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Horse Health 5 – Polysaccharide Storage Myopathy: Getting the Diagnosis of PSSM1

Polysaccharide Storage Myopathy 1 (PSSM1)

Original posted 1/27/17

Polysaccharide Storage Myopathy 1 (PSSM1): What Is it?

Polysaccharide Storage Myopathy 1 (PSSM1) is a disorder where the horse stores too much glycogen in their muscles. When they store this excess glycogen, it’s usually in a strange shape, making the process of breaking it down slower.

Thus, while the horse has too much glycogen/energy stored in their muscles (in the form of physical, enlarged granules inside their muscle tissues), they actually suffer from too little stored glycogen/energy available for the muscles to use, creating an energy crisis.

Polysaccharide Storage Myopathy: Before diagnosis, while on pasture – notice the extremely tight hindquarter/flank muscles

Polysaccharide Storage Myopathy is genetic. Jax is n/P1, meaning he is heterozygous (only one copy) for the P1 gene (Polysaccharide Storage Myopathy or PSSM1 is the disease state, P1 is the nackname for the mutated gene GYS1), so he either inherited it from his dam or his sire (unknown at this time). PSSM1 is semi-dominant, it only takes one gene for the disease to affect the horse. Some PSSM1 horses seem to be unaffected, others are affected, and the cause for this is currently unknown.  

There is ongoing research for another set of genetic diseases known as PSSM2, which may turn out to be muscle wasting diseases rather than a polysaccharide storage myopathy, and there appears to be more than one genetic variant. This is all still very new and very technical, and is yet to be peer reviewed.

However, indications are that these PSSM2 variants can be present with the P1 variant, causing worse symptoms than the P1 variant alone (P1/P1 are usually more symptomatic than n/P1 horses also). At this time, it’s unknown if Jax has any other variants.

[Update: in April, 2018 I tested and received current PSSM2 genetic results – he is n/P1, n/P2, n/Px – he is heterozygous positive for PSSM1, a gene associated with PSSM2, and a gene associated with RER. Hybridizing his management to include these 2 extra genes made a WORLD of difference for him, but that wouldn’t come for another year after this post. See more about testing for Polysaccharide Storage Myopathy].  

Polysaccharide Storage Myopathy 1 (PSSM1): Recap of Symptoms

Well, that’s enough of the brain-jarring scientific info for now. Here’s a recap of progressive issues that Jax has had since July 2015.  Keep in mind I was taking him back and forth to the vet trying to get them to see what I was seeing, but Jax is the stoic type and wouldn’t show lameness – makes sense, it’s painful muscles and not injury, so when he’s feeling good or doesn’t want to show pain he can still use himself like nothing is wrong.

Jax’s Polysaccharide Storage Myopathy symptoms:

  • Sticky stifles, (his stifles stick when he’s not in work, but it was becoming debilitating)
  • Sore back and hindquarters
  • Tight muscles everywhere
  • Behavior changes
  • Possible feed/gut complications (ulcers, low grade laminitis – undiagnosed)
  • Atrophied right shoulder
  • Obvious twist in right hind leg at a walk, catching/hitching every step at a trot
  • Can no longer canter
  • Hindquarters stiff and beginning to look atrophied
  • Slight roach back and pelvis tilted with posty/straight appearance to hind legs (overextending stifles and aggravating them even more)
  • Difficulty lifting hinds for farrier
  • Can no longer trot, barely walks around in pasture
  • SI swelling
  • Early 2016 – Colic; switched from sweet feed to Strategy Healthy Edge and started on supplements, including Vitamin E
  • Won’t fully weight right front foot – now won’t lift fronts for farrier
  • Body soreness everywhere, chiro helps but short-lived
  • Glutes/croup sore – light touch makes him drop his back
  • May 2016 – Long toe / low heel syndrome now apparent (been there for about a year, I just didn’t know enough to see it) – apparently all four of his feet had been sore for a while, but since it was bilateral (left and right) in nature there was no head bobbing or hip hiking

Since making the list above, I’ve studied Polysaccharide Storage Myopathy in great detail, and realized he’s been showing symptoms since the day I got him (as an unbroke four year old, August 2011):

Polysaccharide Storage Myopathy 1 (PSSM1): Early Symptoms of PSSM1

  • Tight-rope walking (gives himself splints, especially with metal shoes on). [Update 2020 – splints are likely due to long-standing calcium deficiency!]
  • Forging, stepping on his front feet with his hind feet.
  • Tight muscles, especially in hindquarters, neck, and shoulders.
  • Tripping while ridden and out in pasture.
  • During training as a four year old, preferred canter to trotting (the trot is a symmetrical gait, and is difficult for horses whose muscles are damaged/asymmetrical).
  • After one intense workout as a four year old, front left leg started trembling.
    • Session was intense because he ran, strung out, through the arena, and I was working on having him slow down and balance himself.  Turns out, it’s hard to get a slower, balanced canter for PSSM horses.  This was the only time I saw spasms until it is mentioned below, but several times he was “shaky” which I attributed to nerves (usually while riding on the trailer).  The shakiness is more likely a pre-spasm, his muscles aren’t completely in crisis mode, but close – it almost looks like hypoglycemia (and very well could be!).
  • Spooky and anxious, especially in the Spring with new grass growth (sugar).  Would get so scared he’d shake… so many things make more sense now.
  • Walked in a bent “c” shape on trail/road rides, took over a year to get straightness.
  • “Soft feet” – many PSSM horses tend to get Low Grade Laminitis (which is also a metabolic/sugar issue).

