Diphtheria Complications Breathing problems. Diphtheria-causing bacteria may produce a poison (toxin). This toxin damages tissue in the immediate area of infection — the nose and throat, for example. This localized infection produces a tough, gray-colored membrane — which is composed of dead cells, bacteria and other substances — on the inside of your nose and throat. This tough membrane, or covering, is dangerous because it can obstruct breathing.
Heart damage. The diphtheria toxin may spread through your bloodstream and damage other tissues in your body, such as your heart muscle. One complication of diphtheria is inflammation of the heart muscle (myocarditis). Signs and symptoms of myocarditis include fever, vague chest pain, joint pain and an abnormally fast heart rate. Damage to the heart from myocarditis may be only slight, showing up as minor abnormalities on an electrocardiogram, or very severe, leading to congestive heart failure and sudden death.
Kidney damage. The diphtheria toxin may damage the kidneys, affecting their ability to filter wastes from the blood.
Nerve damage. The toxin can also cause nerve damage, targeting certain nerves such as those to the throat, making swallowing difficult. Nerves to the arms and legs may also become inflamed, causing muscle weakness. In severe cases, nerves that help control the muscles used in breathing may be damaged, leading to paralysis of these muscles and trouble breathing.
With treatment, most people with diphtheria survive these complications, but recovery is often slow. Diphtheria is fatal in approximately one in 10 cases.
Diphtheria: Complications - MayoClinic.com