Amblyopia - Amblyopia

Japanese: 弱視 - じゃくし（英語表記）Amblyopia

What is the disease?

Amblyopia is a condition that occurs during the period of visual sensitivity (up to about age 8), retina ( Also well ) This is a condition in which a clear image cannot be formed on the eye, which hinders the development of the visual center and makes it difficult to see.

For example, some people may be worried that their child has low vision (0.1), and may be worried that they may have low vision. naked eye ( Ragaan ) In most cases, this means that your vision is 0.1 (i.e., your vision is 0.1 when you take a vision test without wearing any glasses). correction ( Resilience ) If the visual acuity is 1.0 or higher, the ability to see fine objects is considered complete, and the condition is not considered amblyopia.

There are many causes of amblyopia, the most common of which are listed below:

Various types of amblyopia

● Strabismus and Amblyopia

When you have strabismus and your eyes are not facing straight ahead, your retina is the most sensitive area. Macular area ( Half-portion ) The image is no longer focused on the eye, which impedes the development of visual function and results in amblyopia. Since the normal eye is dominant, the eye that is always squinting will become amblyopic. Strabismus will be explained in more detail in the next section.

● Form deprivation amblyopia

Congenital Cataracts ( Disclaimer ) Or eyelid tumors, Ptosis ( Fighting hard ) Amblyopia is caused by visual input being blocked by an eye patch, etc. If such related factors are present in a newborn, it can lead to amblyopia even within a few days, so caution is required.

● Refractive amblyopia

Amblyopia is caused by severe farsightedness, astigmatism, etc. Many people think of farsightedness as "being able to see far away clearly," but eyesight develops by looking at close objects, so severe farsightedness, which cannot focus on close objects, hinders the development of visual function and causes amblyopia. The same goes for severe astigmatism. In the case of myopia, unless it is pathological myopia, it is often not caused by amblyopia, as the eyes can focus on close objects.

● Anisometropia Amblyopia

When the difference in refractive index between the left and right eyes becomes large enough, the visual input from the eye that is easier to focus on takes priority, and the eye that is harder to focus on (the eye with the larger refractive error) becomes amblyopic. Amblyopia is likely to occur when both eyes are farsighted and the difference in refractive index between the left and right eyes is 3D (diopter) or more, but amblyopia can also occur when only one eye is severely myopic.

Treatment methods

We have listed the main types of amblyopia above, but the key to the development of amblyopia is the sensitivity of visual function. Sensitivity increases after birth, reaching its peak at around 3 months of age. This period of high sensitivity continues until around 18 months of age, but after that it gradually declines, almost completely disappearing by around 6 to 8 years of age. Even if treatment for amblyopia is started from around the age of 10, it is difficult to achieve any results because there is almost no sensitivity.

The treatment for amblyopia involves switching the better eye to a different eye for a set period of time. Shielding ( Shahei ) The basic method is to wear glasses that are fully corrected for the amblyopic eye and force the patient to use the amblyopic eye. In cases of refractive amblyopia, where the same degree of vision loss is seen in both eyes, simply wearing glasses that are fully corrected is sufficient.

The earlier treatment is started, the more effective it is, so if an abnormality is suspected during a 3-year-old checkup, it is important to undergo a detailed examination as soon as possible.

Hirofumi Shimojo

Source: Houken “Sixth Edition Family Medicine Encyclopedia” Information about the Sixth Edition Family Medicine Encyclopedia

Japanese:

どんな病気か

　弱視とは視覚の感受性期（8歳くらいまで）の期間内に、網膜(もうまく)上に鮮明な像が結ばないことにより、視覚中枢の発達が妨げられて、視力が出にくい状態をいいます。

　たとえば「私の子どもは視力が0.1ないので弱視ではないか」と不安に思われる人もいるかもしれませんが、「裸眼(らがん)視力が0.1ない（つまり何もつけない状態で視力検査したときの視力が0.1ない）」ということをいっている場合がほとんどです。眼鏡で矯正(きょうせい)すると視力が1.0以上出る場合は、細かい物を見る力は完成していると考えられ、弱視とはいいません。眼鏡で完全矯正しているのにもかかわらず、視力が出ない状態が弱視です。

　弱視にはさまざまな原因がありますが、主なものを以下に示します。

さまざまな弱視

●斜視弱視(しゃしじゃくし)

　斜視があって、眼が正面を向いていない場合、網膜で最も感度の高い黄斑部(おうはんぶ)に像を結ばなくなり、視機能の発達が妨げられ、弱視となります。正常眼のほうが優位にはたらくため、いつも斜視になっている眼は弱視となってしまいます。斜視については次項で詳しく説明します。

●形態覚遮断弱視(けいたいかくしゃだんじゃくし)

　先天性白内障(はくないしょう)や、まぶたの腫瘍、眼瞼下垂(がんけんかすい)、眼帯などにより視覚入力が妨げられることによって起きる弱視です。新生児にこのような連関要因がはたらくと、数日間でも弱視化することがあり、注意が必要です。

●屈折性弱視(くっせつせいじゃくし)

　強度の遠視、乱視などが原因となる弱視です。遠視といえば「遠くがよく見える」というイメージをもっている人も多いと思いますが、視力は近くを見ることにより発達するため、近くにピントの合わない強度の遠視では、視機能の発達が妨げられ、弱視が起きます。強度の乱視も同様です。近視の場合は病的な近視でない限りは近くにピントが合うため、弱視にならないことが多いようです。

●不同視弱視(ふどうしじゃくし)

　左右の眼の屈折度の差がある程度以上大きくなると、ピントを合わせやすいほうの眼の視覚入力が優先され、ピントを合わせにくいほうの眼（屈折異常が大きい眼）は弱視化します。両眼とも遠視で、左右の屈折度の差が3D（ディオプター）以上になると弱視が起きやすいのですが、片眼のみ強度の近視である場合も弱視となります。

治療の方法

　以上主な弱視の種類をあげましたが、弱視の成立の大きな鍵となるのが視機能の感受性です。感受性は出生後上昇し、3カ月くらいでピークをむかえます。1歳半ころまで感受性が高い時期が持続しますが、それ以上は徐々に下降し、6～8歳くらいでほぼ消失します。10歳くらいから弱視の治療を始めても感受性がほとんどないため、効果が得にくいといえます。

　弱視の治療法は、視力のよいほうの眼を決められた時間遮閉(しゃへい)して、弱視の眼に完全矯正した眼鏡をかけ、強制的に弱視眼を使わせるという方法が基本です。両眼とも同程度の視力低下がみられる屈折性弱視の場合には、完全矯正した眼鏡をかけるだけでよいです。

　早期に治療を開始すれば効果が大きいため、3歳児検診などで異常が疑われた場合は早い時期に精密検査を受けることが重要です。

下條　裕史

出典　法研「六訂版家庭医学大全科」六訂版家庭医学大全科について　情報

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