The phenotypic spectrum of GLYT1 encephalopathy has not yet been completely described. However, common features of the disorder include:
• Neonatal onset.
• Very low muscle tone that progresses to stiffness of the legs and arms.
• Altered neurological state with absence of normal neonatal reflexes.
• Breathing problems requiring assisted mechanical ventilation.
• Dysmorphic traits, commonly a long and expressionless face, and a small and receding chin. Other described findings include: abnormal shape of the skull, broad forehead, strabismus, sparse and pronounced eyebrows, long eyelashes, low set ears, abnormal shape of the nose and mouth, increased hair on the body, and increased skin on the back of the neck.
• Muscle and joint abnormalities, including severe contractures (arthrogryposis multiplex congenita).
• Startles caused by sudden loud sounds or tactile stimulation.
• Severe developmental delay.
• During pregnancy, reported ultrasound findings include: increased amniotic fluid (polyhydramnios), abnormal accumulation of fluids in the fetus (hydrops or increased nuchal fold), arthrogryposis, and lack of fetal movements.
• Mildly elevated levels of glycine in cerebrospinal fluid, and normal or slightly elevated levels of glycine in serum.
• Abnormal brain magnetic resonance imaging is common, with a nonspecific pattern.
• Abnormal electroencephalogram has been described in two patients.
• Anomalies of the genitals and kidneys have been reported in two children.
Additional characteristics have been described in single patients, including epilepsy, high blood pressure, heart defects, inguinal hernia, alterations of the sensory and motor nerves, visual alterations or hearing loss.
Further studies reporting new patients will allow deciphering the full spectrum of clinical manifestations of GLYT1 encephalopathy.
SLC6A9