The infraspinatus muscle sits on the shoulder blade in the front leg. This muscle acts to turn the leg outward. Fibrotic myopathy of the infraspinatus is when scar tissue forms in the muscle causing tightness. This often develops secondary to muscle strain from high-intensity exercise It has also been attributed to acute trauma to the infraspinatus muscle and tendon that results in hemorrhage, swelling, fibrosis, and eventual tendon contracture.
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Infraspinatus myopathy is often seen as an acute lameness. Dogs with this condition walk with a very distinct gait due to the tightness of the muscle. It is not entirely clear what causes fibrotic myopathy, but it may be due to trauma, repetitive low grade injury to the muscle, or genetic factors.
During a physical exam, flexibility testing and palpation is helpful in determining if there is fibrosis of the infraspinatus muscle, while comparing it to your pet’s contralateral side. Gait changes are also very distinctive and can help us diagnose this. With your dog’s gait, the elbow is held adducted against the chest while the antebrachium is externally rotated and abducted, with the paw supinated. Shoulder range of motion is restricted, especially in extension, because of tendon contracture. During movement as the limb advances, the carpus/paw is rapidly flipped forward before being placed on the ground. If the diagnosis is not obvious, walking the dog up a flight of stairs exaggerates the gait. Some disuse atrophy of the supraspinatus and deltoid muscles may also be noted. A musculoskeletal ultrasound can confirm a diagnosis.
Physical therapy for fibrotic myopathy focuses heavily on trying to stretch the contracted tissues. We use ultrasound, heat, and massage along with stretching to try to loosen the tissues. We also address any pain and swelling that may be associated with the fibrosis. We then slowly add in strength exercises. We also address any compensatory issues that may arise from the abnormal gait. How much improvement we see may depend on the cause and severity of the fibrosis.
If caught in the early stages of the disease platelet rich plasma injections may be helpful to stop the progression. Surgical treatment involves cutting the tendon of the infraspinatus muscle where it connects to the humerus. This releases the tension of the scarred muscle, allowing the shoulder to move through pain free full range of motion. The body then relies on other muscles to support and flex the shoulder joint. Most dogs can return to working activity after recovery from this procedure. Depending on the extent of injury some patients may do well with physical therapy alone and not need injections or surgery. More severe cases may require injections or surgery in order to improve.
