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| For the medical profession
Hyperekplexia
is a non-epileptic paroxysmal disorder classically characterised by
pronounced startle responses to tactile or acoustic stimuli with
delayed habituation, hypertonia and episodic neonatal apnoea (clinical
criteria shown below). The hypertonia may be predominantly truncal,
attenuated during sleep and less prominent after a year of age.
Although primarily seen as a neurogenetic syndrome with both familial
and sporadic cases, acquired hyperekplexia has also been recognised.
Inheritance is often misreported as solely autosomal dominant, but this
represents a bias of early reports (as dominant families often present
sooner, with greater numbers of affected members). In fact, autosomal
recessive inheritance is at least as prevalent and is common in certain
ethnic backgrounds.
Hyperekplexia appears to be a rare
disorder, but the true incidence is unknown. A lack of familiarity with
this condition leads to under-reporting and misdiagnosis. Adults with
less pronounced symptoms can receive a delayed diagnosis, sometimes via
family screening launched following a positive gene test in a neonate.
Despite the classical features being present from birth (and in some
reports exaggerated startle responses were felt in utero) the majority
of definite diagnoses are made during infancy. Once appreciated, the
clinical pattern appears to be relatively straightforward to recognise,
which may explain why hereditary hyperekplexia was described so
accurately prior to gene testing. Corroborating this, the majority of
requests for genetic testing come from a small number of tertiary
centres worldwide.
Proposed diagnostic criteria for hyperekplexia:
Required:
Startle:
Exaggerated
startle reflex to unexpected (particularly auditory) stimuli. The
startle response can be prolonged and be present before birth.
Consciousness is unaltered during startle episodes. Nose-tap-test is
positive (does not habituate).
Stiffness:
Generalized
stiffness immediately after birth, normalising during the first years
of life. The stiffness can be predominantly truncal or lower limb,
increases with handling and disappears during sleep. Short period of
generalised stiffness following the startle response during which
voluntary movements are impossible. This can result in falls in adults.
The combination of stiffness (often in lower limbs) and startle can
result in frequent falls. Mimics excluded Normal MR imaging, no
dysmorphism or congenital deficits noted. Normal EEG during startle
episode. Autonomic features of paroxysmal extreme pain disorder absent.
Supportive:
Inguinal,
umbilical, or epigastric hernia (secondary to high abdominal tone and
splinting) Anxiety and epileptic seizures are occasionally seen
Cyanotic apnoea attacks in infancy.
Rarely:
Some families have seen delayed motor milestones, speech delay or mild autistic features.
Note:
Antibodies
to the glycine receptor and have been discovered in a single case to
date, and were associated with progressive encephalomyeleitis and
hyperekplexia-like symptoms. The fact that the glycine receptor
antibodies were detected at the cell surface and that aggressive
immunosuppression alleviated symptoms provides an eloquent argument
that they are causative and demonstrate evidence of non-genetic
hyperekplexia.
We have contacts with Professor Angela Vincent's team at Oxford University who can perform these tests. 
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