I think the doctor might have used the English abbreviation for Dissociative Disorder. It could also be "Dépersonnalisation et Dérealisation", one of the dissociative disorders.

In any case, I'd keep DD.

differential diagnosis

another possibility

https://www.sciencedirect.com/science/article/abs/pii/S00109...
“Developmental Dysmnesia”: A Case Report
Abstract
We report a 9-year-old girl who presented from an early age a severe and specific verbal memory disorder, in the absence of a definite etiology. The results of an extensive neuropsychological assessment showed a dissociation between normal visuo-spatial memory abilities and a marked short-term and long-term verbal memory impairment. We argue that our case is affected by “developmental dysmnesia”, a condition so far described in two cases only (De Renzi and Lucchelli, 1990, and Temple, 1997). The disorder affected episodic as well as semantic verbal memory. The long term effects on learning and cognition of a memory deficit occurring early in childhood are discussed.
Dysmnésies Développementales
Est-ce que peut-être c'est là qu'il se trouvent les initiales DD?
https://www.cairn.info/revue-de-neuropsychologie-2017-3-page...
C’est ainsi le cas pour les dysmnésies développementales, c’est-à-dire les troubles de la mémoire, notamment la mémoire épisodique et/ou sémantique. Le terme de dysmnésie n’apparaît pas non plus dans le thésaurus Medline, certainement car cette pathologie est sous-diagnostiquée. De ce fait, aucune définition précise de cette pathologie n’est proposée. Si on transpose la « trame diagnostique » habituelle, la dysmnésie pourrait se définir comme un trouble persistant de la mémoire à long terme explicite ne s’expliquant ni par une déficience intellectuelle, ni par un trouble sensoriel ou un trouble cognitif autre, et ce en l’absence de carence éducative et sociale. Ce trouble apparaît au cours du développement et n’a pas de cause organique identifiée (contrairement à l’amnésie développementale, pour laquelle un évènement dans la vie du patient peut être la cause des troubles de mémoire : ischémie, AVC, traumatisme crânien, épilepsie…).

--------------------------------------------------
Note added at 1 ώρα (2023-04-18 13:15:31 GMT)
--------------------------------------------------

***https://pubmed.ncbi.nlm.nih.gov/2245300/
Developmental dysmnesia in a poor reader
E De Renzi 1, F Lucchelli
Affiliations expand
PMID: 2245300 DOI: 10.1093/brain/113.5.1337
Abstract
A 22-yr-old subject with moderate developmental dyslexia experienced poor memory from an early age. In spite of normal intelligence he had great difficulty in learning historical and geographical names, mathematical formulae, poems and songs, and verbal series such as the letters of the alphabet and the months of the year. His recognition of familiar faces was also faulty at times. A battery of memory tests confirmed a moderate deficit of verbal as well as visual memory, whereas spatial memory was preserved. Other cognitive functions were intact, except for a mild reading disability. No evidence of brain pathology was provided by his history, neurological examination and CT scanning.
https://academic.oup.com/acn/article/14/8/688/3887
Developmental dysmnesia in an LD clinic cohort
W.R. Levick
Archives of Clinical Neuropsychology, Volume 14, Issue 8, November 1999, Page 688, https://doi.org/10.1093/arclin/14.8.688
Published: 01 November 1999


--------------------------------------------------
Note added at 1 ώρα (2023-04-18 13:24:11 GMT)
--------------------------------------------------

https://www.sciencedirect.com/science/article/abs/pii/S02229...
(parce que le lien précédent ne marchait pas)