Psoriatic Arthritis (PsA) is a chronic inflammatory disease affecting the joints and connective tissues, often occurring in individuals with psoriasis. This autoimmune process targets the musculoskeletal system, and the feet are a common site of involvement. Up to 70% of people with PsA experience foot or ankle abnormalities during the course of their disease.
Inflammation can manifest in diverse ways, affecting the toes, skin, nails, and the areas where tendons attach to bone. Since these symptoms are easily mistaken for more common foot ailments, knowing the specific visual signs of PsA is important. The unique patterns of inflammation in the feet provide significant clues that help confirm the diagnosis.
Swollen Toes and Joint Inflammation (Dactylitis)
One characteristic visual sign of PsA in the feet is dactylitis, often described as a “sausage digit.” This condition involves the uniform, painful swelling of an entire toe, not just the joint capsule. The inflammation affects all soft tissues within the digit, including joints, tendons, and ligaments.
The affected toe appears diffusely swollen, warm, and tender, sometimes limiting movement due to enlargement. This is a distinguishing feature because other forms of arthritis, such as rheumatoid arthritis, typically cause swelling localized only to the individual joints. Dactylitis frequently presents asymmetrically, meaning it may affect one toe on one foot but not the corresponding toe on the other.
This inflammation can involve the metatarsophalangeal joints (at the base of the toes) and the interphalangeal joints (between the toe bones). Repeated episodes of dactylitis can lead to permanent changes over time, including joint damage or the shortening and deformity of the toes. Dactylitis is specific to spondyloarthropathies like PsA and can sometimes be the first or only symptom before other joint issues appear.
Skin Plaques and Nail Abnormalities
Psoriatic Arthritis is closely linked to the skin condition psoriasis, which can manifest in distinct ways on the feet and toenails. When psoriatic plaques appear on the soles of the feet, it is often referred to as palmoplantar psoriasis. These plaques typically present as thick, raised patches of discolored skin covered with silvery-white scales.
Unlike plaques on other areas of the body, those on the weight-bearing plantar surface can be intensely painful and prone to developing deep, open cracks or fissures. The constant pressure and friction from walking can exacerbate the scaling and thickness. In individuals with darker skin tones, the patches may appear more purple, gray, or dark brown rather than the classic red.
Nail abnormalities, or psoriatic onychodystrophy, are common in individuals with PsA. These changes are caused by inflammation in the nail matrix and nail bed, and they can sometimes be confused with a fungal infection.
The toenails may display several visual signs:
- Pitting, which are tiny indentations that make the nail surface look like a thimble.
- The “oil drop” or “salmon patch,” appearing as a yellowish-red or brown discoloration visible beneath the nail plate.
- Subungual hyperkeratosis, where the nail becomes significantly thickened due to a buildup of chalky debris under the nail.
- Onycholysis, where the nail begins to separate from the underlying nail bed, starting at the tip and progressing inward.
Tendon and Ligament Pain (Enthesitis)
Psoriatic Arthritis also causes a specific type of inflammation called enthesitis, which affects the entheses—the points where tendons or ligaments attach directly to the bone. This is considered a hallmark feature of the disease and is particularly prominent in the lower extremities. Enthesitis causes tenderness, soreness, and swelling at these insertion sites, which is distinct from swelling within a joint.
In the foot and ankle, two locations are most frequently involved: the Achilles tendon and the plantar fascia. Achilles enthesitis causes pain, tenderness, and visible swelling at the back of the heel where the tendon connects to the heel bone. The area may look puffy and be warm to the touch.
Inflammation of the plantar fascia insertion, the ligament running along the bottom of the foot, causes heel pain that can mimic common plantar fasciitis. However, PsA-related enthesitis pain is often associated with persistent localized swelling and tenderness. Over time, chronic enthesitis can lead to the formation of bone spurs, which are abnormal bone growths that develop at the insertion point.