What is Myelomalacia?
A "softening" or necrosis of the spinal cord. Typically, it affects the center part (gray) first and then affects the outer part (white matter) as it progresses.
What Causes Myelomalacia?
Myelomalacia occurs after a loss of spinal cord blood flow. What causes a loss of blood flow, you ask? Typically an acute disc herniation that cause either a rupture or compression of the cord vessels. We can occasionally see this with fibrocartilagenous emboli and other spinal cord diseases too.
What's the Clinical Presentation?
Here's the scenario: 5 year old Dachshund presents to your clinic with an acute onset paraplegia about 1 hour prior to your evaluation. You perform an exam and find the following:
Mentation: normal
Cranial nerves: Normal
Gait: Paraplegia with intact nociception in all limbs and tail
Postural reactions: absent both pelvic limbs, normal in both thoracic limbs
Reflexes: Reduced c. trunci to T12 bilaterally, normal limb reflexes.
Palpation: Focal pain T12
Lesion localization?? (see below to check yourself)
You call me for a consult. I examine the dog about 2 hours after you and find the following:
Mentation: normal
Cranial nerves: Normal
Gait: Paraplegia with absent nociception in all limbs and tail
Postural reactions: absent both pelvic limbs, normal in both thoracic limbs
Reflexes: Reduced c. trunci to T7 bilaterally, absent patella reflexes bilaterally with intact withdrawal and perineal reflex. Normal thoracic limb reflexes.
Palpation: focal pain T12
Neuroanatomic lesion localization??
So now, you're kicking yourself because there is a difference in our two exams. (I've taken so many phone calls over the years from vets kicking themselves for this...read on to see why you must stop self beratement!)
The neuroanatomic lesion localization from exam 1 was a T3-L3 myelopathy with a focus at T10-T11 based on the cut off of the cutaneous trunci. However, the next exam has a C.trunci that has moved cranial to T7, which means we should have a focus at T5-6 for the lesion. (C. trunci is present 1-2 segments caudal to the lesion). Wait! (You shout) The loss of patella reflexes should indicate a lesion in the L4-6 spinal cord segment because that is where femoral nerve cell bodies are located. You're right!
So now we have exam findings that don't allow us to localize to one segment. AND, more specifically there is suggestion of diffuse spinal cord disease because the entire segment from T5/6 to L4-6 could be affected based on the cutaneous trunci. Discs rarely herniated cranial to T10 so when I see a cutaneous trunci cutoff cranial to T10 in a deep pain negative dog I start talking about myelomalacia to the client.
This is clinical myelomalacia: Evidence of neuroanatomic lesion localization that appears to ascending and/or descending from the initial findings.
This dog had a peracute disc herniation which resulted in rupture of spinal cord vessels, loss of blood flow to the spinal cord, and subsequent necrosis of the gray, and a bit of the white matter. You happened to catch the dog on the downward spiral which is why your exam showed intact deep pain and reflexes. Even if you had referred the dog instantly, we magically did surgery within 5 minutes, the process of malacia was started and things are destined to get worse. Dogs that are walking and, then suddenly pelgic, are at increased risk of myelomalacia!
Cell bodies are in the gray matter. The cutaneous trunci sensory cell bodies are getting liquefied as the malacia ascends therefore we cannot perform the cutaneous trunci reflex from T7-L6 anymore. Furthermore, the cell bodies for the femoral nerve have undergone necrosis thus this reflex cannot be performed anymore. If we let this go on and on, eventually some dogs will get necrosis of the entire pelvic plexus (meaning no pelvic limb reflexes, lower motor neuron bladder, no anal reflex) and it will ascend cranial to involve the thoracic limbs. At that time, dogs begin to hypoventilate because the cell bodies that innervate the intercostal muscles are located in the thoracic spinal cord and they are gone.
What is the Treatment for Myelomalacia
Sadly, nothing. Many smart people are trying to find compounds that will stop the advancement of myelomalacia but as of yet we don't have any treatment. Furthermore, this is one of the few times in which animals with disc herniation are best served by humane euthanasia. Hypoventilation is not reversible, myelomalacia can be painful to endure, and there is no hope of recovering motor function. If those aren't reasons enough for humane euthanasia, consider that the phrenic nerve (which innervates the diaphragm) comes out about C5 so if the malacia continues to advance cranial they will die of asphyxiation. We all can agree that isn't fair I think.
This TidBit was stimulated from on a case I saw this week so keep those consults coming! Your cases are helping your colleagues to remember things they once knew, and they give us something to talk about!
Thanks for reading. I hope all of you have a wonderful, peaceful holiday season and a safe, happy New Year!