Myelomalacia is a condition that can occur after a spinal cord injury. It is more likely to develop in dogs that have complete paralysis and sudden loss of deep pain sensation (Grade 5).
It is a progressive condition caused by an impaired blood supply to the spinal cord after an injury. Myelomalacia may take time to develop, even after what may appear to have been successful surgery.
Paralysis develops forward and backward from the site of the injury, resulting in complete paralysis (tetraplegia). As the death of the spinal cord moves towards the head, the dog will increasingly be in pain and in distress. If this is diagnosed, your vet will often recommend euthanasia to prevent further suffering.
This paper, published in 2017, gives more information on the condition:
Prevalence and Risk Factors for Presumptive Ascending/Descending Myelomalacia in Dogs after Thoracolumbar Intervertebral Disk Herniation. Balducci et al 2017
Research paper on treatment:
"EHLD appears to be effective in halting the progression of presumptive PMM and preventing morbidity even in dogs which had the interval between MRI and EHLD"
Note that this is a very small sample of dogs and very specific PMM cases. The treatment may not be generalisable across all cases of Myelomalacia.