Cristalino

[dropcap class="dropcap"]El cristalino es la lente transparente, incolora, biconvexa y avascular, que encontramos en el ojo , justo detrás de la pupila e iris. Está compuesto de fibras provenientes de células epiteliales. De hecho, el citoplasma de estas células conforma la substancia transparente de esta lente tan especial[/dropcap]

El cristalino está formado por 4 capas, de superficie al interior:

[list class="circle"]
  • cápsula o cristaloides
  • epitelio subcapsular
  • corteza
  • núcleo
  • [/list]

    La cápsula del cristalino o cristaloides tiene estructura de membrana, es muy elástica, está conectada al músculo ciliar por la zónula de Zinn. Por ello el cristalino tiende a una formo globular, redondeada, la forma que debe adoptar para la visión lejana.

    La zónula de Zinn o ligamentos suspensores o zónula, sujetan la cristaloides a los procesos ciliares. Estos ligamentos mantienen el cristalino en su posición.

    El cristalino está formado por células alargadas (fibras), compuestas principalmente por unas proteínas llamadas cristalinas. Estas fibras se continúan produciendo durante toda la vida humana, por diferenciación de las células originadas en la región germinal del epitelio, cerca del ecuador. Como consecuencia de ello, el espesor de la lente crece con la edad del sujeto: en la corteza anterior y posterior, las nuevas capas de fibras se superponen a las viejas formando estructuras concéntricas estratificadas, de modo similar a lo que sucede en una cebolla. Las fibras del interior van perdiendo los orgánulos intracelulares, en lo que parece ser un proceso de apoptosis. Este hecho ayuda a reducir la absorción y a mejorar la transparencia del medio, a la que también puede contribuir la regularidad de las fibras (transversalmente, siguen una configuración hexagonal). Además, como consecuencia de este crecimiento también se produce un endurecimiento del cristalino.

    El cristalino presenta unas líneas de sutura que parten de los polos y se extienden radialmente. Estas líneas se corresponden con las regiones en las que coinciden fibras con direcciones de alargamiento contrarias. En el feto, en la cara anterior hay tres líneas dispuestas en ángulos de 120 grados, en forma de "Y", mientras que en la posterior configuran otra "Y" invertida. Con la edad, como se van añadiendo nuevas fibras, la estructura se complica.

    Acomodación

    acomodacionEl cuerpo ciliar es circular, el músculo ciliar dentro de el, es un músculo esfínter. El diámetro interior del músculo se hace menor cuando este se contráe y mayor cuando se relaja.

    Cuando el ojo está viendo un objeto lejano (los rayos luminosos entran paralelos al ojo), el músculo ciliar se relaja. Los procesos ciliares tiran de los ligamentos suspensores, que a su vez tiran de la cristaloides, lo que causa que el cristalino adopte una forma mas plana, alejando el foco del mismo y por lo tanto enfocar objetos lejanos.

     

    Cuando necesitamos ver un objeto a corta distancia, se crea una demanda de acomodación, que lo que consigue es que el músculo ciliar se contráe, tensando la zónula, lo cual consigue hacer el cristalino mas convexo (y por lo tanto mas potente), lo que le permite enfocar objetos situados a cortas distancias. (View another Accommodation of the Crystalline Lens graphic.)

    El ajuste de forma que se realiza sobre el cristalino, para adapatarlo a la visión a diferentes distancias es conocido como acomodación y está asociado con cambios en el tamaño de la pupila. El diagrama superior muestra el cambio producido en el cuerpo ciliar, cristalino y pupila.

    La amplitud de acomodación del ojo es la cantidad que el cristalino puede acomodar indicado en dioptrías (D). Esta cantidad varía con la edad de las personas, disminuyendo con la edad.

    La amplitud de acomodación es el equivalente a la inversa de la distancia a la que el ojo emétrope puede enfocar con claridad.

     

    Rangos de Acomodación por edad
    Edad Amplitud de acomodación Punto cercano de acomodación (ojo emétrope)
    5 16.00 dioptrías 6.3 cm
    10 14.00 dioptrías 7.1 cm
    15 12.00 dioptrías 8.3 cm
    20 10.00 dioptrías 10.0 cm
    25 8.50 dioptrías 11.8 cm
    30 7.00 dioptrías 14.3 cm
    35 5.50 dioptrías 18.2 cm
    40 4.50 dioptrías 22.2 cm
    45 3.50 dioptrías 28.6 cm
    50 2.50 dioptrías 40.0 cm
    55 1.75 dioptrías 57.0 cm
    60 1.00 dioptría  100.0 cm
    65 0.50 dioptría  200.0 cm
    70 0.25 dioptría  400.0 cm
    75 0.12 dioptría  infinito

     

    Astenopía acomodativa

    Normalmente el proceso de acomodación es suave y no necesita de un gran esfuerzo. Cuando cambiamos el foco de lejos a cerca, el músculo ciliar se contrae con rapidez, provocando la acomodación. Volver a enfocar un objeto lejano, hace que el músculo ciliar se relaje.

