What causes shooting pain in back, outside the left knee and left ankle?
I read your question carefully and I understand your concern.
From what I understand the shooting pain is on the left and spreads downwards to left lower limb up to the ankle, correct me if I am wrong.
Back pain in itself may at times radiate towards the lower limbs which is what I suppose your doctor thinks. However if it reaches to your ankle I would say that it is something more than local inflammation radiating, radiation of back pain doesn't usually go further than the knee.
So I would say that a nerve root compression (sciatica is a term used for compression of the sciatic nerve, but there are other compression sites as well) is well possible in your case. Causes can be several, most common are disc herniation or arthritic changes of the spine with bony spurs leading to compression of the nerve root, based on your ankylosing spondylitis diagnosis I would say that the latter is more likely. Local inflammation does also play a role as it increases the pressure and compression degree.
In terms of diagnosing that I am afraid MRI imaging of the lumbar spine is necessary, clinical exam does give some indication, but cannot fully assess the degree of compression.
It doesn't change much the initial therapy, initial management whatever the cause is still anti-inflammatory medication and physical therapy, in most cases improvement is seen over the next few weeks. Local corticosteroid and analgesic injections may also be tried at times. Only if the pain persists over the weeks is imaging to consider surgery done, or if there are symptoms such as bladder control issues, paralysis of the legs which may require emergency surgery, otherwise for the pain itself (as intense as I know it is) it is managed conservatively with pain killers at first.
I remain at your disposal for other questions.
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