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Hyperekplexia (Startle Disease): Symptoms, Genetics & Treatment

Hyperekplexia (formerly "hyperplexia") is a rare genetic disorder characterized by exaggerated startle responses and neonatal hypertonia.

Clinical Features

Classic Triad

  1. Exaggerated startle: Sudden, brief, generalized stiffening and flexion/extension in response to unexpected tactile, auditory, or visual stimuli.
  2. Generalized hypertonia: Present from birth, worse when awake, improves with sleep. "Stiff baby" syndrome.
  3. Nocturnal myoclonus: Jerks during non-REM sleep.

Additional Features

  • Apnea episodes triggered by startle (risk of sudden death in infancy).
  • Developmental delay (mild, in some).
  • Cataracts, umbilical/inguinal hernia, hip dysplasia (in some genetic subtypes).
  • Adult-onset: persistent startle, falls, anxiety, social impairment.

Genetics

GeneProteinInheritanceFrequency
GLRA1Glycine receptor α1 subunitAD (mostly), AR~80% of cases
GLRBGlycine receptor β subunitARRare
SLC6A5Glycine transporter GlyT2AR~10–15%
GPHNGephyrin (scaffolding)ARVery rare
ARHGEF9CollybistinX-linkedRare

All impair glycinergic inhibition in spinal cord/brainstem → disinhibited motor neurons → exaggerated startle.

Diagnosis

  • Clinical: Excessive startle + hypertonia from birth.
  • Nose tap test: Sustained stiffening without habituation.
  • Genetic testing: Confirmatory; guides recurrence risk.
  • EEG/EMG: Normal (differentiates from epilepsy).

Treatment

Pharmacologic

  • Clonazepam (first-line): Enhances GABAA inhibition; reduces startle and hypertonia. Start low (0.01–0.03 mg/kg/day).
  • Alternatives: Valproate, levetiracetam, gabapentin (less evidence).

Non-Pharmacologic

  • Vigevano maneuver: Forceful head flexion → activates vestibulospinal inhibition → aborts stiffening spell.
  • Physical therapy: Gait training, fall prevention.
  • Genetic counseling.
  • Avoid startle triggers (loud alarms, sudden touch).

Prognosis

Hypertonia improves in childhood; startle persists lifelong. Normal lifespan with treatment. Early diagnosis prevents infant apnea deaths.

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