First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
The symptoms you describe are concerning for cervical myelopathy, though other possibilities do exist. The cervical spinal canal is normally 12-15mm in diameter, with age related wear and tear, arthritis, injury etc., this canal can become smaller (stenosis). At 10mm and less, the spinal canal is considered at risk for compression/myelopathy (squeezed spinal cord) and at 7-8mm surgery is often initiated. However, depending on the patients symptoms and imaging findings, a patient may have surgery with larger canals.
Cervical stenosis symptoms generally start with neck pain, arm pain/numbness and then progress to decreased grip strength and arm weakness. As the stenosis worsens it can cause imbalance, clumbsiness (triping etc) and leg weakness. In the worse stages it can cause incontinence of bowel/bladder and paralysis. From the symptoms you describe, I am concerned that you have severe, symptomatic cervical myelopathy (neck/arm pain, arm tingling/numbness, decreased grip strength and incontinence), unless your sympotms are caused by alternative explanations. As for the cause, you will need additional tests, but it is likely that you have age related disc buldges with a congentially small canal to start with. Although no blood test is specific for psoriatic arthritis(PA), I would reccomend that you have an ESR (erythrocyte sedimentation rate) ANA with full subtype analysis, RF (rheumatoid factor-is usually negative in PA). Your rheumatologist will be able to discuss further options reguarding this possible condition.
I would recommend an SSEP, somatosensory evoked potential to assess whether the wires connecting your leg and brain are being damaged by your cervical stenosis. I would also recommend an EMG to evaluate for other causes of your symptoms, for example-carpal tunnel syndrome, radiculopathy (nerve root compression) etc. Most importantly I would recommend that you see a neurosurgeon for consultation on possible decompressive laminectomy (relieving the pressure on your spinal cord) as soon as possible. He may require further imaging such as a CT Myelogram (dye study of the spinal canal) to further assess your need for surgery. I hope this has been helpful.