Yet another possible reason for your heel pain is underlying systemic diseases - although rare, it's always still a possibility.
People suffering from heel pain may also be suffering from an underlying disease - whether they know it yet or not. Systemic arthritic conditions such as rheumatoid arthritis and psoriatic arthritis can cause heel pain. Other possible conditions causing heel pain include crystal deposition arthritides, Paget's Disease, Diffuse idiopathic skeletal hyperostosis, Reiter's disease, diabetes, and sickle cell anemia. These conditions are briefly defined below and there is also some information about what is actually happening in these diseases that can manifest as heel pain.
* Rheumatoid Arthritis: This is an autoimmune disease that causes chronic inflammation that damages the structures around joints. While typically this disease presents with pain in the small joints of the hands and feet, it can also cause heel pain. The pain in the heels is due to the inflammatory factors in the body, which can cause pain and swelling not only in the joints, but also bursae (fluid-filled sacs that are present in many parts of the body) or tendons (tendonitis). There are 2-3 main bursae in the area of the heel and the Achilles tendon also inserts onto the heel bone, predisposing this area in a few ways. Rheumatoid arthritis also causes damage and wearing down of the protective fat pads in the feet, which are located at the heel and the ball of the foot. More often pain is in the ball of the foot due to this, but it can occur in the heel too - simply because the fat pad is no longer able to protect the underlying heel bone.
* Psoriatic Arthritis: This is another autoimmune disease that causes pain and swelling within joints. It is similar to rheumatoid arthritis in some ways, but affects a different pattern of joints. It also commonly presents with changes in the appearance and condition of the skin and nails. The underlying reason that psoriatic arthritis can cause heel pain is nearly the same as the reason discussed above for rheumatoid arthritis. It stems from the inflammation of the surrounding structures of the heel bone.
* Crystal deposition arthritides: This type of arthritis includes gout and a condition referred to as CPPD (or calcium pyrophosphate deposition). Both of these arthritic conditions are caused by the body creating crystals within joints and tendons. CPPD and gout differ in the composition of the crystals that are deposited. Because these crystals can settle into joint capsules and surrounding tendons, the heel is predisposed to be affected because the Achilles tendon inserts here. Gout is more commonly seen overall than CPPD, and gout usually affects men while CPPD usually affects women.
* Paget's Disease: This is a chronic condition of the bone that causes osteoclast (bone-absorbing) activity to be more active than osteoblast (bone-making) activity. As the osteoblasts attempt to lay down more bone, the activity becomes more chaotic and the bone that is layed down is haphazard. This makes the bone more large and dense, but also then more brittle and weak (unable to recover from stresses as well as normal). Pain in the heel that is caused by Paget's is due to the excessive bone activity as well as (in worse cases) fractures in the brittle bone.
* Diffuse Idiopathic Skeletal Hyperostosis: Also know as DISH or Foreistier's disease, this is a condition in which the body forms bone in areas that ligaments attach to bones. This is demonstrated well on an x-ray and looks like bone spurs. The pain from this condition is due to the pressure of the spurs created and also from the stiffness that comes along with bone being created in areas that otherwise have flexibility and motion (Achilles tendon for instance).
* Reiter's Disease: This is also known as a "reactive" arthritis. It is typically caused by an underlying infection. Initially it was described during World War I, when a doctor was noticing that soldiers were developing joint pain after bouts of diarrhea (from an intestinal infection). Nowadays, the infections that cause Reiter's are typically genitourinary or sexually transmitted diseases. X-rays of people with heel pain due to Reiter's will actually reveal large bone spurs or multiple small bone spurs in the heel in patients that would otherwise seem too young to have developed these spurs.
* Sickle Cell Anemia: This is a disease that alters the shape of red blood cells. Due to this change in shape, the red blood cells are more likely to stick together and clump up. It is a genetic disease and can cause heel pain because of the blood cells clumping together and thus not effectively supplying the heel bone with oxygen. This can cause death to some of the cells of the heel bone and this death of cells (necrosis) is very painful. It is basically starving/strangling the heel bone.
* Diabetes: Diabetes is a disease that affects nearly every organ and part of the body at some point, if not well controlled. Heel pain among people with diabetes is often a reflection of poorly controlled blood sugars which over time damage the conductive lining around nerves. Nerves have a layer around them called myelin, which insulates the nerve to and potentiates nerve signals through the body. When myelin is damaged, the nerves are unable to transmit normal signals and thus produce painful symptoms such as burning, tingling or numbness.
While many of the above mentioned conditions tend to be the outliers in heel pain, or the "zebras" among the "horses," it is important to keep them in the back of your mind. This is especially important if treatments for plantar fasciitis are not as effective as expected. The diseases described here are diagnosed with help of blood work and full assessment by your primary care doctor.