    A veces, este proceso puede provocar un stress, tras largos períodos de trabajo en distancia corta. A veces, observamos que personas con buena AV en distancia larga, ven como su AV disminuye gradualmente en esa distancia. A menudo este problema se manifiesta en uno de los ojos, aunque suele sobrevenir en el parejo con el tiempo. Este problema se conoce como astenopía acomodativa, que puede manifestarse como una miopía y/o astigmatismo.

    Es un problema muy comñun en estudiantes adolescentes, aunque puede ocurrir en cualquier edad en personas con trabajos a distancia corta exhaustivos. Se recomienda establecer períodos de descanso para evitar este mal.

    Además de parpadeo contínuo, dolores de cabeza, etc uno de los síntomas mas comunes es ver los objetos borrosos despues de levantar la vista tras un trabajo intensivo en cerca. En estadíos iniciales, los objetos lejanos borrosos se enfocan poco a poco, tras un breve lapso de tiempo (el músculo ciliar se relaja gradualmente).

    No obstante, una astenopía acomodativa contínua, puede provocar que el músculo ciliar pierda la capacidad de relajarse al completo. Lo cual provoca una pseudomiopía que comprometerá la visión lejana.

    La mecánica de la astenopía acomodativa varía según el caso. En la mayoría de los casos, el músculo ciliar sufre un espasmo (espasmo ciliar). En un principio puede ser tempoarl, con el tiempo puede hacerse crónico; estro puede hacer que el cristalino se deforme en todos sus meridianos, provocando una miopía, o que lo haga en un meridiano, provocando un astigmatismo.

    El trabajo excesivo en distancia próxima, sin descansos para el músculo ciliar, provocan un calentamiento muscular que puede ser transmitido al humor vítreo, lo que puede provocar pequeños desprendimientos de partículas en el mismo, comunmente conocidas como moscas flotantes o miodesopsias. Estas partículas flotan en el humor vítreo proyectando sus sombras sobre la retina, este es el por qué vemos esas moscas "volando"

    En algunas ocasiones, uno de los ojos puede verse mas adfectado por este proceso, cuando el ojo dominante de una persona hace un trabajo desproporcionado frente a su parejo. Esto puede provocar en ese ojo una progresión de miopía/astigmatismo.

    Un examen completo por un especialista puede detectar leves grados de astenopía. El especialista puede sospechar al comparar los datos con datos anteriores.

    Para reducir la aparición de la astenopía, lo primero es reducir o eliminar la presión sobre el mñusculo ciliar por períodos grandes de tiempo. Es muy importante tambien no acercar demasiado los objetos cercanos, mantenerlos siempre a la máxima distancia posible. Una buena iluminación es importante tambien. Cuanto mas cercanos los objetos, mayor es el esfuerzo del músculo ciliar.

    [list class="circle"]
  • sentarse erguido, derecho al escribir/leer
  • mantener lo mas alejado posible los objetos/ibros
  • mantener al menos la distancia de un antebrazo entre el objeto y el ojo
  • hacer descansos regulares (mirar objetos lejanos)
  • [/list]

    En el caso de los miopes, se precriben lentes negativos para permitriles ver enfocado. Este tipo de lentes son usados para toda distancia, es importante conocer el hecho de que estos lentes incrementan el trabajo del músculo ciliar.

    Si todos los síntomas arriba descritos empiezan a notarse, es recomendable el uso de gafas de descanso, prescritas por un especialista para evitar el sobresfuerzo del músculo ciliar. Otra opción son los lentes progresivos.

    The idea behind a reading prescription of convex (plus) lenses, in either single vision glasses or bifocals while performing near visual tasks, is that the light entering entering the eyes will be refocused by the lenses.  That is, the lenses do a small amount of the focusing for the person’s crystalline lenses, reducing some or even all of the ongoing stress on the ciliary muscles.  For example, the demand of print being read at 20 inches on the accomodative system of an eye (with no refractive error) is about +2.00 D (dioptría).  If +0.50 D glasses are prescribed, the eye needs to provide only +1.50 of the work—that is, only 75%, rather than 100%, of the workload.

