Chez l’enfant, les troubles de la mémoire à long-terme sont très peu identifiés et décrits dans la littérature scientifique. Ainsi, doit-on comprendre que les dysmnésies développementales ont une prévalence très faible ou bien est-ce un sous-diagnostic ? À partir d’une revue de la littérature et de deux études de cas, nous postulons que la dysmnésie développementale est un trouble du neurodéveloppement sous-diagnostiqué en raison de deux facteurs : la présence d’autres troubles du neurodéveloppement ; et l’absence de difficultés scolaires massives. La recherche systématique de troubles mnésiques chez des enfants présentant un trouble des apprentissages scolaires devrait permettre de recenser, à l’avenir, plus de dysmnésies développementales.

Memory disorders in adults are common after cerebral stroke, brain tumor, or major neurocognitive disorder. But in childhood, these disorders are less described in scientific literature and are perhaps less frequent. Many studies have described long-term memory disorder after hypoxic–ischemic events (see articles from Vargha-Kadhem and colleagues). This disorder is named developmental amnesia because the origin of difficulties is identified. But in a few cases (4 cases described currently) causes are unknown despite the same memory disorders. This neurodevelopmental disorder could be named: developmental dysmnesia. So, the question is: is the prevalence of developmental dysmnesia in the general population really low or is this an under-diagnosis? Temple and Richardson (2006) stated prevalence at around 5–6%, but in fact neuropsychologists never diagnose developmental dysmnesia.

From literature review and description of two new cases which have have recently been identified (one with episodic dysmnesia and one with mixed dysmnesia), we postulate that developmental dysmnesia is an underdiagnosed neurodevelopmental disorder because of the presence of other developmental disorders such as ADHD, SLI… which become the tree that hides the forest; and because of low scholastic difficulties which are compensated for by more consequent personal effort.

Long-term memory can be perturbed in children without brain damage, which we have named developmental dysmnesia. We have demonstrated that many parameters generate few diagnostics of developmental dysmnesia which could be a neurodevelopmental disorder with the same prevalence as others. The diagnostic of memory disorder in children is difficult because complaints are not about memory, and/or scholastic difficulties are compensated. We have identified three types of developmental dysmnesia, based on the nature of the memory system that is disturbed: episodic dysmnesia, semantic dysmnesia and mixed dysmnesia (episodic and semantic memory disorders). Moreover, we could distinguish two sensorial modalities: verbal and/or visuo-spatial developmental dysmnesia. The systematic investigation of long-term memory problems in children with neurodevelopmental disorders (notably ADHD and dyslexia) must permit listing more cases of developmental dysmnesia in the future. We encourage all neuropsychologists and others professionals which working with children to assess long-term memory despite lack of complaints.