Table 4.

Main causes for abnormal facial expressions

Cause	Definition and example
Gene-related factors	Gene-related factors are causes for individuals’ abnormal facial changes that root in the presence or mutation of one or a set of genes.  Examples: Down syndrome (genetic root: presence of a third copy of chromosome 21) or Cornelia de Lange syndrome (genetic root: NIPBL or SMC1A, SMC3, RAD21 or HDAC8, BRD4 and ANKRD11 genes).5 41 42 55
Neurological factors	Neurological factors are defined as reasons that are associated with individuals’ congenital or acquired disorders of nerves and the nervous system. Neurological factors can either be related to genetic or non-genetic factors, caused by irregularity in nerves associated with the brain or the face.  Examples: Neurological factors with genetic causes (eg, Rett syndrome, MECP2 gene; Cervical or Cranial dystonia, GNAL gene) and without (eg, embouchure dystonia, Oromandibular dystonia)46 47; due to nerves associated with the brain (eg, stroke) or the face (Bell’s palsy or facial paralysis, Hemifacial Spasm).43–45
Psychiatric conditions	Psychiatric conditions, especially psychotic disorders, have the potential to cause abnormal facial expressions among individuals. Psychiatric conditions could be broadly defined as mental illnesses, whereas psychotic disorder factors are causes to abnormal facial expressions that root in individuals’ impaired sense of reality.  Examples: Non-drug-related Tourette syndrome (facial tics) or autism (facial expression limitation).48–50
Medication-induced triggers	Medication-induced triggers could be understood as causes to individuals’ short-term or long-term abnormal facial changes due to their adverse reactions to a certain medication of a type of medications.  Examples: Neuroleptic malignant syndrome (antipsychotic drugs), tardive dyskinesia (antipsychotic medications) or drug-related Tourette syndrome.51–54