Psoriasis is a common inflammatory dermatosis, associated with an increased risk for heart attack and stroke, a relationship that may be related to a chronic inflammatory state.
Psoriatic arthritis is a progressive, erosive, chronic, heterogeneous, and systemic inflammatory disease that develops in up to 30% of patients with psoriasis. 

Causes of Psoriatic Arthritis:

Although the exact causative mechanism is not well known. Psoriasis is a T cell-mediated inflammatory disease. Both genetic (HLA types and other susceptibility loci) and environmental factors contribute to its occurrence and risk. Population of T cells namely CD4+ TH17 and TH1 cells and CD8+ T cells accumulate in the outermost layer of skin called epidermis. These cells secrete cytokines and growth factors that induce keratinocyte hyperproliferation, resulting in the characteristic lesions. Psoriatic lesions can be induced in susceptible individuals by local trauma. This process is known is Koebner phenomenon. The major complication of psoriasis is arthritic psoriasis and is related to somewhat similar causes to psoriasis. The familial relation psoriatic arthritis is indicative of a genetic basis. The major histocompatibility complex is a known susceptibility locus for psoriatic arthritis. Nearly 25% of patients with psoriatic arthritis are positive for human leukocyte antigen (HLA)-B27. Specific HLA alleles are associated with different manifestations, including symmetric sacroiliitis, asymmetric sacroiliitis, enthesis, dactylitis. The major pathogenic involvement in the cause and progression of psoriatic arthritis is cytokines in the interleukin (IL)-23-Th17-IL-17 and tumor necrosis factor (TNF) pathway (e.g., IL-12, IL-17, IL-23, and TNF-α), as well as cytokines such as IL-22.

Signs, Symptoms and clinical features about Psoriatic Arthritis:

Peripheral and axial joints, entheses, skin, and nails are usually affected with lesions and crusts formation. Severe pain in joints with skin manifestation, Scalp lesions, intergluteal and perianal lesions are common in psoriatic arthritis. Nail lesions are major finding in up to 80% of cases. Inflamed, swollen, and painful joints, often in the fingers and toes. Deformed joints from chronic inflammation. These are all the manifestation of psoriatic arthritis. If disease progresses, Severe inflammation that damages the joints in the hands and feet, resulting in deformation and movement problems. Bone loss (osteolysis) at the joints may lead to shortening (telescoping) of the fingers and toes. Neck and back pain may also occur. 
The typical lesion is a well-demarcated, pink to salmon–colored plaque covered by loosely adherent silver-white scale There is marked epidermal thickening (acanthosis), with regular downward elongation of the ridges. The pattern of this downward growth has been likened to “test tubes in a rack.” Increased epidermal cell turnover and lack of maturation is associated with disease. Also seen is thinning of the epidermal cell layer overlying the tips of dermal papillae (supra papillary plates), and dilated and tortuous blood vessels within the papillae. These vessels bleed readily when the scale is removed, giving rise to multiple punctate bleeding points (Auspitz sign). Neutrophils form small aggregates within both the spongiotic superficial epidermis and the parakeratotic stratum corneum. 
Many cases with psoriatic arthritis are seen with tiny dents, called pitting, and ridges in their nails. Pain in muscles and joints are common in psoriatic arthritis.
Treatment for Psoriatic Arthritis:
 

Treatment for psoriatic arthritis aims at following:

• Relieve symptoms
• Slow the condition's progression
• Improve quality of life

These goals can be achieved by the following strategies and medications:

1. Non-steroidal anti-inflammatory drugs (NSAIDs)
2. Corticosteroids
3. Disease-modifying anti-rheumatic drugs (DMARDs)
4. Enzyme Inhibitor
5. Immunosuppressants
6. Biological therapies
7. Light Therapy
8. Surgery

1. Non-steroidal anti-inflammatory drugs or NSAIDs: 
NSAIDs will limit the inflammation occurring in joints and peripheries. The main NSAIDs used are Aspirin, Ibuprofen and Naproxen.

2. Corticosteroids:
Corticosteroids work alongside NSAIDs by reducing pain and swelling. Those who cannot take NSAIDs take corticosteroids. The main corticosteroid used is Prednisone. They do have side effects so are used with caution.

3. Disease-modifying anti-rheumatic drugs or DMARDs:
If NSAIDs and corticosteroids fail to work. DMARDs are used. These can slow or stop pain, swelling, and joint and tissue damage. They're stronger than NSAIDs, and they may take longer to work. The main DMARDs used are Leflunomide, Methotrexate and Sulfasalazine.

4. Enzyme Inhibitor:
Enzyme inhibitors works by blocking a specific enzyme, a kind of protein, called PDE-4. That helps to slow other reactions that lead to inflammation. The main Enzyme inhibitor used is Apremilast.

5. Immunosuppressants:
Immunosuppressants are necessary evil in psoriatic arthritis. They suppress the immune system so that our own immune system doesn’t destroy out organs and bones. The main Immunosuppressants used are Azathioprine and Cyclosporine.

6. Biological therapies:
Biological treatments work by stopping particular chemicals in the blood activating the immune system to attack the lining of the joints. The main biological agents used are Adalimumab, Abatacept, Golimumab, Certolizumab.

7. Light therapy:
Narrowband UVB phototherapy, Excimer laser and PUVA are used in light therapy. This treatment is expensive and may require several visits to the laser center.

8. Surgery: 
Synovectomy removes the diseased lining of a joint. If the damage is very severe, Joint replacement is done by a procedure called arthroplasty. When a joint can't easily be replaced, joint fusion might make it stronger, more stable, and less painful.

If you wanna read more about psoriasis, read this.
Written by Amanda Samuelsson