RESUMEN La Hemihidranencefalia es una enfermedad congenita cerebral grave y relativamente rara caracterizada por la ausencia unilateral completa o casi total de la corteza cerebral dentro de un craneo normal con preservacion de estructuras como las meninges, ganglios basales, cerebelo, protuberancia, bulbo raquideo y la hoz cerebral. Los hemisferios cerebrales inicialmente toman la forma normal y luego se plantea que mas tarde se da una destruccion cortical por un grave proceso encefaloclastico, en particular por una lesion focal hipoxico-isquemica, lo cual lleva a que el lugar que ocupaba el tejido destruido sea sustituido en gran parte por espacios llenos de liquido cefalorraquideo que se asemeja a un gran quiste intracraneal. Se presentan dos casos clinicos, el primero con una hemihidranancefalia derecha con un pronostico desfavorable desde el momento del diagnostico, no obstante actualmente ha alcanzado un desarrollo cognitivo que rebasa las expectativas para su condicion; El segundo caso es un paciente con hidranencefalia global, este no alcanzo un desarrollo cognitivo aceptable, pero contradijo el pronostico y tiempo de vida acorde con su grave malformacion. Palabras clave: Hemihidranencefalia, Hidranencefalia, plasticidad neuronal, malformaciones congenitas del sistema nervioso. CONGENITAL BRAIN MALFORMATIONS: UNEXPECTED PROGNOSIS ABSTRACT Hemi-hydranencephaly is a serious and extremely rare congenital brain disease characterized by complete or almost complete unilateral absence of the cerebral cortex within a relatively normal skull with preservation of structures such as the meninges, basal ganglia, cerebellum, Pons, medulla oblongata and the falx. In this condition, cerebral hemispheres are formed initially, but later destruction occurs through a serious encephaloclastic process, in particular by a focal hypoxic – ischemic lesion, which leads to the place occupied by the destroyed tissue is replaced to a large extent by spaces fi lled with cerebrospinal fl uid, which resembles a large intracranial cyst. Two clinical cases are presented, the fi rst with a right hemi hydranancephaly with an unfavorable prognosis since his diagnosis nevertheless has currently reached a cognitive development that goes beyond expectations for his condition; The second case is a patient with a global hydranencephaly, this did not reach an acceptable cognitive development, but contradicted the prognosis and life according to his severe malformation. Key words: Hemihydranencephaly. Hydranencephaly. Congenital malformations nervous system, neuronal plasticity. Diego Fernando Alegria Portilla2. RESUMEN La Hemihidranencefalia es una enfermedad congenita cerebral grave y relativamente rara caracterizada por la ausencia unilateral completa o casi total de la corteza cerebral dentro de un craneo normal con preservacion de estructuras como las meninges, ganglios basales, cerebelo, protuberancia, bulbo raquideo y la hoz cerebral. Los hemisferios cerebrales inicialmente toman la forma normal y luego se plantea que mas tarde se da una destruccion cortical por un grave proceso encefaloclastico, en particular por una lesion focal hipoxico-isquemica, lo cual lleva a que el lugar que ocupaba el tejido destruido sea sustituido en gran parte por espacios llenos de liquido cefalorraquideo que se asemeja a un gran quiste intracraneal. Se presentan dos casos clinicos, el primero con una hemihidranancefalia derecha con un pronostico desfavorable desde el momento del diagnostico, no obstante actualmente ha alcanzado un desarrollo cognitivo que rebasa las expectativas para su condicion; El segundo caso es un paciente con hidranencefalia global, este no alcanzo un desarrollo cognitivo aceptable, pero contradijo el pronostico y tiempo de vida acorde con su grave malformacion. Palabras clave: Hemihidranencefalia, Hidranencefalia, plasticidad neuronal, malformaciones congenitas del sistema nervioso. CONGENITAL BRAIN MALFORMATIONS: UNEXPECTED PROGNOSIS ABSTRACT Hemi-hydranencephaly is a serious and extremely rare congenital brain disease characterized by complete or almost complete unilateral absence of the cerebral cortex within a relatively normal skull with preservation of structures such as the meninges, basal ganglia, cerebellum, Pons, medulla oblongata and the falx. In this condition, cerebral hemispheres are formed initially, but later destruction occurs through a serious encephaloclastic process, in particular by a focal hypoxic – ischemic lesion, which leads to the place occupied by the destroyed tissue is replaced to a large extent by spaces fi lled with cerebrospinal fl uid, which resembles a large intracranial cyst. Two clinical cases are presented, the fi rst with a right hemi hydranancephaly with an unfavorable prognosis since his diagnosis nevertheless has currently reached a cognitive development that goes beyond expectations for his condition; The second case is a patient with a global hydranencephaly, this did not reach an acceptable cognitive development, but contradicted the prognosis and life according to his severe malformation. Key words: Hemihydranencephaly. Hydranencephaly. Congenital malformations nervous system, neuronal plasticity.