Polysaccharide Storage Myopathy 1 (PSSM1): Four year old Jax in training – notice the bell and splint boots

Most of these things could be explained away as a young horse trying to find his feet, which is exactly what happened. As he got older and fitter, all of these things fixed (with regular exercise) and I assumed wouldn’t come back.  

Polysaccharide Storage Myopathy 1 (PSSM1): Increasing Symptoms of PSSM1

Fast forward to mid-2016 (Jax is now 9, has been showing symptoms for about a year).  Just before the diagnosis of Polysaccharide Storage Myopathy and after getting his feet back in good shape, I began hand-walking and free lunging Jax to try to get him to build up some strength. In May I took him off grain completely, but left him on his supplements with soaked alfalfa cubes. He started doing much better, stifles got stronger with minimal catching, SI looked normal again, and he was able to trot and canter again.

However, his back was still hurting and right hind was still twisting. There was muscle wasting in his flanks and lumbar area (a combination of being out of work for months and avoiding those muscles/compensating for pain?  Or a hint of n/P2 beginning to show, since PSSM2 is adult onset).  

He also went on pasture full-time for the first time since I had bought him (he’s boarded, he had to be moved to a new area so other horses could benefit from a low-grass pasture).  This is probably why he was still having problems – grass can be too high in sugar for a PSSM1 horse [Update: it’s clover and high potassium that tears him up!]. At the end of August, I decided to rip out 40 mane hairs and mail them to Animal Genetics, a company in Florida that does 5 panel testing (HYPP, PSSM1, MH, GBED, HERDA).

I opted for the 5 panel as HYPP and MH can be similar in appearance to PSSM. Before I could get the results of his test back (it only took 6 days), he got under an apple tree and ate a few apples (again, sugar). The next day he was having spasms (see video below), and I knew what he had 2 days before the results came back. At this point, I knew I needed to change some things:

  • No pasture (Update 2019: he handles pasture just fine if it’s the right type!  No clover, ryegrass, or anything high in potassium.  Otherwise it helps him move more and is really good for his muscles!).
  • Exercise 7 days/week, even in winter
  • Trails needed onsite, as trailering him triggers episodes 
Polysaccharide Storage Myopathy: Very mild muscle spasm in August, 2016, days before his n/P1 diagnosis was received.

Polysaccharide Storage Myopathy 1 (PSSM1): PSSM Trigger – Moving

I had a place where we’d trailer frequently to ride the trails, and it was also a boarding facility. I was able to move him there the next day. The next couple weeks were extremely difficult, as the excitement of a move made him worse (it took a full month for him to mentally settle). Finally, he settled and we were able to begin working towards getting him as healthy as possible.

First hurdle – abscesses. He was still having abscesses from his feet being mis-shaped. The worst abscess he’s ever had came a couple weeks after moving him – it blew out the entire lateral (outside) bar from the sole of his hoof. He was lame a couple weeks prior to it blowing and until a few days after (by the end of September 2016 he was fine).

It’s now been 4 months on a PSSM1 diet with 7 days/week exercise, and he’s not had another abscess – in fact, his feet are better than they’ve ever been, and for the first time since I’ve had him, barefoot trail rides haven’t been a problem though he can’t handle rockier areas without boots.

[edit to add: this is a cycle that continued with him in 2017-2019 and has been going on since 2015, possibly sooner; approx. 4 weeks after receiving vaccines he gets sensitive hooves, and it takes about 6 months for them to get back to normal.  Abscessing did continue in 2017 until I started trimming with the ABC method.]

[2nd Update: I didn’t vaccinate Jax in 2020 after moving him home to my own land, and he while he got mild hoof sensitivity in June (his normal time of year for this to happen) he didn’t have any laminitis episodes or major abscesses. He’s usually prone to thrush as well which was also very mild this year.

By the end of 2020 he was crushing rocks on gravel roads barefoot for the first time ever! I’ve also started to let him self-trim since he’s handling rough terrain barefoot, though I do touch up the soles, bars, and frogs to keep pressure spots from forming. I’m not sure how long this will continue but it’s been a fascinating change for sure!]

Polysaccharide Storage Myopathy: Coming back into ridden work just before diagnosis of n/P1, Summer of 2016, before the move to another facility with trails and an indoor onsite.  Notice the tight, rigid muscles still in the hindquarters, and the left leg stretched back.  This common PSSM stance has been with us since the beginning and usually occurs after long rides or after playing in the field – it turns out this is a very mild PSSM symptom indicating discomfort, but it does lessen and even disappear when he’s in high work levels and his muscles are kept strong (read as worked and holding less glycogen).  Stretching out with both is a much stronger sign, especially if they refuse to move as that could be the beginnings of a tie up.

Polysaccharide Storage Myopathy 1 (PSSM1): MAIN TAKEAWAY

One of the most important things I can stress about Polysaccharide Storage Myopathy, is that it’s not all about tying up.  I resisted testing Jax, even though I suspected, for a long time because ‘tying up’ is the main symptom – but with more research I’m learning that many horses don’t fully tie up.  I believe the colic he had in January 2016 was the closest he’s come to a full-blown tie up. 

I didn’t like the way he was moving his feet, or the 3-legged gallop he took when he got too nervous to stand still – I mentioned it to the vet and said I thought he was tying up, but was pretty much ignored because Jax was still moving around.   He was given mineral oil and 10cc of Banamine  (which helps tying up), and within a few minutes he was better.  Since switching his grain (which I started the process of switching him the day after the colic), he’s never had another episode this bad, hopefully he never will.

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