Hyperexplexia or stiff baby syndrome

The stiff baby syndrome or hyperexplexia is an unusual neurological disease, occurring via dominant autosomal transmission, and distinguished by a permanent hypertonia that is heightened by the slightest stimulus. The diagnosis can further be ascertained by the clinical and electromyographic aspects, as well as the occurrence of a family history. The evolution of the disease is benign with myorelaxing treatment. The recognition of this syndrome is of great importance in order to avoid an erroneous diagnosis of epilepsy, to warn the parents against the risk of recurrence in case of later pregnancies, and to indicate the necessity of constant supervision during the neonatal period as attacks of hvpertonia may lead to apnoeas that can endanger the child's life.

Hyperexplexia was described for the first time in 1962 by Kok and Bruyn in 29 members of one family.[1] The disease was characterised by the occurrence of a hypertonia from birth, which became less pronounced during the first year of life but could later lead to repeated falls. Those characteristics were later reported by Suhren et al on 25 members of a family.[2] In 1972, Klein described a family of 10 who presented with a syndrome distinguished by attacks of hypertonia heightened by the slightest physical stimulus and in whom an electromyogram showed a persistent activity even on resting; this activity vanished on treatment with diazepam.[3] In 1981, Lingam et al described the same symptoms and named the disease: stiff baby syndrome.[4] Another case was reported by Melki et al in 1988.[5] In all cases a muscular stiffness appearing from birth is reported, which gives the child a peculiar attitude described as 'fetal', and there is a flexion of the forearms and legs and closed fists. The expression of the child is also characteristic: the stare is fixed and gives the child an expression of anxiety. The hypertonia diminishes during sleep but increases with the slightest psychic or tactile stimulus. Attacks of hypertonia can involve respiratory muscles and lead to apnoeas, endangering the child's life. Digestive troubles, such as vomiting, are also reported. Vomiting is, in most cases, in association with a hiatal hernia. It is also noted that umbilical, inguinal, and diaphragmatic hernia are more frequent; they are attributed to the hypertonia.

Electroencephalography is normal but electromyography gives a characteristic result and shows an almost permanent muscular activity with, however, periods of electric quietness.[6] The nerve conduction velocity is normal. The evolution of the disease is characterised by a delay in reaching motor 'milestones' (for example, walking). The hypertonia is reduced during the first two years, but abnormal reactions such as involuntary starts remain after the slightest stimulus. There is neither mental nor neurological deficiency.[7]

It is important to distinguish the stiff baby syndrome from other neurological diseases manifest by an increase in muscle tone. First, this syndrome must not be confused with the stiff man syndrome described bv Gordon et al in 1967.[8] The stiff man syndrome is not hereditary and appears at the age of 40 to 60; evolution is slow and progressive. It is distinguished by a permanent contracture, an intense dysphagia, the electromyogram shows a permanent electric activity, even during rest, and it is magnified by the slightest tactile or nociceptive stimulus.[9] The Isaacs-Mertens syndrome, usually occuring around the age of 30, and for which neonatal forms have been reported, presents a mostly distal hypertonia, accompanied by fasciculations; the eclectromyogram shows a permanent activity on resting as well as numerous discharges. The syndrome described by Stevens in 1965 under the name 'Jumping Frenchman of Maine' is different, as violent starts, produced by the slightest stimulus, are associated with an echolalia and an echopraxia.[10] Gilles de la Tourette's syndrome chiefly presents spasmodic twitchings and startling reactions. Several other conditions can produce abnormalities in tonicity: among them are the encephalomyelites, Creutzfeldt-Jakob disease, metoclopramide or strychnine poisoning, and myoclonic epilepsy.

The physiopathological mechanism of hyperexplexia is controversial: Markand thinks that the hypertonia is linked to a hyperactiviry of cortical neurons,[11] but Suhren et al and Morley et al invoke an abnormality in the inhibitory system of the brain stem.[2] [12] Andermann et al think that the problem is with the serotoninergic system.[6] Myorelaxing treatments such as diazepam are efficient as they enable a lowering of the hypertonia. There can be a reappearance of the symptoms when the treatment is stopped to carry out electromyography.

The recognition of the stiff baby syndrome is of great significance in order to avoid an erroneous diagnosis of epilepsy and consequent treatment with anticonvulsants. Recognition is also important so that treatment with myorelaxants can be started to avoid the abnormal startle reactions that produce falls in older children. It is also important to warn the parents about the risk of reoccurence of the disease, and to indicate the necessity of constant supervision during the neonatal period, as attacks of hypertonia can lead to serious apnoeas endangering the child's life,[13] although the evolution of the disease is benign when treated.


Service der Neonatologie,
Pavillon de la Mere de l'Enfant,
CHR de Nantes,
44035 Nantes Cedex 01,

[1] Kok O, Bruyn G. An unidentified hereditary disease. Lancet 1962;i:1359.

[2] Suhren O, Bruyn W, Tuynman JA. Hyperexplexia: a hereditary sstartle syndrome. J Neurol Sci 1966;3:577-605.

[3] Klein R, Haddow J, Deluca C. Familial congenital disorder resembling stiff man syndrome. Am J Dis Child 1972;124:730-1.

[4] Lingam S, Wilson J, Hirt E. Hereditary stiff baby syndrome. Am J Dis Child 1981;135:909-11.

[5] Melki I, Rizkallah E, Akatcherian C. L'hyperexplexiam: la maladie du sursaut. Pediatrie 1988;43:35-7.

[6]Andermann F, Keene D, Andermann E, Quesny LF. Startle disease or hyperexplexia. Further delineation of the syndrome. Brain 1980;103: 985-97.

[7] Weaver D, Morley D. Garg B, Markand O. Hyperexplexia: no hereditary stiff baby syndrome. Am J Dis Child 1982;136:562.

[8] Gordon E, Janusko D, Kauffman LA. Critical survey of stiff-man syndrome. Am J Med 1967;42:582-99.

[9] Chaunu MP, Rivrain Y, Thenint JP, et al. Le syndromne de l'homme raide ou 'stiff-man' syndrome. Journal de Medecine de Caen 1982;17:23-9.

[10] Stevens H. Jumping Frenchmen of Maine. Arch Neurol 1965;12:311-4.

[11] Markand O. Familial startle disease(hyperexplexia). Electrophysiological studeis. Arch Neurol 1984;41:71-4.

[12] Morley D, Weaver D, Garg B, Markand O Hyperexplexia: an inherited disorder of the startle response. Clin Genet 1982;21:388-96.

[13] Vigevano F, Diapva M, Bernardina B. Startle disease: an unavoidable cause of sudden infant death. Lancet 1989;i:216.