🧬 Polysaccharide Storage Myopathy (PSSM) Vet Guide 2025 by Dr Duncan Houston

Polysaccharide Storage Myopathy (PSSM) is a metabolic muscle disorder affecting many breeds, causing glycogen buildup in muscles and episodes of “tying-up.” In this comprehensive guide, we’ll dive into the distinct types (Type 1 & Type 2), genetic testing, clinical signs, diagnosis, diet‐and‐exercise management, prognosis, and essential vet–owner collaboration strategies. Let’s keep your horse sound and performing at its best! 🐴✨

1. What Is PSSM & Why It Matters

PSSM is a glycogen storage disorder causing excessive and abnormal polysaccharide deposits in muscle. Clinically, this results in painful muscle cramping, stiffness, sweating, reluctance to move, and poor performance. It falls under the umbrella of exertional rhabdomyolysis syndromes :contentReference[oaicite:3]{index=3}.

2. Type 1 vs Type 2 PSSM

• Type 1 PSSM: Caused by a mutation in the GYS1 gene leading to overproduction of glycogen and abnormal polysaccharide chains :contentReference[oaicite:4]{index=4}. Found in over 20 breeds including Quarter, Draft, Paint, Appaloosa, Warmbloods, Arabs, Thoroughbreds :contentReference[oaicite:5]{index=5}.
• Type 2 PSSM: Similar clinical presentation without GYS1 mutation; diagnosed via muscle biopsy. More common in Warmbloods, Arabians, and some drafts :contentReference[oaicite:6]{index=6}.

3. Breed Predisposition & Prevalence

• Quarter Horses: 6–12 % in general; up to 28 % in halter lines :contentReference[oaicite:7]{index=7}.
• Paints/Appaloosas: ~6–8 % :contentReference[oaicite:8]{index=8}.
• Draft breeds: Up to 35–62 % carry the mutation (e.g., Belgians, Percherons) :contentReference[oaicite:9]{index=9}.
• Warmbloods: Type 1 rare; Type 2 more common :contentReference[oaicite:10]{index=10}.
• Low prevalence in Arabians, Thoroughbreds :contentReference[oaicite:11]{index=11}.

4. Signs & Clinical Presentation

Clinical signs include:

• Muscle stiffness/cramping—“tying-up” after light exercise :contentReference[oaicite:12]{index=12};
• Sweating, pawing, reluctance to move, shifting lameness, firm muscles :contentReference[oaicite:13]{index=13};
• Muscle atrophy, gait abnormalities, weakness, elevated CK even at rest :contentReference[oaicite:14]{index=14};
• Some horses are subclinical until triggered by exercise, stress, or high-NSC diet.

5. Diagnosis: Genetic Testing & Muscle Biopsy

• Genetic test: Samples via hair or blood confirm Type 1 mutation :contentReference[oaicite:15]{index=15}.
• Muscle biopsy: Required for Type 2; shows PAS-positive glycogen clumps :contentReference[oaicite:16]{index=16}.
• Other causes of rhabdomyolysis (e.g. MYHM, malignant hyperthermia) should be ruled out :contentReference[oaicite:17]{index=17}.

6. How PSSM Affects Muscle & Metabolism

Horses with PSSM1 have up to 2–4× higher muscle glycogen and produce excess polysaccharide; Type 2 has similar abnormal deposits :contentReference[oaicite:18]{index=18}. Insulin sensitivity increases glycogen, especially in stock-type breeds :contentReference[oaicite:19]{index=19}.

7. Management & Treatment Strategies

Dietary Control

• Forage with <10–12 % NSC (<100 g/kg) baked into all meals :contentReference[oaicite:20]{index=20};
• High-fat calories (15–20 % of DE) from oils or beet pulp reduce reliance on glycogen :contentReference[oaicite:21]{index=21};
• Cool-season, mature grass hay; hay analysis recommended :contentReference[oaicite:22]{index=22};
• Avoid grains/sweet feeds; use low-sugar balancers if needed :contentReference[oaicite:23]{index=23};
• Vegetable oil or rice bran as safe energy boosters :contentReference[oaicite:24]{index=24}.

Exercise & Conditioning

• Daily turnout or regular exercise—helps muscles utilise glycogen :contentReference[oaicite:25]{index=25};
• Progressive conditioning avoids triggering muscle damage;
• Avoid stall confinement and sudden intense work

Supplements & Adjuncts

• Vitamin E, selenium, magnesium may aid muscle health :contentReference[oaicite:26]{index=26};
• Acetyl-L-carnitine may support energy metabolism :contentReference[oaicite:27]{index=27};
• Monitor electrolytes closely during work.

Veterinary Monitoring

• Monthly condition scoring, weight tracking;
• Periodic lameness and gait evaluation;
• Blood CK testing after bouts of stiffness;
• Recheck diet/hay analysis regularly.

8. Prognosis & Long-Term Outlook

While there is no cure, adherence to diet and exercise regimens reduces rhabdomyolysis by ~90% in clinical cases :contentReference[oaicite:28]{index=28}. Mild cases may remain asymptomatic; severe or homozygous animals require stricter compliance.

9. Owner–Vet Collaboration Tips

1. Test early—genetics or biopsy—to guide management;
2. Work with vet to analyse hay NSC and set diet plan;
3. Keep daily logs: feed, exercise, behavior, manure, CK numbers;
4. Plan gradual exercise progression;
5. Review progress quarterly and adjust program;
6. Discuss breeding plans—avoid passing Type 1 allele;
7. Use Ask A Vet app for 24/7 support and logging.

10. Quick Reference Summary Table

🔚 Final Thoughts

PSSM is a lifelong metabolic condition—but with science-based diet, tailored exercise, and vigilant monitoring, most affected horses thrive. Early diagnosis, genetic screening, and close vet-owner partnership are key. Interested in custom diet audits, exercise journals, or breeding guidance? Our Ask A Vet team is here to support you. Download the Ask A Vet app for 24/7 expert guidance, feeding calculators, and performance trackers. 🌟