While you may know that 1 in 3 people with psoriasis develop psoriatic arthritis (PsA), did you know that PsA is under an umbrella of diseases known as spondyloarthritis (SpA)? The unique features of SpA include inflammation and pain in the spine and pelvic joints and the involvement of the entheses, or where ligaments and tendons join to bones. 
SpA is typically split into two categories, axial spondyloarthritis or peripheral spondyloarthritis, although it is possible to experience symptoms of both.  The primary symptoms of axial SpA are inflammation and stiffness in the spine and hips, particularly in the sacroiliac joints, which link the spine and the pelvis.
Ankylosing spondylitis (AS) is the most common form of axial SpA and is characterized by changes in the bones that are visible by X-ray. In the most severe cases, new bone can form, which fuses parts of the spine together in a permanent rigid position. While other joints can be involved in AS, it is more typical of peripheral SpA that other joints are involved. 
There are several diseases that fall into the peripheral SpA category including PsA, enteropathic arthritis (EnA), and reactive arthritis. Both EnA and reactive arthritis involve the intestine, but EnA is chronic and presents with inflammatory bowel disease, while reactive arthritis is thought to be caused by an infection of the intestine and is typically resolved within a year. 
In PsA, while inflammation may be present in the spine and pelvis, common locations of enthesitis, or inflammation of the entheses, are the bottoms of the feet, the Achilles' tendons, and above the knee. A common symptom unique to PsA is known as dactylitis, where an entire finger or toe swells and appears as a “sausage digit.”  Also associated with PsA is nail psoriasis, which includes pitting of the nails or lifting of the nails from the nail bed. Most people with PsA experience symptoms of psoriasis before the development of PsA, but it is not necessary for diagnosis. 
Treatment for these diseases depends on your diagnosis and severity, but can include nonsteroidal anti-inflammatory drugs which can help manage the pain and stiffness of arthritis, as well as or in addition to biologics and disease-modifying anti-rheumatic drugs.  Diet and exercise have also been shown to improve SpA symptoms. Talk with your rheumatologist about your symptoms and your treatment goals.