Thank you Rob so much for all these additional information! I see that you have been extensively investigated so far, however since I do not have a list of the actual tests that have been already conducted so far so what I will do is provide you with some information below and if you see any of this is not yet done or not addressed by your doctors then you may please ask them to address it now asap.
First of all have you had blood Vitamin B12, Folate, Vitamin D, Vitamin E etc checked?
Fish eating may cause an infection with parasite called as Diphyllobothrium Latum. This parasite can cause deficiency of vitamin B12, and chronic vitamin b 12 deficiency can cause the various types of gastrointestinal & neurological symptoms you are having. It can cause myelopathy (damage to the spinal cord) and can cause the symptoms like legs giving out etc, also the other symptoms you have mentioned. If you had no vit B12 test then you need it now asap. Deficiency of this vitamin can be treated easily and once vit b12 level becomes normal you should see improvement in your symptoms.
Other vitamin deficiencies that can cause similar symptoms include Folate, Vitamin D, Vitamin E deficiency etc and blood testing can easily diagnose it. If you have deficiency then treating with appropriate vitamin should help you.
The parasite diphyllobothrium latum itself can be treated with Praziquantel or Niclosamide, which helps in eradicating this parasite from your body.
There is another condition called as HAM (HTLV virus associated Myelopathy) which is injury to the spinal cord through a virus called as HTLV-1. Please see this is not HIV, this is a different virus. This can be diagnosed through a spinal tap and collecting CSF (Cerebro Spinal Fluid) which can show viral titer – if you have this disease. This condition is treated with one or more of the following – Steroids,
Zidovudine, Interferon etc
The CSF can also reveal if you have any other chronic infection or inflammatory conditions that may be causing your symptoms.
Certain kinds of neuropathies (peripheral nerve damage), for example CIDP (Chronic Inflammatory Demyelinating Polyneuropathy) too can cause your symptoms, so if not tested already with a nerve & muscle test called as EMG/Nerve conduction test then you need this test too. This condition can be treated with steroids or immunoglobulins, or plasmapheresis etc.
Whipple disease is another infection which is acquired through poor hygienic eating and it can affect the gastrointestinal tract as well as nervous system and present with chronic diarrhea, difficulty with walking, fatigue, numbness, blurring of vision etc. This disease is usually diagnosed with intestinal biopsy and staining and culture or PCR testing and this infection can be treated with antibiotics.
If not already done then you may also ask your Drs for an MRI of the brain & spinal cord too - which can show any lesions in the brain or spinal cord causing your symptoms.
If you see any of the above has not already been addressed by your doctors then please bring it to their notice so that they can arrange for tests to diagnosis and then treated appropriately.
Please let me know if you have any further questions; and if you feel you have received the answer to your question please rate me so that I can receive credit for the time and effort put by me in answering your question.