Raised intracranial pressure (ICP) cause diplopia because of compression of the 6th cranial nerve. The pressure comes from above the nerve.
During episodes of raised ICP, the bony skull prevents upwards and sidewards expansion.
Downwards expansion is possible because of the presence of softer tissue. The expansion helps keep the pressure in the normal range until there is no room for further expansion.
The raised ICP pushes down on the 6th cranial nerve and squashes it against the petroclinoid ligament on which it rests in this part of the skull. This ligament has a sharp edge. There is also an element of stretching of the nerve from this excessive pressure.
This causes an interruption in the innervation of this nerve supplying the lateral rectus extra-ocular muscle and this ultimately leads to a sudden onset esotropia in the affected eye and diplopia.