The autopsy found the 92 year-old had died of dehydration and starvation.
The judge in charge of the case described the elderly woman’s death as “inexplicable”, “horrifying” and of “unimaginable suffering”, Spanish daily Heraldo de Aragón reported.
The man’s girlfriend discovered the 92-year-old’s body last December and alerted emergency services immediately.
The autopsy later revealed the old woman had been neglected for approximately a month prior to her death, national daily ABC reported.
The forensic report also suggested she was left lying in the same position for several weeks, as infected wounds and bed sores were found all over her body.
The grandmother used to live in Asturias, before the young couple took her to live with them in Zaragoza.
The pair had reportedly bought a special bed and wheelchair for the disabled woman.
Zaragoza’s public prosecutor has now decided to launch proceedings against the grandson Mario D.A. and his girlfriend Tamara C.A, both in their thirties.