Spectroscopic imaging of frontal neuronal dysfunction in hyperekplexia

Hyperekplexia in two siblings

Hyperekplexia is a rare, hereditary, non-epileptic disorder characterized by an exaggerated startle reaction to unexpected auditory, somatosensory and visual stimuli. The authors describe a one-day-old term neonate, who presented with jitteriness and episodic tonic spasms, and his elder sister with hyperekplexia. Hyperekplexia though is a rare disorder is one of the differential diagnoses for refractory tonic spasms in infancy. The prognosis is generally good in hereditary hyperekplexia. Recent molecular studies have revealed many associated mutations in the glycine receptor alpha and beta subunit genes.

Startle syndromes

Startle syndromes consist of three heterogeneous groups of disorders with abnormal responses to startling events. The first is hyperekplexia, which can be split up into the "major" or "minor" form. The major form of hyperekplexia is characterised by excessive startle reflexes, startle-induced falls, and continuous stiffness in the neonatal period. This form has a genetic basis: mutations in the alpha1 subunit of the glycine receptor gene, GLRA1, or related genes. The minor form, which is restricted to excessive startle reflexes with no stiffness, has no known genetic cause or underlying pathophysiological substrate. The second group of startle syndromes are neuropsychiatric, in which excessive startling and various additional behavioural features occur. The third group are disorders in which startling stimuli can induce responses other than startle reflexes, such as startle-induced epilepsy. Diagnosis of startle syndromes depends on clinical history, electromyographic studies, and genetic screening. Further study of these disorders may enable improved discrimination between the different groups.

Magnetic resonance spectroscopy of cerebral cortex is normal in hereditary hyperekplexia due to mutations in the GLRA1 gene

Excessive startling and stiffness in hereditary hyperekplexia has been attributed to lack of inhibition at either the cortical or brainstem level. Six patients with hereditary hyperekplexia (HH) and a confirmed mutation in the gene encoding the alpha(1) subunit of the glycine receptor (GLRA1) underwent single voxel (1)H magnetic resonance spectroscopy (MRS) of the brainstem and an area of frontal cortex and white matter using a method that allows absolute quantification of metabolites. The results of MRS were within normal limits, although there was a tendency for the neuronal marker N-acetyl aspartate to be reduced in the brainstem of patients compared with that in controls. Thus, we found no evidence to support a deficit in the cerebral cortex in patients with hereditary hyperekplexia due to mutations in the GLRA1 gene.

Hyperekplexia: a treatable neurogenetic disease

Hyperekplexia is primarily an autosomal dominant disease characterized by exaggerated startle reflex and neonatal hypertonia. It can be associated with, if untreated, sudden infant death from apnea or aspiration pneumonia and serious injuries and loss of ambulation from frequent falls. Different mutations in the alpha1 subunit of inhibitory glycine receptor (GLRA1) gene have been identified in many affected families. The most common mutation is Arg271 reported in at least 12 independent families. These mutations uncouple the ligand binding and chloride channel function of inhibitory glycine receptor and result in increased excitability in pontomedullary reticular neurons and abnormal spinal reciprocal inhibition. Three mouse models from spontaneous mutations in GLRA1 and beta subunit of inhibitory glycine receptor (GLRB) genes and two transgenic mouse models are valuable for the study of the pathophysiology and the genotype-phenotype correlation of the disease. The disease caused by mutation in GLRB in mice supports the notion that human hyperekplexia with no detectable mutations in GLRA1 may harbor mutations in GLRB. Clonazepam, a gamma aminobutyric acid (GABA) receptor agonist, is highly effective and is the drug of choice. It enhances the GABA-gated chloride channel function and presumably compensates for the defective glycine-gated chloride channel in hyperekplexia. Recognition of the disease will lead to appropriate treatment and genetic counseling.

Major and minor form of hereditary hyperekplexia

Hyperekplexia is a hereditary neurological disorder characterized by excessive startle responses. Within the disorder two clinical forms can be distinguished. The major form is characterized by continuous generalized stiffness in the first year of life and an exaggerated startle reflex, accompanied by temporary generalized stiffness and falls, whereas in the minor form only excessive startle and hypnic jerks have been described. Mutations in the gene encoding the alpha-1 subunit of the glycine receptor (GLRA1) are responsible for the major form of hyperekplexia but no mutation was detected in patients with the minor form in the large Dutch pedigree originally described by Suhren and colleagues. Here we describe the genetic analysis of the GLRA1 gene of two English families in which both forms of hyperekplexia were present. Mutation analysis revealed no genetic defect in the GLRA1 gene in patients carrying either the minor or major forms. This is further evidence that the minor form of hyperekplexia is seldom due to a genetic defect in the GLRA1 gene.

Hyperekplexia in neonates

Hyperekplexia (startle disease) is a rare non-epileptic disorder characterised by an exaggerated persistent startle reaction to unexpected auditory, somatosensory and visual stimuli, generalised muscular rigidity, and nocturnal myoclonus. The genetic basis is a mutation usually of the arginine residue 271 leading to neuronal hyperexcitability by impairing glycinergic inhibition. Hyperekplexia is usually familial, most often autosomal dominant with complete penetrance and variable expression. It can present in fetal life as abnormal intrauterine movements, or later at any time from the neonatal period to adulthood. Early manifestations include abnormal responses to unexpected auditory, visual, and somatosensory stimuli such as sustained tonic spasm, exaggerated startle response, and fetal posture with clenched fists and anxious stare. The tonic spasms may mimic generalised tonic seizures, leading to apnoea and death. Consistent generalised flexor spasm in response to tapping of the nasal bridge (without habituation) is the clinical hallmark of hyperekplexia. Electroencephalography may show fast spikes initially during the tonic spasms, followed by slowing of background activity with eventual flattening corresponding to the phase of apnoea bradycardia and cyanosis. Electromyography shows a characteristic almost permanent muscular activity with periods of electrical quietness. Nerve conduction velocity is normal. No specific computed tomography findings have been reported yet. Clonazepam, a gamma aminobutyric acid (GABA) receptor agonist, is the treatment of choice for hypertonia and apnoeic episodes. It, however, may not influence the degree of stiffness significantly. A simple manoeuvre like forced flexion of the head and legs towards the trunk is known to be life saving when prolonged stiffness impedes respiration.