Final answer:
Mannitol's therapeutic action in reducing intracranial pressure is signified by decreased cerebral edema, as it draws excess water out of the brain through its function as an osmotic diuretic.
Step-by-step explanation:
The therapeutic action of mannitol when administered to a client with increased intracranial pressure following a head injury is indicated by decreased cerebral edema. Mannitol, an osmotic diuretic, promotes water loss through osmosis, drawing water out of the brain tissue and thereby reducing swelling.
Unlike antidiuretic hormone (ADH) which reduces urine volume and maintains blood pressure, mannitol facilitates the elimination of excess water, indirectly helping to lower increased intracranial pressure.