Peroneal Tendonitis is an rare issue with the tendons on the lateral side of the rearfoot. The problem typically occurs in runners where the stresses on these tendons are therefore much higher. There are two peroneal muscles on the outside of the lower limb whose tendons go round the outside of the ankle joint with one tendon attaching on the lateral side of the foot at the base of the fifth metatarsal. The other tendon passes under the foot to attach to an spot near the middle of the arch of the foot. The muscles have many different actions, however a primary one is to prevent the rearfoot rolling outwards and ending up having a ankle sprain. As they work hard at that task, the stress on the tendons could be too much for the tendon to tolerate and they end up with a peroneal tendonitis.
Usually the problem starts off with discomfort either above or just below the lateral ankle bone with or without some inflammation. In some the swelling develops later. With ongoing exercise the symptoms gets more persistent and gradually worse. A typical finding in those with peroneal tendinopathy is a lower supination resistance. This means it's easy for the rearfoot to supinate or roll laterally. This leads to the peroneal muscles to be very active, so if you then combine it with higher level of sporting activity, then the tendon is at high risk for an overuse injury.
Dealing with peroneal tendonitis generally starts with minimizing the strain by lessening physical activity levels and also the use of shoe wedging or foot inserts to pronate or tip the foot inwards so the muscle does not have to work as hard. Ice and anti-inflammatory drugs will also help reduce the pain and inflammation. Over the medium to long term increasing load by the way of exercise needs to be placed on the tendon so that it can get accustomed to the loads placed on it. In some cases, surgery is indicated.