Polysaccharide Storage Myopathy (PSSM) is an inherited muscle disorder in horses that leads to problems in energy metabolism of the muscles. This condition primarily occurs in draft and Quarter Horse breeds, as well as related horse breeds. Studies show that about 6-10% of Quarter Horses and up to 36% of Belgian draft horses may be affected. Horses with PSSM often exhibit symptoms such as muscle stiffness, trembling, and reluctance to move, particularly after rest periods or light exercise. Early diagnosis, targeted feeding, and exercise programs can help alleviate symptoms and improve the quality of life for affected horses.
Symptoms
PSSM typically presents specific symptoms, especially after periods of rest or with excessive exertion. Affected horses tend to show:
- Muscle stiffness and cramps
- Trembling and muscle twitching, particularly in the hindquarters
- Reluctance to move and difficulty in starting to trot
- Increased sweating
- Partial muscle atrophy (muscle wasting) in chronic cases
These symptoms can vary and often appear at the onset of training, making early diagnosis and targeted treatment especially important.
Causes
The causes of PSSM mainly lie in genetic factors that affect the energy metabolism of the muscles. The key causes include:
- Genetic predisposition, particularly in draft breeds and Quarter Horses
- Disturbed storage of polysaccharides (glycogen) in muscle cells
- Abnormally increased accumulation of sugar and glycogen in the muscles
- Changes in insulin metabolism that affect muscle development
These genetically-based metabolic disorders lead to inefficient energy supply to the muscles, resulting in the typical symptoms of the disease.
Diagnosis
The diagnosis of PSSM is usually made through a combination of clinical examinations, genetic tests, and muscle biopsies. Initially, the veterinarian assesses the symptoms and movement patterns of the horse. If PSSM is suspected, a genetic test can be performed to identify genetic changes typical for this condition. A muscle biopsy, where a small tissue sample is taken and examined for abnormal polysaccharide storage, can confirm the diagnosis. These examinations reliably establish PSSM and help develop an individual treatment strategy.
Treatment
The treatment of PSSM aims to alleviate symptoms and optimally support muscle function. An important component is feeding: Horses with PSSM benefit from a tailored, low-carbohydrate, and fat-optimized diet that provides the necessary energy without overburdening the muscles. This often involves switching to high-quality hay, reducing grain amounts, and supplementing with oils or specific fat sources.
Regular, controlled exercise also plays a crucial role. A structured training plan without long rest periods helps stabilize muscle supply and reduce symptoms. In some cases, physiotherapy or targeted medications may also provide support. Individual therapy and care from veterinarians and experts help improve the horse's quality of life in the long term.
Post-Treatment Care
After diagnosis and treatment adjustment, horses with PSSM often need to maintain an adapted lifestyle to remain stable and symptom-free in the long run. This primarily includes a continuous exercise program without extended breaks, as regular movement helps prevent muscle tension and supports energy utilization in the muscles.
Feeding remains crucial: Horse owners should ensure to maintain low-carbohydrate and high-fat feeds and consistently implement feeding plans. Stress should be minimized, as it can exacerbate muscle issues. With this ongoing support and regular veterinary check-ups, many horses with PSSM can lead active and fulfilling lives.