Startle syndrome: causes, symptoms, diagnosis

Startle syndrome unites a large group of diseases characterized by an increased startle reaction (startle - flinching) to unexpected external stimuli.

The startle reaction ("generalized motor activation reaction") is a universal component of the orienting reflex for mammals. Its latent period is less than 100 ms, and its duration is less than 1000 ms. The physiological startle reaction is characterized by a habituation reaction. As a benign phenomenon, the startle reaction occurs in 5-10% of the population.

The enhanced startle reaction is a stereotypical response (startle) to light, sound and other unexpected stimuli. The predominant element of this startle is a generalized flexion reaction of the head, trunk and limbs (although sometimes an extension reaction is observed). Like the physiological startle reaction in healthy people, it is mediated mainly by the reticular formation of the brainstem (as well as the amygdala and hippocampus), has an extremely wide receptive field and is caused by increased excitability of spinal motor neurons. The startle reaction is modulated by cortical mechanisms. The state of anxiety enhances the startle reaction. The pathological (enhanced) startle reaction differs from the physiological one in its severity.

An increased startle reaction can also be a consequence of various diseases affecting the nervous system. In this regard, it can be primary and secondary.

The main forms and causes of startle syndrome:

I. Physiological startle reaction of healthy people (shuddering in response to light, sound and other unexpected stimuli).

II. Enhanced (pathological) startle reaction:

A. Primary forms:

1. Hyperekplexia.
2. Culturally conditioned syndromes such as miryachit, lata, "jumping Frenchman from Maine" and others.

B. Secondary forms:

1. Non-progressive encephalopathies.
2. Startle epilepsy.
3. High damage to the spinal cord and brainstem (brainstem reticular reflex myoclonus).
4. Arnold-Chiari malformation.
5. Occlusion of the posterior thalamic artery.
6. Creutzfeldt-Jakob disease.
7. Myoclonic epilepsies.
8. Rigid person syndrome.
9. Tourette's syndrome.
10. Hyperthyroidism.
11. Hyperactive behavior.
12. Mental retardation.
13. Iatrogenic forms (drug-induced).
14. Psychogenic diseases.

A. Primary forms of startle syndrome

Primary forms include benign enhanced startle reaction, hyperekplexia (startle diseases), startle epilepsy and some so-called culture-mediated disorders (the pathophysiology of the latter remains poorly understood and their place in the classification may change).

Hyperekplexia is a sporadic (with a later onset) or (more often) hereditary disease with an autosomal dominant type of inheritance, characterized by onset in early childhood, congenital muscular hypertension ("stiff-baby"), which gradually regresses with age, and the presence of pathological startle reactions. The latter are the dominant clinical symptom. In the same families, there are expanded and less pronounced forms of startle reactions, which, unlike muscular rigidity, persist throughout life and often cause the patient to fall (sometimes with repeated fractures). A demonstrative startle reaction is a shudder when tapping the tip of the nose, to which no addiction is formed. In this case, unlike startle epilepsy, consciousness is not impaired. Patients with hyperekplexia are characterized by increased nocturnal myoclonus. It has been suggested that hyperekplexia represents a reticular stimulus-sensitive (reflex) myoclonus. Good response to clonazepam is often observed.

Culture-related syndromes, which can be both familial and sporadic, include such as “lata”, “mirachit”, “jumping Frenchman from Maine”, “imu”, “mali-mali”, “yaun”, “hiccup” and others (there are more than 10 of them), which have been described in different countries of the world, starting from the 15th century.

The two most studied forms are "lata" and "jumping Frenchman of Maine syndrome". They occur in both familial and sporadic forms. The main manifestations are pronounced startle reactions in response to unexpected sensory (usually auditory) stimuli, which are combined with such phenomena (not necessarily all of them) as echolalia, echopraxia, coprolalia and automatic execution of orders or movements that imitate the behavior of others. These syndromes are currently rare.

B. Secondary forms of startle syndrome

Secondary forms are found in a large number of neurological and mental diseases. They include non-progressive encephalopathies (post-traumatic, post-hypoxic, perinatal anoxia), degenerative diseases, high spinal cord injuries, Arnold-Chiari syndrome, posterior thalamic artery occlusion, brain abscess, Chiari malformation, Creutzfeldt-Jakob disease, myoclonic epilepsies, rigid person syndrome, sarcoidosis, viral infections, multiple sclerosis, Tourette syndrome, hyperthyroidism and "hyperadrenergic states", Tay-Sachs disease, some phacomotoses, paraneoplastic brainstem lesions, hyperactive behavior, mental retardation, and some other conditions. Enhanced startle reactions are also found in the picture of psychogenic neurotic diseases, especially in the presence of anxiety disorders.

A special variant of secondary startle syndrome is "startle epilepsy", which does not denote a nosological unit and unites several phenomena in epilepsies of different origin. This includes epileptic seizures that are provoked by unexpected sensory stimuli ("stimulus-sensitive epilepsy"), causing a startle. Such epileptic seizures have been described in various forms of cerebral palsy, as well as in patients with Down syndrome, Sturge-Weber disease, and Lennox-Gastaut syndrome. Startle-induced epileptic seizures can be either partial or generalized and are observed in lesions of the frontal or parietal region. Clonazepam and carbamazepine have a good effect (especially in children).