What physically happens when there is dehiscence of the lateral canthus (aka "fishmouthing syndrome")?

I have reviewed your photos from your prior posts.  You do not have to live with your results.  You eyelids are fixable.  Standard upper and lower blepharoplasty as performed by the majority of surgeons is a bad procedure.  Simply put, there is not enough customization of these surgeries for the individual patient.  Seventy percent of my surgical practice is devoted to the aesthetic repair of other surgeon's cosmetic eyelid surgeries.  When a transcutaneous lower blepharoplasty is performed, the surgery damages the motor nerves that supply the lower eyelid margin.  Removing "excess" skin and the scaring that occurs with the surgery stresses the lower eyelid and puts traction on the lateral canthal tendon.  Surgeons attempt to compensate for these changes by often performing a lateral cathopexy or a somewhat stronger procedure called a lateral canthoplasty.  These are often very unnatural due to the tension required to support the eyelid after this surgery.  The skin incision pulls on the weakened lid margin causing it to rotate down. I call this lid roll.  Overtime, the lower eyelid becomes horizontally short, there is rounding of the lateral canthus, The lower eyelid is effectively short of skin so these eyelids are also vertically short.  The solution is to recruit skin and soft tissue back into the eyelid.  My free ebook on eyelid surgery details how these eyelids can be restored.  The video also shows how this is done.  There is help for you.

Hello Bleph Leppard
Your problem is intriguing, transcutaneous  quadbleph was done by whom , meaning what type of surgeon? Oculoplast, dermatologist  ,plastic surgeon?? 
The lateral canthus is the confluence of 2 tendons one from the upper and one from the lower eyelid. It is attached to the inner part of the lateral orbital rim in line with your pupil. Its hard to imagine that being disrupted by a transcutaneous procedure. When I fix lateral canthal problems I am usually working on bone inside the orbit.
It is impossible to tell you what has happened without photos and a physical exam but I can tell you it's possible to improve if not fix entirely in the right hands.
Best wishes, don't despair
Rebecca Jackson M. D.