In PIH an impending sign of eclampsia is:
As you know, severe preeclampsia progresses to eclampsia if not treated promptly,hence symptoms of severe preeclampsia can also be called as indicators of Impending eclampsia.
Signs of Impending or Imminent eclampsia/:

Headache (severe)

Symptoms of severe preeclampsia/

Blurring of vision/flashing lights

Symptoms associated with poor prognosis

Epigastric pain
Brisk deep tendon reflexes and ankle clonus
Diminished urinary output (<400 ml/24 hours).
Visual symptoms
Weight gain of 2 lb per week
Severe proteinuria of 10 g
Pedal edema