Cognitive Decline in a Young Adult with Pre-Existent Developmental Delay – What the Adult Neurologist Needs to Know

INTRODUCTION Developmental delay is a common problem in children, and often no specific cause is found. It is also not unusual for other neurological problems, such as epilepsy, to complicate the picture. The question is always whether the intellectual problems are due to a static defect, or are part of a progressive neurodegenerative disorder. Many conditions declare themselves with the passage of years. However, a small but significant group of conditions are sufficiently indolent in their progress to cause no obvious deterioration until late adolescence or adulthood. In this situation, a parent or carer may ask the adult neurologist to assess whether the perceived decline in functioning is real, and if so, what the cause is. This situation is often far from clear-cut, and so I will try and provide some guidance for when this difficult diagnostic problem arises. THREE POSSIBLE CAUSES OF COGNITIVE DECLINE Pseudo-regression A patient may have