Does prazosin or diazepam work for urinary incontinence secondary to a thoracolumbar myelopathy?

Let's back it up a moment... animals with thoracolumbar injury often have some degree of urinary incontinence if the injury is severe enough to result in a non-ambulatory status.

Here is what I expect:
No motor = no urine function.
Weak motor = +/- urine function.
Good motor = normal urine function.

What is the neuroanatomy here?

I'm glad you asked! (If you didn't you've probably stopped reading now.) Urination is managed through a coordinated effort between the sympathetic (hypogastric nerve), parasympathetic (pelvic nerve) and somatic (pudendal nerve) function. See details below.

Sympathetic innervation:
1. Purpose: management of the storage phase of urination and pain receptors for distention in the bladder.
2. Nerve involved: Hypogastric nerve
3. Cell body location: L2-5 in the cat, L1-4 in the dog
4. Neurotransmitters utilized: Beta receptors in the urine bladder and alpha receptors in the urethra are stimulated with norepinephrine to maintain a relaxed bladder muscle and tonic internal sphincter of the urethra.

** Prazosin works to block sympathetic function, thus decreasing STORAGE phase, increasing express-ability.

Parasympathetic innervation:
1. Purpose: management of the voiding phase of urination and stretch receptors in the detrusor for distension.
2. Nerve involved: Pelvic nerve
3. Cell body location: S1-S3
4. Neurotransmitters utilized: Acetylcholine is released at the nerve terminals in the bladder muscle (detrusor). There is questionable innervation of the internal sphincter with parasympathetic fibers. The most compelling research suggests that rhythmic contraction of the detrusor muscle during voiding results in stretching and opening of the urethral smooth muscle rather than direct innervation.
Somatic Innervation:
1. Purpose: Maintenance of storage phase through contraction of the external urinary sphincter and some stretch receptors of the urethra.
2. Nerve involved: pudendal
3. Cell body location: S1-S3
4. Neurotransmitters: acetylcholine

** Diazepam works here to block the external sphincter contraction.

If an animal sustains injury cranial to the pudendal/pelvic nerves (S1) they will have an "upper motor neuron" bladder. In this case, they cannot voluntarily evacuate urine. And this is when we try drugs to help these poor dogs (and owners that are trying to express them!).

A recent article by an author with a very similar name to mine* recently published their answer to the question at the top of this mailer.

Bottom line, there was no statistical difference between the control (no drug) and drug group in terms of length of hospitalization or degree of urinary incontinence at discharge (p=0.14).

Shoot. Don't toss those drugs down the drain, however. Despite statistics, we may have some clinical improvement in individuals so it could be worth trying on a short term (7 days or less) basis. However, if it's not helping, don't continue just because it should be helping!

Incidentally, they found that with each 1 kg increase in body weight the chances of incontinence increased by 14%. Yet another reason to encourage healthy-weight dachshunds!

* Barnes KH, Aulkah KS, Liu C: Retrospective evaluation of prazosin and diazepam after thoracolumbar hemilaminectomy in dogs; Vet J:253(November),2019