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The estimated birth prevalence is between 1/1,650-20,000. Both sexes are affected, with a higher incidence in males.

Epileptic spasms are the typical type of seizures observed. They consist of brief tonic contractions of axial muscles, each typically lasting less than 3 seconds, which may be flexor, extensor or mixed. These usually occur in series or clusters, with increasing prominence of the motor features through the cluster, often over a period of minutes (though clusters may last 30 minutes or longer) and are often seen on awakening. Spasms may be symmetric or asymmetric and some might be subtle, with minor head nods, eye or chin movements. Prior to onset of spasms, development can be normal or abnormal depending on etiology. Developmental delay, arrest or regression is typically seen with the onset of spasms. Isolated regression in visual attention or altered social responsiveness may occur in the days or weeks preceding onset of spasms. Developmental plateauing and regression typically worsen without rapid, effective treatment. Infantile spasms syndrome may have different etiologies and can be a feature of other syndromes such as Down syndrome, tuberous sclerosis complex, inverted duplicated chromosome 15 syndrome, and rarely metabolic diseases.

Pathogenic variants in genes and chromosomal abnormalities have been associated with infantile spasms syndrome; amongst syndromes frequently featuring infantile spasms, they include STXBP1, TSC1, TSC2 and trisomy 21. Other common genetic etiologies include ARX, CDKL5, SPTAN1. Structural abnormalities include acquired antenatal and perinatal lesions and malformations of cortical development.

Diagnosis is based on the presence of epileptic spasms, which may be witnessed in person or observed in home video and should be confirmed by electroencephalography (EEG) /video-EEG with electromyography (EMG). Interictal EEG shows either hypsarrhythmia that is observed in the awake and/or sleep recording, or focal and multifocal epileptic anomalies with a less disorganized background. The ictal recording is characterized by a fast activity that might precede a high amplitude, generalized sharp or slow wave followed or super-imposed by low amplitude. EMG helps to distinguish epileptic spasms from myoclonic seizures and tonic seizures. Neuroimaging is useful to determine the syndrome etiology: MRI is abnormal in half to two-thirds of children and can show either acquired or congenital focal, multifocal or diffuse lesions. Metabolic investigations exclude metabolic diseases. Genetic studies may include chromosomal microarray, gene panels or exomes and should be considered in all patients mainly those without a known acquired structural etiology.

Brief paroxysmal events which occur in clusters, both epileptic and non-epileptic, should be differentiated from epileptic spasms. These include myoclonic epilepsy in infancy, hyperekplexia, shuddering, infantile self-stimulation and gastro-esophageal reflux (Sandifer syndrome).

Counseling is important for subsequent pregnancies when a genetic etiology is identified. The pattern of inheritance depends on the etiology.

First line pharmacological treatment is vigabatrin or a combination of corticosteroids and vigabatrin. Treatment should be introduced as early as possible when the diagnosis is established. Early referral to a tertiary epilepsy center should be considered in order to identify candidates suitable for surgery.

Infantile Spasms syndrome may evolve to other epilepsy types or syndromes, including Lennox-Gastaut syndrome, or drug-resistant focal epilepsies. The majority of infants have a poor developmental outcome, regardless of seizure outcome. Severity of developmental delay relates predominantly to etiology and time to treatment from spasm onset. In a small subgroup, patients can have a full recovery with freedom from spasms and no cognitive impact.