An 8-month-old boy presents with clusters of brief, symmetric flexor spasms occurring predominantly on arousal. His parents report developmental arrest over the past 2 months. EEG shows the pattern marked **A** in the diagram. Which of the following is the MOST COMMON identifiable etiology of this condition in this patient?

Question

Accepted Answer

A. Tuberous sclerosis complex. ## Why Tuberous sclerosis complex is right

The pattern marked **A** — hypsarrhythmia (chaotic, high-amplitude multifocal spike-wave activity) — is the pathognomonic interictal EEG hallmark of West syndrome. The clinical presentation of epileptic spasms in clusters on arousal with developmental arrest in an 8-month-old is classic for West syndrome. Among the diverse symptomatic etiologies of West syndrome (which account for 60–90% of cases), tuberous sclerosis complex (TSC) is the single most identifiable cause, responsible for 10–25% of all West syndrome cases. The UKISS and Nelson Textbook of Pediatrics both emphasize TSC as the leading identifiable etiology, making it the most common specific diagnosis to screen for in a child presenting with hypsarrhythmia. Vigabatrin is the first-line agent specifically for TSC-related spasms because of its >90% response rate in this subgroup.

## Why each distractor is wrong

- **Hypoxic-ischaemic encephalopathy**: While HIE is a recognized symptomatic cause of West syndrome, it is not the single most common identifiable etiology. TSC accounts for a higher proportion of identifiable cases.
- **Pyridoxine-dependent epilepsy**: This is a rare inborn error of metabolism that can cause infantile spasms but is far less common than TSC. It would be a cryptogenic/unknown case initially and requires specific pyridoxine trial for diagnosis.
- **Centrotemporal focal cortical dysplasia**: Focal cortical dysplasias are recognized etiologies of West syndrome, but they are less frequently identified than TSC and do not account for 10–25% of cases.

**High-Yield:** Tuberous sclerosis complex is the single most identifiable cause of West syndrome (10–25% of cases); hypsarrhythmia on EEG is the diagnostic hallmark; vigabatrin is first-line for TSC-related spasms.

[cite: Nelson Textbook of Pediatrics 21e Ch 611; UKISS Lancet 2004;364:1773]

Answer

B. Pyridoxine-dependent epilepsy

Answer

C. Hypoxic-ischaemic encephalopathy

Answer

D. Centrotemporal focal cortical dysplasia

Explanation

Why Tuberous sclerosis complex is right

Why each distractor is wrong

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