title: Discriminative Thresholds in Facial Asymmetry: A Review of the Literature.
abstract: Facial symmetry is intimately correlated with attractiveness. Perfect facial symmetry is disconcerting and a degree of facial asymmetry is considered normal. There is a lack of data on the limits of normality across facial subunits.
  
  
  
  This systematic review aims to establish categories of facial asymmetry perception for facial aesthetic units by establishing a discriminative threshold of "deformity perception" across facial subunits and a threshold for intervention (unacceptable asymmetry).
  
  
  
  A review of the literature was performed across Medline and Embase databases using OvidSP. All prospective studies evaluating the perception of progressive facial asymmetry in laymen or clinicians using a two- or three-dimensional model were included. Studies that did not evaluate rates of perception at varying degrees of asymmetry were excluded as these did not allow for the identification of a perceptive threshold.
  
  
  
  Each facial feature possesses a unique threshold of perception defined by an abrupt, statistically significant increase in detection. Asymmetry of the eyelid position at rest is the most sensitive facial feature (perceptive threshold, 2 mm) (P < 0.02). This is followed by deviations of the oral commissure (3 mm) (P < 0.001), brow position (3.5 mm) (P < 0.001), nasal tip deviation (4 mm) (P < 0.001), and chin deviation (6 mm) (P < 0.001). Desire for surgery for worsening deformities beyond the intervention threshold is characterized by an exponential, rather than linear, correlation.
  
  
  
  Categories of facial asymmetry perception establish a framework to counsel patients with facial asymmetries, and are a valuable adjunct to clinical judgment in the management of static and dynamic facial deformities.
authors.affiliation: Department of Plastic, Reconstructive, and Hand Surgery, Concord Hospital, Sydney, Australia.
authors.forename: Tim T
authors.initials: TT
authors.lastname: Wang
authors.affiliation: Department of Plastic, Reconstructive, and Hand Surgery, Concord Hospital, Sydney, Australia.
authors.forename: Louis
authors.initials: L
authors.lastname: Wessels
authors.affiliation: Department of Plastic, Reconstructive, and Hand Surgery, Concord Hospital, Sydney, Australia.
authors.forename: Gazi
authors.initials: G
authors.lastname: Hussain
authors.affiliation: Department of Plastic, Reconstructive, and Hand Surgery, Concord Hospital, Sydney, Australia.
authors.forename: Steve
authors.initials: S
authors.lastname: Merten
mesh_terms: D003357:Cosmetic Techniques / Q000592:standards*; D005145:Face / Q000033:anatomy & histology* / Q000601:surgery; D005146:Facial Asymmetry / Q000175:diagnosis* / Q000601:surgery; D005260:Female; D006801:Humans; D008297:Male; D017410:Practice Guidelines as Topic; D012684:Sensory Thresholds; D012737:Sex Factors; D014796:Visual Perception