    In most cases, the relief of tension on the focusing system, via the use of the proper plus lens prescription for near work, is enough to prevent over-contraction of these intraocular muscles and, thus, avert a nearpoint-stress event.  Sometimes, a regimen of certain vision therapy techniques can be done to reverse the effects (including low myopia and/or astigmatism) of the nearpoint stress.

    Prevention or reversal of nearpoint-stress-induced myopia and/or astigmatism, though, is much more likely to take place very soon after the onset of the condition, rather than at a later point.  If the myopic and/or astigmatic refractive error has been embedded too deeply into the eyes’ focusing system, it can be very difficult, or impossible, to reverse.

    It has been theorized that in some cases of nearpoint stress, the cornea of the eye takes on a “steeper” (more convex) shape in one or more meridians, due to prolonged pressure behind it.  If so, the pressure may be due to frequent anterior-to-posterior expanding of the eye’s crystalline lens from focusing too much at near, inducing a compression of the aqueous fluid anterior to the lens and, thus, resulting in pressure on the posterior cornea.

    In many cases, rigid gas permeable contact lenses (RGPs) can retard or stop the progression of myopia and/or astigmatism.  This may be evidence that a change in corneal shape can be a factor in some types of nearpoint stress.  Apparently, in such cases, the rigid lens prevents the anterior cornea from becoming more convex, thus arresting the advancement of myopia in the eye.

    If the aforementioned pressure on the cornea can occur from nearpoint stress, similar pressure, theoretically, could occur behind the crystalline lens of the eye, being transmitted through the vitreous gel and then to the retina.  If so, it may be, in some cases, that the retina and the back of the eye (the sclera) gradually are pushed posteriorly, eventually resulting in a lengthening of the eyeball and in the onset or increase of myopia.

    presbyopia

    After age 40 in most people, and by age 45 in virtually all, a clear, comfortable focus at a near distance becomes more difficult with eyes which see clearly (whether with or without glasses) at a far distance.  This normal condition is known as “presbyopia,” and it is due both to a lessening of flexibility of the crystalline lens and to a generalized weakening of the ciliary muscle which causes the lens to accommodate (change focus).

    By the time one reaches age 65 or so, the crystalline lens is virtually incapable of changing shape.  Unless one is nearsighted, it is not possible to focus objects (such as print on a page) clearly at even an arm’s length distance.

    Interestingly, the first symptom of presbyopia often is not blurred print or eyestrain while reading.  Rather, one may observe that objects across the room appear momentarily blurry after looking away from a near distance (that is, after reading, writing, or viewing a computer screen for awhile).  This is because the crystalline lenses within the eyes have become less flexible than they used to be, resulting in their being less able to accommodate (change focus) from near to far.

    With time, it will take longer and longer to refocus objects far away after having done close work.  Nearpoint stress can intensify and accentuate this process.

    Eventually, if presbyopic eyes are forced to continue to focus unwillingly at near, one’s far vision will become and remain noticeably blurry.  For a person who never had to wear glasses to see clearly far away, myopia (nearsightedness) and/or astigmatism will have set in, requiring a far-distance prescription in glasses or contact lenses to see clearly again.  For a person who already is myopic, the degree of nearsightedness will have increased, requiring a stronger lens prescription to regain clear vision.

    A myopic (nearsighted) person with presbyopia often can remove his/her glasses to focus clearly at near.  If this is too inconvenient, he/she can obtain multifocal or progressive addition lenses to be able to focus clearly at far and near with the same pair of glasses.

    When a person wearing single-vision contact lenses develops presbyopia, it will be necessary to wear some type of reading prescription (in glasses) over the contacts to achieve and maintain a clear, unstrained focus at near.  In many cases, bifocal or aspheric contact lenses can provide adequate vision at far and near distances, without the use of glasses.

    For some people, one eye (usually the dominant eye) may be fit with a contact lens focusing that eye for far away viewing and the other eye fit with a lens focusing that eye for near viewing.  This is called a “monovision” fit.  However, with this arrangement, one’s depth perception (which is important when driving) may be compromised to some extent.

    Note that “presbyopia” is not the same as “hyperopia” or farsightedness.  Presbyopia is an age-related condition, resulting in difficulty keeping a clear, comfortable focus at a near distance, even with an eye which is not hyperopic (farsighted).  On the other hand, hyperopia is a refractive error which makes it more difficult than normal to maintain a focus at a near distance than at a far away distance at any age (although, if one has a moderate to high degree of hyperopia, even maintaining a clear focus far away is difficult).

