jueves, 29 de abril de 2010

EL NERVIO OPTICO. DISTROFIA DEL NERVIO OPTICO




El nervio óptico es un nervio craneal y sensitivo, encargado de transmitir la información visual desde la retina hasta el cerebro.

Se origina en la capa de células ganglionares de la retina, siendo su origen aparente el ángulo anterior del quiasma óptico.


Recorrido y relaciones [editar]

Mide aproximadamente 4 cm de longitud, y se orienta en sentido anteroposterior.

Se describen en él cuatro segmentos:

* Primer segmento: intraocular. Los axones de las células ganglionares de la retina convergen en la papila óptica, desde allí el nervio perfora las capas superficiales del ojo (esclerótica y coroides) en un sitio llamado zona cribosa.
* Segundo segmento: intraorbitario. El nervio queda comprendido en un cono formado por los músculos rectos del ojo y se sumerge en la grasa retroocular. En el vértice de la órbita por el anillo fibroso que presta inserción a los músculos rectos (anillo de Zinn). En este segmento el nervio se relaciona por arriba con la arteria oftálmica y por fuera con el ganglio oftálmico.
* Tercer segmento: intracanalicular. El nervio atraviesa el foramen óptico acompañado por la arteria oftálmica. En este sitio, es frecuente la lesión del nervio por fracturas que comprometan la base del cráneo y el vértice de la órbita.
* Cuarto segmento: intracraneal. Mide 1 cm y está comprendido entre el foramen óptico y el quiasma óptico. En este segmento el nervio está situado sobre la tienda de la hipófisis y sobre el canal óptico del esfenoides.

Semiología [editar]

La exploración del II par craneal debe incluir maniobras que evaluen la agudeza visual, los campos visuales, y la visión de color.[1] La agudeza visual suele explorarse con la gráfica de Snellen, la cual tiene dos tiras de colores con la cual se explora la visión de color. El examen del fondo de ojo debe ser parte de la exploración del nervio óptico.

El reflejo fotomotor o pupilar se explora iluminando cada ojo y se observa la contracción de la pupila. En el reflejo consensual, la iluminación de un ojo causa contracción de la pupila del ojo contralateral.

Atrofia del nervio óptico

Es la necrosis del nervio que lleva la información relacionada con la vista desde el ojo hasta el cerebro.

Causas, incidencia y factores de riesgo

La atrofia óptica tiene muchas causas que no tienen relación entre sí y la más común de ellas es la insuficiencia en el flujo sanguíneo, denominada neuropatía óptica isquémica, la cual afecta con mayor frecuencia a los ancianos. El nervio óptico también puede resultar con daño a causa de un shock, diversas sustancias tóxicas, radiación y traumatismo.

Diversas enfermedades oculares, más comúnmente el glaucoma, también pueden causar atrofia del nervio óptico. Además, esta afección puede ser causada por enfermedades del cerebro y del sistema nervioso central, tales como arteritis craneal (algunas veces llamada arteritis temporal), esclerosis múltiple, tumor cerebral y accidente cerebrovascular.

Existen también varias formas hereditarias y poco comunes de atrofia del nervio óptico que afectan a niños y adultos jóvenes.

Síntomas

La atrofia del nervio óptico ocasiona disminución de la visión y reducción del campo visual, al igual que pérdida de la capacidad para ver detalles finos. La reacción de la pupila a la luz disminuirá y es posible que finalmente se pierda por completo.

Signos y exámenes

La atrofia del nervio óptico se puede detectar rápidamente con una evaluación completa de los ojos. La búsqueda de la causa puede requerir un examen físico completo y exámenes específicos.

Tratamiento

El daño por la atrofia del nervio óptico es irreversible una vez que haya ocurrido. En lo posible, se debe encontrar y tratar la enfermedad subyacente para prevenir una pérdida mayor.

Expectativas (pronóstico)

La visión que se pierde a causa de la atrofia del nervio óptico no se puede recuperar, pero si la causa se puede identificar y controlar, se puede prevenir la pérdida visual mayor que lleva a la ceguera.

Complicaciones

Las complicaciones estarán relacionadas con la enfermedad que causa la atrofia.

Situaciones que requieren asistencia médica

Los pacientes con atrofia del nervio óptico deben someterse a un control estricto por parte de un oftalmólogo con experiencia en neurooftalmología. Cualquier cambio en la visión debe informarse de manera inmediata.

Prevención

Muchas causas de la atrofia del nervio óptico no se pueden prevenir. Las lesiones faciales, la mayoría de las cuales están relacionadas con accidentes automovilísticos, se pueden prevenir con medidas de seguridad estándar como el uso del cinturón.

La toxina que más comúnmente ocasiona atrofia del nervio óptico es el metanol, el cual se puede encontrar en el alcohol de fabricación casera. Por esta razón, este tipo de alcohol, al igual que otras formas de alcohol que no son para beber, nunca se deben consumir.

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