Left hypoglossal nerve injury and paralysis of the left side of the tongue.
A month before this picture was taken, this patient's left hypoglossal nerve was injured during surgery on his carotid artery (carotid endarterectomy). This was immediately followed by deviation of the tongue to the same side as the injury.
On tongue protrusion, the muscle on the weak side cannot balance the forward push of the muscle on the intact side, and as a result, the tongue deviates to the weak side.
Unilateral damage to the nerve supply leads to wasting, weakness and fasciculation of that side of the tongue. The wasting and fasciculation are best observed with the tongue lying in the floor of the mouth. This is a picture of a recent injury in which wasting has not set in yet.