Clinical features of ascending/descending myelomalacia

The hallmark of ADM is the progression of an initially focal SCI to one that can no longer be localized to a specific segmental area. The neurological examination and process of neurolocalization (Chapter 10) help first define an anatomic zone where a focal lesion exists, based on evaluation of the gait if present, muscle tone, segmental reflexes, and nociception. These zones include C1–C5, C6–T2 (the cervical intumescence), T3–L3, L4–L5, L6–S1, and S2–caudal (coccygeal) segments. Further refinement of localization is possible in some patients by identifying a deficit in the cutaneous trunci (nee “panniculus”) reflex and/or determination of a line of analgesia or sensory level where there is hypoalgesia or analgesia caudal to this line and normal nociception cranial to it. ADM can be recognized when the history or initial neurological examination clearly demonstrated localization to one of these zones (almost always T3–L3), but a subsequent examination discloses deficits that cannot be explained by this localization. ADM can progress bidirectionally, or may only progress cranially (ascend) or only caudally (descend).

Patients with focal T3–L3 IVDH and neurologic deficits will have general proprioceptive ataxia and upper motor neuron paresis in the pelvic limbs or paraplegia. The paresis is usually associated with hyperreflexia and hypertonia in the pelvic limbs. Their breathing and thoracic limb functions will be normal. They may have loss of the cutaneous trunci reflex caudal to the lesion, and in severe cases may have loss of nociception caudal to a line of analgesia or sensory level that corresponds to a level just caudal to the spinal cord lesion. With descending myelomalacia that destroys the lower motor neuron gray matter throughout the lumbar, sacral, and caudal segments, subsequent examination of this patient would reveal loss of tone/flaccidity in the abdominal muscles, pelvic limbs, tail, anus, and bladder. These same areas would be analgesic from the loss of the general somatic afferent pathways. With ascending myelomalacia, the line of analgesia will migrate cranially from the initial localization, intercostal paralysis will be noted, and eventually there may be lower motor neuron paresis or paralysis of the thoracic limbs. A Horner’s syndrome may become apparent due to the involvement of the intermediate gray matter in the cranial thoracic spinal cord segments. With further ascension there may be complete respiratory paralysis and death as more of the cervical spinal cord is affected.

Dogs with ADM are typically reported to exhibit signs of excruciating discomfort, and in some cases fever. This is believed to be due to meningeal and subarachnoid hemorrhage at the cranial aspect of the ascending lesion. It has been reported that dogs with ADM have an “anxious” expression or “looking like they know they’re going to die.” Dogs with ADM have also been described as appearing depressed and may rarely assume an opisthotonic posture [4–6].

The progression of signs usually occurs over a period of hours or up to several days. Most cases of ADM are recognized within 24–72 h after an initial acute onset of SCI, although there have been sporadic reports of occurrence beyond 72 h of initial paralytic signs [6]. The occurrence of ADM appears to be unaffected by any medical and/or surgical intervention that was employed for the paralyzed patient prior to its onset. Hence, it is quite possible to see ADM in a dog that has already undergone decompressive spinal cord surgery or has been treated with pharmaceuticals [7].

Myelography of ADM typically will reveal more than 4 segments where there is disruption of typical subarachnoid columns, and often, there will be contrast seen in the central part of the vertebral canal where the spinal cord should be outlined if it were intact. Magnetic resonance imaging will disclose parenchymal hyperintensity on T2-weighted images and diffuse hypointensity on gradient echo images cranial and caudal to the focal area of extradural mass representing the inciting disc herniation [8, 9].

There is no known effective treatment for ADM. The ascending form is usually fatal, and even the descending-only form would result in diffuse lower motor neuron deficits in the pelvic limbs, bladder, and bowel such that euthanasia is usually the appropriate intervention.

Epidemiology

The true incidence of ADM is not known. Published reports indicate that it may occur in 3–9% of dogs with acute thoracolumbar SCI [5, 6, 10–18]. Personal experience of the authors and communication with other neurologists and neurosurgeons would suggest a possibly lower incidence, perhaps on the order of 1%. A review of myelograms at one institution identified only five dogs with lesions consistent with ADM (lesions greater than four segments) over a 7-year span, which represented fewer than 1% of all the dogs that underwent myelography there [19]. A retrospective study of 46 dogs with surgical decompression of focal IVDH in the thoracolumbar area identified 1 dog with ADM [20]. Perhaps the fairest statement to make is that ADM is (fortunately) an uncommon sequel to disc herniation but occurs enough and is of such devastating clinical significance that clinicians need to be familiar with the syndrome and recognize it when present. If the characteristic clinical signs are recognized on the initial examination of the patient there is no need for any imaging studies.

There are no data to suggest any gender predilection. Most reported cases have been associated with IVDH, but there are a few instances where other forms of spinal cord trauma have been the initial insult [14]. There have been rare cases of ADM reported with no known inciting cause of SCI, no prior history of surgery or other iatrogenic cause, and no vertebral canal lesions [16, 21]. The majority of reported patients have been dachshunds, but it is unclear if this is a true breed predilection or simply a reflection of the disproportionate number of dachshunds seen in the population of dogs affected with IVDH.

Neuropathology

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