Down syndrome is the most common chromosomal abnormality identified at birth. These individuals will have multiple comorbid illnesses that require complex medical care throughout their lives. In recent years, specific patterns of regression have been detected in these individuals; most notably, developmental regression with language, behavior, and cognitive skills that they had previously acquired, which now affects both the quality of life and autonomy. These development regression patterns are referred to as Down syndrome disintegrative disorder.The case that we are reporting occurred in a 17-year-old female presenting with significant weight loss, altered mental status, and loss of functional skills over a period of one month. Co-occurring hypothyroidism and hyperthyroidism symptoms exacerbation may have triggered this patient's regression. Our case highlights the importance of conducting a thorough investigation for life-threatening and non-lifethreatening illnesses that can present with similar symptoms, in order to make the correct diagnosis of Down syndrome disintegrative disorder and find appropriate therapies and future care.

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The case that we report occurred in a previously healthy, fully immunized 13-year-old female presenting with severe unilateral headaches and generalized tonic-clonic seizures and subsequently diagnosed with herpes simplex virus type 1 (HSV-1) meningoencephalitis. The patient was successfully treated with acyclovir and seizures were controlled with valproic acid. The HSV meningoencephalitis has high morbidity and mortality. Our case highlights one of the severe presentations of HSV meningoencephalitis in young adolescents that can manifest with headaches, fever, seizures, focal neurologic signs, and altered mental status. We also highlight the need for a thorough workup among pediatric providers in the emergency and inpatient departments to avoid delays in diagnosis that can lead to poor outcomes.