デジタルデバイスの過剰使用により発症した急性内斜視における水平直筋付着部の形態

The conformation of horizontal extraocular muscles in acute esotropia

要約　目的：近年，急性後天共同性内斜視（AACE）のうち，スマートフォンを含むデジタルデバイスの過剰使用によるAACE（SAACE）の発症が注目されている。SAACEは過剰な近見作業から輻湊痙攣による内直筋の緊張が増大し，内斜視になると考えられている。発症背景には，近視や輻湊と開散の不均衡などが挙げられているが，解剖学的な検討は少ない。今回，手術となった症例における術中水平直筋の所見を検討した。

対象と方法：手術となったSAACE患者14例において術中に内直筋および外直筋の輪部から付着部までの距離（付着部距離）と付着部での筋幅（付着部幅）を計測した。斜視のない網膜剝離患者21例（対照群）も同様に計測し比較した。内直筋の計測値を外直筋の計測値で除した値（内/外直筋付着部距離比および内/外直筋付着部幅比）も算出し，両群間で比較した。

結果：両群間に外直筋付着部距離および幅の差はなかったが，対照群に比べ，SAACE群の内直筋付着部幅は大きい傾向にあり，付着部距離は有意に短かった（p＜0.05）。内/外直筋付着部距離比は有意に小さく，内/外直筋付着部幅比は有意に大きかった（p＜0.05）。

考察と結論：SAACE患者は，長時間の近見作業によって，輻湊痙攣を起こし，内直筋緊張の増大に加え，内直筋の付着部が輪部に近く，内直筋の筋幅が大きいため，内・外直筋の不均衡により内斜視が発症しやすくなる可能性が示唆された。

Abstract　Purpose：Acute acquired concomitant esotropia（AACE）induced by excessive digital device usage, especially smartphones（smartphone associated AACE：SAACE）, has been increasing over the past few years. Convergence spasm induced by excessive near work is one of the suggested mechanisms. Anatomical differences could also potentially contribute to the onset of SAACE. The present study was performed to investigate the anatomical differences in horizontal recti between SAACE patients and normal subjects.

Subjects and Methods：The distances between limbus and the insertion of horizontal recti（LI distance）were measured intraoperatively in 14 patients with SAACE（SAACE group）. The widths of horizontal recti at the insertion（insertion width）were also measured. Twenty-one patients who underwent surgery for retinal detachment were considered as controls. The differences in the LI distances and insertion widths between the SAACE and the control groups were investigated.

Results：There were no differences in the LI distances and insertion widths of lateral recti between both groups. However, the LI distances of medial recti were significantly shorter in the SAACE group（p＜0.05）. There was also a tendency of larger insertion widths of medial recti in SAACE group. Compared to the control group, the medial/lateral ratio of LI distances was significantly lower and that of insertion widths was significantly higher in SAACE group（p＜0.05）.

Conclusion：The findings of more anterior insertion and larger muscle width indicate stronger force of medial recti in the SAACE group. In addition to excessive accommodation followed by increases in the tonus of medial recti, anatomical imbalance between lateral and medial recti was suggested to contribute to the onset of esotropia following excessive near work.