2d - How can I implement a directional melee attack?

I am attempting to make a top down game similar to the SNES Zelda games, in Unity 4.3, in 2D. When my character swings their sword, every enemy within a radius takes damage. I want to make sure that melee attacks only damage enemies the player is facing.

How can I do this?

---

The relevant code in PlayerAttack.cs looks like this:

// Update is called once per frame
void Update () {
    // have the player initiate an attack on the object
    if(Input.GetKeyUp(KeyCode.F)){
        MeleeAttack();
    }
}


private void MeleeAttack(){
    // create a variable called distance to make sure the distance is close enough
    // to inflict damage from a melee attack target.transform.positon = the position
    // of the object to attack and transform.position is the position of the players
    // transform
    float distance = Vector3.Distance(target.transform.position, transform.position);

    // make sure the distance is not greater than 1.5 so the player cannot
    // melee an enemy from to far away
    if(distance < 1.5f){

            // get a reference to the enemy's health script and type cast it.
            EnemyHealth eh = (EnemyHealth)target.GetComponent("EnemyHealth");
            // now call the function that will decrease the enemy's health
            eh.AdjustCurrentHealth(-10);
    }
}