Still Undiagnosed

My left big toe went numb about 6 months ago.  The doctor has put me through so many tests ... neck and spine MRI, echocardiagram, pulmonary function test, sleep study...now they say I do not have sleep apnea but need oxygen because my oxygen level drops to low 80s when I sleep.  I'm now on oxygen when I sleep.  In the meantime, still no answer about my left big toe.  They still have scheduled a stress test.  They say my tests look good except for the oxygen.  Now I'm also having some dizziness and occassional numbing sensations in my left arm and hand.  Any idea what I should have checked next or what could be going on?  I'm exhausted all the time.  At one point the doctor mentioned multiple sclerosis but hasn't since.  Any thoughts are appreciated.

[COBOB]

Sound like you get hypoxic hypercapnia (too much carbon dioxide) while sleeping but without apnea.  Here is the problem.  Most humans do not breath do to a lack of oxygen, they breath because Carbon Dioxide gets too high.  This is a sympathetic response driven by the lower brain.  There is a backup system called Hypoxic drive, but this is prone to failure and the administration of O2 can actually suppress breathing in asthma and  COPD patients.  

Blood gases and chemistry also have an effect.  Metabolic alkalosis can cause a "base excess" that can cause lower CO2 levels and increase the pH of the blood to arrive at the lower brain, slowing hypercapnic drive.  

You really need to be seen by a pulmonary medicine specialist.  In Denver, there is a hospital that does a lot of research in this area "National Jewish Hospital".  It is rated the number 1 respiratory hospital in the US.

Follow the docs recommendations on the Oxygen.  I would bet that they have prescribed somewhere from 1-2.5 Liters per min.  More than that and you may be at risk to shut down the Hypoxic drive.

Good luck and let us know how things go.