Generally a clinical diagnosis
For traumatic etiology, obtain CT face to evaluate for fracture
Also obtain CT IAC if concern for posterior<unk> 
Examine external auditory canal, especially in posterior<unk> 
Evaluate the cranial nerves to rule out concomitant injury
Also obtain CT IAC if concern for posterior<unk>
Mandible dislocation
Acute tetanus
Mandible fracture
Akathisia
Pilon fracture
Conversion disorder
Odontoid fracture
Drug toxicity 