Infantile Epileptic Spasms Syndrome (IESS)

Infantile epileptic spasms syndrome (IESS) is a developmental and epileptic encephalopathy in which epileptic spasms begin in infancy, usually in clusters. The modern term includes both classic West syndrome and infants who have epileptic spasms but do not meet every element of the older West syndrome triad.

IESS is a neurological emergency in the sense that diagnosis and effective treatment should not be delayed. Shorter treatment lag is associated with better developmental outcome.

Why the name changed

West syndrome classically described three features: epileptic spasms, hypsarrhythmia on EEG, and developmental stagnation or regression.

The problem is that some infants have true epileptic spasms without classic hypsarrhythmia, or before developmental slowing is obvious. The ILAE therefore uses IESS to include the broader clinical syndrome and retains West syndrome as a more specific historical/electroclinical pattern.

What an epileptic spasm looks like

An epileptic spasm is a brief contraction of axial and limb muscles, usually lasting less than a few seconds. Spasms may be flexor, extensor or mixed.

They often occur in clusters, commonly around waking, and can become more prominent as the cluster continues. Some are dramatic, while others consist only of subtle head nods, eye movements, chin movements or brief body crunches.

Why spasms are often missed

Subtle spasms can be mistaken for reflux, startle, colic, normal infant movements or behavioural events.

Home video can be extremely valuable when the movement is intermittent and not seen during a clinic appointment.

Typical age at onset

The ILAE diagnostic framework places IESS onset between 1 and 24 months.

Onset around 1–2 months is considered an alert feature because very early presentations can overlap with early infantile developmental and epileptic encephalopathies.

The EEG

Hypsarrhythmia is a classic EEG pattern involving high-amplitude, chaotic, multifocal epileptiform activity, but it is not present in every infant with IESS.

Other infants may show multifocal or focal epileptiform discharges. If the interictal EEG is non-diagnostic, recording an actual suspected spasm can be necessary.

MRI

MRI is not required to prove that the spasms are epileptic, but it is strongly recommended to identify an underlying structural cause.

Causes can include cortical malformations, perinatal brain injury, tuberous sclerosis complex and other developmental abnormalities.

Genetic and metabolic causes

IESS can result from many genetic disorders, including genes involved in synaptic function, ion channels and brain development.

Metabolic causes are less common but important because some are specifically treatable. Genetic testing is increasingly central when MRI does not explain the syndrome or when the clinical picture suggests a genetic condition.

Tuberous sclerosis complex

Tuberous sclerosis complex is a particularly important cause because vigabatrin has a well-established role in treatment of spasms associated with TSC.

Infants with TSC may also be followed closely before spasms begin because EEG abnormalities can precede clinical seizures.

First-line treatment

Current international practice recognises hormonal therapy with ACTH or high-dose oral corticosteroids and vigabatrin as the principal established first-line treatments.

The preferred treatment depends on cause, local availability, infant-specific risks and regional practice. Vigabatrin is especially important in TSC-associated spasms.

Why treatment needs rapid reassessment

Clinicians do not simply start treatment and wait months to see what happens. Early reassessment is important to confirm that both clinical spasms and the pathological EEG pattern have resolved.

Persistent spasms or persistent severely abnormal EEG activity usually trigger rapid treatment adjustment.

Combination treatment

Evidence has supported combined hormonal therapy and vigabatrin in some settings, particularly for achieving early spasm cessation.

However, treatment protocols differ internationally and depend on cause, adverse-effect profile and access to ACTH, prednisolone or vigabatrin.

Other treatment options

If first-line therapy fails, options may include another hormonal approach, vigabatrin if not already used, ketogenic dietary therapy, other antiseizure medicines and epilepsy surgery in selected structural cases.

There is no single second-line treatment that is correct for every infant.

Epilepsy surgery

If spasms arise from a focal or hemispheric structural abnormality, surgery can be highly important even when the EEG appears widespread.

Early referral to a specialist epilepsy-surgery programme is appropriate when imaging and electroclinical findings suggest a surgically remediable cause.

Developmental impact

IESS can affect development through both the underlying brain disorder and the epileptic activity itself.

Developmental outcome is generally more favourable when development was normal before spasms, no severe underlying cause is identified, and effective syndrome-specific treatment begins quickly.

What remission means

Stopping spasms does not guarantee that epilepsy has ended permanently. Some children later develop other seizure types or another epilepsy syndrome.

Long-term neurological and developmental follow-up remains important.

Worldwide diagnosis

The ILAE recognises that EEG may be unavailable in some regions. If an experienced clinician directly witnesses typical clustered spasms or reviews a convincing video and the remaining criteria fit, a diagnosis may still be made so treatment is not unnecessarily delayed.

This flexibility is important because IESS outcomes are time-sensitive and access to paediatric EEG is highly unequal worldwide.

The central point

IESS is not simply a type of baby movement. Epileptic spasms are a time-sensitive epilepsy syndrome requiring rapid diagnosis, investigation of the underlying cause and prompt effective treatment.

Sources

Medical note: suspected infantile spasms require urgent medical assessment. Treatment choice depends on cause and should be directed by a paediatric neurology team.

Evidence checked: September 2026.