Introduction:
Ring 20 [r (20)] is a rare chromosomal disorder that can present as a subacute epileptic encephalopathy in a previously healthy child. Recognition of the evolution of characteristic electroclinical features by reviewing serial video EEGs can help to narrow the broad differential diagnosis; guiding early genetic diagnosis and management.

Case report:
An 8-year-old girl regressed over a period of two months from a reported high-functioning baseline to intractable focal seizures with cognitive impairment and marked behavioural change. The seizures began with episodes of impaired awareness (prolonged altered consciousness, oral automatisms, urinary incontinence) which evolved within a month to a clinical picture of sleep related frontal lobe seizures presenting as night terrors with hallucinations. These evolved to non-convulsive status epilepticus (NCSE) with intractable focal motor seizures in the day and night. She became encephalopathic and exhibited challenging behaviour. Serial EEGs deteriorated over time; evolving to an epileptic encephalopathy with Electrical Status Epilepticus in Sleep (ESES). The awake EEG initially showed focal runs of left fronto-temporal sharp waves, which within a month progressed to prolonged runs of bilateral very high voltage, anteriorly dominant, 1-3Hz (slow) spike-and-wave discharges. However, the posterior alpha rhythm on the EEGs remained preserved. Ictal EEGs showed an onset with diffuse low voltage fast activity, lateralising to the left hemisphere. Striking peri-ictal behavioural disturbance was noted. Investigations including CSF, MRI and oligoclonal bands were negative. She was resistant to immunosuppressants and all anti-epileptic drugs, eventually gaining partial seizure control with a ketogenic diet.

Discussion:
This was a challenging case. The patient had an aggressive clinical course and there was uncertainty about the diagnosis. A link between epilepsy onset and neuropsychological impairment in three children presenting with abrupt onset epileptic encephalopathy with [r (20)] have been described by Vignoli et al(1) with similar EEG features. The key learning point from this case was that despite worsening of the clinical picture and corresponding EEGs evolving into an epileptic encephalopathy/NCSE, the posterior alpha rhythm was preserved. It was because of this that the possibility of a non-autoimmune genetic condition was raised. Genetic results were negative but were repeated, which confirmed the diagnosis of [r (20)] with karyotyping.

Conclusion:
Ring 20 chromosome syndrome is probably underdiagnosed; serial video EEGs can help in the recognition of the unique electroclinical features which strongly support the hypothesis of involvement of frontal lobes-basal ganglia networks in this unique syndrome (2).