contact with the skin - hence the term "phantom scratching." Primary secretory otitis media often accompanies this condition in CKCS, potentially contributing to signs of pain.

Atlanto-Axial Subluxation

Abnormal development of the dens (or infrequent fracture of the dens) can result in subluxation. Small or toy breed dogs less than a year of age have either hypoplasia or aplasia of the dens and may be clinically occult until a traumatic event occurs with failure of the remaining supporting ligaments and subluxation. Radiographs are usually diagnostic, although occasionally MRI or CT is required for confirmation. Surgical reduction and stabilisation affords the best prognosis.

Synovial Cysts

Synovial cyst development with protrusion into the spinal canal and compression of the cord through the cervical and occasionally thoracolumbar spine are recognised. Cervical synovial cysts are often noted in the large to giant breeds and may occur as part of CSM. In addition, these cysts may be present at the lumbosacral junction and contribute to lumbosacral pain. Surgical removal is warranted where pain cannot be alleviated or neurological deficits are appreciated.

References

1.  Mathews KA. Neuropathic pain in dogs and cats: if only they could tell us they hurt. Vet Clin North Am Small Anim Pract. 2010;38:1365–1414.

2.  Webb AA. Potential sources of neck and back pain in clinical conditions of dogs and cats: a review. Vet J. 2003;165:193–213.