ECT is shock treatment. It is used to treat psychological disorders. How much of it causes how much of retrograde amnesia, is there any quantification?
Permanent loss of memory seems only occur in the memory of the treatment and events leading up to the treatment.
The results indicated that ECT can initially disrupt recall of events that occurred many years previously, but recovery of these memories was virtually complete by seven months after treatment. It was also clear that persisting memory loss for information acquired only a few days before treatment can occur. For information acquired one to two years prior to treatment, recovery was substantial, but the results suggested that some memory problems might persist for events that occurred during this time period.
Squire LR, Slater PC, Miller PL. Retrograde amnesia and bilateral electroconvulsive therapy. Long-term follow-up. Arch Gen Psychiatry. 1981
This study comes to the same conclusion:
Our results are consistent with the possibility that ECT as currently practiced does not cause significant lasting retrograde amnesia, but that amnesia is mostly temporary and related to the period of impairment immediately following ECT.
Martijn Meeter, Jaap M.J. Murre, Steve M.J. Janssen, Tom Birkenhager, W.W. van den Broek, Retrograde amnesia after electroconvulsive therapy: A temporary effect?, Journal of Affective Disorders, Volume 132, Issues 1–2, 2011
The following extract from The Practice of Electroconvulsive Therapy: Recommendations for Treatment, Training and Privileging suggests that almost all patients in fact do experience retrograde amnesia, but the severity differs and can be assessed with a Mental State Exam which should be conducted prior:
Following ECT, patients also display retrograde amnesia. Deficits in the recall of both personal (autobiographical) and public information are usually evident, and the deficits are typically greatest for events that occurred temporally closest to the treatment
Below is a link to the full text.