Miguel experienced a sudden, inexplicable blindness. tests have shown no physical basis for his symptoms. the most likely diagnosis is

The blindness is considered sudden if the loss of vision develops within a few minutes to a couple of days. It may affect one or both eyes and all or part of a field of vision. Sudden loss of vision can occur for a number of reasons.
The most common causes of sudden loss of vision are physical: damaged retina, Blockage of a major artery of the retina,Eye injury... 
But, tests have shown no physical basis for his symptoms. So, the diagnosis can some serious medical conditions such as a stroke or brain tumour.

How how am i supposed to know