PSSM Type 1 & 2 Overview, Foxwatch Equine Nutrition Hub

What PSSM is, in two paragraphs

PSSM is a genetic muscle disease that causes horses to store abnormal amounts of sugar (glycogen) inside their muscle cells. When the horse exercises, the muscle cannot release that sugar normally, so it tears itself up instead. The visible result is "tying up": stiffness, reluctance to move, sweating, and dark urine after exercise. In severe cases, the horse cannot get up.

There are two distinct forms. PSSM Type 1 is caused by a single, well-mapped mutation in a gene called GYS1[1]. A simple hair or blood test confirms it. PSSM Type 2 (also called MFM, myofibrillar myopathy) looks similar but does not have the GYS1 mutation; the genetics are still being worked out and most "PSSM2 panels" sold commercially are scientifically controversial[28].

Is my horse at risk?

PSSM Type 1 is concentrated in specific breeds. The mutation runs roughly:

GYS1 mutation prevalence by breed

Approximate carrier frequency among genetically tested horses. Single source for orientation only; breed-by-breed values vary by region and registry.

Source data composed from lit_review_01_pssm1_dietary and lit_review_28_genetic_testing_carrier. Specific cohort numbers vary by region and tested population. Values are approximate, intended for orientation only. Always confirm with current literature for breeding decisions.

How is it diagnosed?

For PSSM1, a genetic test (UC Davis VGL, Animal Genetics Inc, Etalon Diagnostics, EquiSeq) gives a clear yes/no answer for around $40-$80[28]. Pull tail or mane hairs, send them in, results in two to three weeks.

For PSSM2/MFM, the only definitive diagnosis is a muscle biopsy showing the characteristic protein-aggregate pattern[4]. The polygenic test panels you may see advertised (P2, P3, P4, P8, K1) are scientifically controversial and the equine veterinary community has not endorsed them[28].

The good news

Both forms respond well to dietary management. The cornerstone is the same: low non-structural carbohydrate (NSC) intake combined with elevated dietary fat[1]. With proper feeding and consistent exercise, most horses go from chronically tying up to nearly asymptomatic. We have a separate Feeding the PSSM horse guide that walks through targets, meal timing, and supplement priorities.

What this article does not do: diagnose your horse. Suggest specific feeds. Override your vet. If you suspect PSSM, get a genetic test and talk to your veterinarian; they will interpret the results in your horse's specific context.

Operational essentials

If you are running a barn with a confirmed PSSM horse, the management protocol is well-defined:

Diet: NSC under 10% of total digestible energy, with fat providing more than 12% of calories[1]. Forage tested for sugar and starch; concentrate (if needed) chosen with NSC less than 12%.
Exercise: Daily turnout. Consistent work, ideally six days a week, gradually increasing intensity. Sudden rest days followed by hard work are the most predictable trigger for tying-up episodes.
Vitamin E: 1.8-2.0 IU/kg body weight daily, particularly for horses with confirmed marginal serum levels[13]. Selenium concurrently in selenium-deficient regions.
Hay testing: Required, not optional. Visual inspection cannot tell you NSC. Equi-Analytical or Dairy One are the two industry-standard labs in the US[7].
Pasture management: Sugar peaks in pasture grass occur in afternoon and after frost. Limit turnout during peak NSC, use grazing muzzles, or use dry-lot management for severely affected horses[9].

Differential diagnosis a barn manager should know

Not every tying-up horse has PSSM. Recurrent Exertional Rhabdomyolysis (RER) is a separate condition, more common in Thoroughbreds and Standardbreds, with a different metabolic basis[6]. The treatment overlap is partial; getting the diagnosis right matters because RER responds to different exercise and dietary protocols than PSSM does.

Also rule out: severe vitamin E or selenium deficiency (which can mimic PSSM), white muscle disease in foals, exertional rhabdomyolysis from overtraining alone, and electrolyte imbalances. Genetic test is the cheapest fastest way to confirm or rule out PSSM1; muscle biopsy for PSSM2 if PSSM1 is negative but symptoms persist.

Co-occurring conditions

PSSM horses frequently develop equine gastric ulcer syndrome (EGUS), partly because the high-fat dietary management increases gastric acid retention[33]. Hindgut acidosis is another concern with high-concentrate diets. The standard management protocol therefore typically includes:

Fat sources delivered in oils or stabilized rice bran rather than high-concentrate-fat blends
Frequent small meals rather than two large ones
Free-choice forage when possible
Periodic gastroscopy in horses with persistent stiffness despite good dietary control

See lit_review_20_gut_microbiome_hindgut and lit_review_33_gi_comorbidity for the full GI co-management protocol.

Pathophysiology

PSSM Type 1 is an autosomal dominant glycogenosis caused by the R309H founder mutation in the equine GYS1 gene[1]. The mutation results in constitutive activation of glycogen synthase and abnormal accumulation of glycogen plus amylase-resistant polysaccharide within skeletal muscle fibers. Glucose clearance in PSSM1 horses is approximately 1.5x faster than healthy controls during IVGTT, with lower circulating insulin suggesting enhanced peripheral insulin sensitivity[1].

PSSM Type 2 / MFM is heterogeneous. Histologic findings include cytoplasmic and subsarcolemmal protein aggregates with desmin and αB-crystallin co-localization. The genetic basis is incompletely characterized; commercially marketed polygenic panels (P2 through P8, K1) lack peer-reviewed validation as of the corpus completion date[28].

Diagnosis

For PSSM1: hair-bulb or whole-blood PCR for the GYS1 R309H mutation. Sensitivity and specificity approach 100% for the homozygous and heterozygous genotypes. Clinical penetrance is incomplete and breed-modulated; homozygous horses show higher resting CK and AST activities than heterozygotes (rho = 0.816, P = 0.01 for AST vs subsarcolemmal vacuolation severity)[1].

For PSSM2/MFM: muscle biopsy of the semimembranosus or semitendinosus, with periodic acid-Schiff staining and amylase digestion. Look for amylase-resistant polysaccharide inclusions and protein aggregates. Histologic interpretation requires an experienced equine pathologist; misclassification with normal age-related glycogen variability is common.

Dietary intervention: evidence base

The standard low-NSC, high-fat protocol traces to Valberg et al. and subsequent observational and controlled work synthesized in lit_review_01[1]. Key parameters:

Starch < 10% of digestible energy
Fat > 12% of digestible energy (commonly 15-20% in active competition horses)
Forage NSC ideally below 10-12% on dry matter basis[2]
Vitamin E supplementation at 1.8-2.0 IU/kg BW/day; selenium concurrent in deficient regions[13, 15]
Daily exercise; consistent training schedule; avoid abrupt loading after stall rest

Outcomes are heterogeneous. CK reduction of 70-90% is typical with full protocol adherence; episode frequency reduces from monthly to less than annual in most controlled cohorts. Long-term return-to-work data are summarized in lit_review_30_longterm_outcomes_qol.

Methodological caveats

The PSSM evidence base is dominated by small sample sizes, heterogeneous outcome measures, and limited blinding. SYRCLE risk-of-bias assessment of the controlled feeding trials is summarized in lit_review_24_methodology_pssm; most studies are at moderate-to-high risk of bias. Strength-of-evidence grading for the primary low-NSC/high-fat dietary intervention is moderate-to-strong, but specific supplement claims (antioxidant blends, branched-chain amino acids, proprietary "PSSM-specific" formulas) are at much weaker evidence levels. See lit_review_18_commercial_feeds_claims for the claim-validation table.

Polysaccharide Storage Myopathy (PSSM): Type 1 & Type 2 / MFM