We normally experience symptoms of hyponatremia when sodium levels fall below 120 mEq/L.  However, an exception to that is during states of increased intracranial pressure (as in a head injury). 

The Monro-Kellie hypothesis states that the cranial compartment is incompressible, and the volume inside the cranium is a fixed volume. The cranium and its constituents (blood, CSF, and brain tissue) create a state of volume equilibrium, such that any increase in volume of one of the cranial constituents must be compensated by a decrease in volume of another.^[5]

So, if somebody gets hit in the head with a bat and start bleeding (thus having an increased ICP) and then on top of that has hyponatremia...which causes the brain to swell (brain cell uptake water)... the ICP will rise even more... so the patient will experience symptoms of hyponatremia earlier than somebody without increased ICP to begin with.

Increased ICP is a clinical feature of hyponatremia (so somebody without any increase in ICP will eventually have an increased ICP given that the Na goes down enough to cause the brain to swell).  Increased ICP and cerebral edema are the cause of the neurological features in hyponatremia:  headache, delirium, brisk DTRs, weakness, seizures, coma.  Increased ICP will also cause hypertension.