HNP is an outdated and essentially incorrect term. In veterinary medicine pathologies of the intervertebral disc are currently referred to as 'intervertebral disc disease (IVDD). Furthermore, the term "cervical disc disease" (CDD) is used for lesions in the cervical region, and "thoracolumbar disc disease" (TLDD) for lesions in the thoracolumbar region. The neurological states based on the neurological examination are classified as follows:
| Grade | Neurological state | Proprioception | Bladder control | Deep pain perception |
|---|---|---|---|---|
| I | Pain | + | + | + |
| II | Paraparesis | - | + | + |
| III | Paraparalysis | - | + | + |
| IV | Paraparalysis | - | - | + |
| V | Paraparalysis | - | - | - |
(Neurological grading scale according to Griffith, 1982)
AVOM presumes that degeneration of the intervertebral disc results in an instability of the vertebrae. This instability can cause misalignment of successive vertebrae and as a consequence of this misalignment and disc degeneration, extrusion or protrusion of the nucleus pulposus or the annulus fibrosis results. The resulting pressure on the spinal cord, or trauma induced by the kinetic energy component of the extruded disc material may cause pain and neurological dysfunction. It is theorised that correcting the misalignment of the vertebrae diminishes the pressure on the intervertebral disc and creates an environment that facilitates an improvement in the neurological state. The realignment contributes to pain relief as well.
In AVOM the recovery of the patient is assessed on the basis of neurological grading. Full recovery is defined as: grade 0, with a completely normal gait and proprioception and without residual paresis. Clinical recovery with some residual motor deficiency (paresis and / or delayed proprioceptive reflexes) is defined as recovery to grade I.