    A hyperopic (farsighted) person with presbyopia generally must acquire reading glasses for near work, or else multifocal or progressive addition lenses for full-time wear.  In some cases, a “monovision” contact lens arrangement also may be appropriate for a hyperopic person with presbyopia.

    For some people with presbyopia, store-bought (non-prescription) reading glasses may be an option.  However, store-bought glasses have equal strengths in the right and left lenses.  Since most people’s eyes have at least slightly unequal refractive errors, the focusing between their two eyes will not be balanced when wearing non-prescription readers.  Thus, one or both eyes may experience eyestrain.  Headaches also may result.

    The amount of presbyopia inevitably increases with age.  Therefore, the additional “plus power” of the lens strength required to maintain a clear, unstrained focus at near will need to be increased every few years to compensate for the irreversible effect of the presbyopia.

    cataract

    Normally, all the layers of the crystalline lens are clear, and light passes through it unobstructed.  However, with age or due to certain systemic diseases, as well as with a cumulative absorption of ultraviolet radiation over many years, the lens material can become cloudy, yellow, brown, and even opaque.  Anything in the lens which obstructs entering light is referred to as a “cataract.”

    More than 50% of people over the age of 60 have some form of a cataract.  It has been said that if one lives long enough, he/she will develop a cataract.  Even some infants are born with a “congenital” cataract which, if left untreated, can cause permanent visual impairment or blindness, even if the cataract is removed years later.

    It is not possible to remove a primary cataract without irreparably damaging the crystalline lens within which the cataract is contained.  A laser cannot be used successfully to remove a cataract, except as described later (in the case of a secondary cataract).  Therefore, cataract surgery involves removing most or all of the lens of the eye and replacing it with an artificial “intraocular lens” or “lens implant,” made of a hard plastic (polymethyl methacrylate or PMMA), silicone, acrylic, or hydrogel material. 

    An “extracapsular” cataract extraction (ECCE) is the routine type of cataract removal.  In an ECCE procedure, an opening is made in the front of the lens capsule.  Through this opening, the lens nucleus is removed, either as a whole or by dissolving it into tiny pieces and vacuuming out the pieces, a procedure called “phacoemulsification.”  Next, the lens cortex also is sucked out, leaving the lens capsule in place, and into the lens capsule is inserted the artificial lens implant.

    Prior to the 1980’s, the entire crystalline lens was removed in a cataract surgery, called an “intracapsular” cataract extraction (ICCE).  Usually, this was performed using “cryoextraction,” where a cryoprobe froze the entire lens, permitting its complete removal.  Now, in the unusual case of an intracapsular lens extraction, or ICCE, the implant lens is placed in front of the iris, rather than behind it, because there is no lens capsule to hold the implant in place.  Rarely is this procedure done anymore.

    Approximately 1-2% of post-cataract extraction patients develop swelling in the area of the retina responsible for central vision (the macula).  This swelling occurs in cystoid spaces, and is referred to as cystoid macular edema.  After an initial improvement following surgery, these patients subsequently will describe blurred vision.  Cystoid macular edema can occur as early as days, or as late as several years, following surgery.  Treatment options include observation, topical therapy, periocular injections, and surgery.

    Naturally occuring carotenoids in the crystalline lens—lutein and zeaxanthin (molecular cousins of beta carotene and vitamin A)—have been shown to reduce the risk of cataracts.  These pigments act as antioxidants within the lens, inhibiting the formation of free radicals, which can damage lenticular material and contribute to the development of cataracts.

    Thus, it may be that the greater the amount of antioxidants such as lutein and zeaxanthin in the system, the less the risk of cataract formation.  These two antioxidants are found particularly in yellow fruits and in green leafy vegetables (especially xanthophyll-rich vegetables such as spinach, kale, collard greens, and broccoli), in eggs, and as nutritional supplements.

    secondary cataract

    Not uncommonly, following an “extracapsular” cataract extraction (ECCE), a few cells of the crystalline lens cortex remain adhered to the inner surface of the posterior lens capsule.  After a few weeks or months, these cells can become opaque, resulting in a secondary cataract.  Fortunately, the eye does not have to be reopened for this simple cataract to be removed.

    Rather, a YAG (yttrium aluminum garnet) laser is used, in a procedure taking only a few minutes, to fire through the clear cornea and pupil and to obliterate the secondary cataract (and a small portion of the capsule behind it).  This enables light to pass into the eye again, unobstructed.  If this laser procedure is successful, a cataract never again should pose a problem for that eye.

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