Diplopia is a common clinical condition, and we see such patients almost every day in our clinical work. Diplopia can significantly affect the life and work of patients and needs to be treated actively. Diplopia is commonly referred to as seeing one object as two, also known as double vision. If you block one eye with your hand, diplopia disappears completely, or if you only have diplopia when you look at both eyes together, it is called binocular diplopia, often due to extraocular muscle disease, or fusion abnormalities; if you block one eye and look with the other eye monocularly, you still have diplopia, in other words, there is no change in the original diplopia, it is called monocular diplopia, and monocular diplopia is mostly related to refractive abnormalities, for example, patients with adolescent myopia often complain of seeing For example, adolescents with myopia often complain of double vision in the distance and look at the words on the blackboard as if there is an imaginary edge, i.e., it is related to refraction and not really seeing one as two. In this case, blocking one eye does not change the perception of diplopia. If, when masking the eye, diplopia is found in one eye and not in the other, some ophthalmic conditions need to be excluded, such as lens dislocation in one eye, corneal astigmatism, retinal and ocular surface diseases, etc. Binocular diplopia is the most common clinical condition and is more complicated to manage. Because diplopia is caused by abnormalities of the extraocular muscles, patients with diplopia are treated by strabismus and pediatric ophthalmologists, as is the case worldwide. Interestingly, some of our patients ask, “Doctor, at my age, I’m also seeing a doctor with a child?” . We classify diplopia into horizontal diplopia, vertical diplopia, and rotational diplopia, depending on the direction of the diplopia. Horizontal diplopia is mostly seen when there is insufficient horizontal external or internal rotation of one or both eyes, and there was no strabismus, but now there is horizontal internal or external strabismus. Horizontal internal strabismus, mostly due to abducens nerve palsy, is manifested by the inability to turn one eye into place when it is turned extremely outward. Alternatively, both eyes cannot be turned outwardly into place. The sudden onset of abnormal eye movements is a cause that needs to be sought. Clinically, trauma is common, followed by vascular diseases such as hypertension, and diabetic patients. Tumor compression is rare and still needs to be excluded. A proportion of patients with external rectus muscle paralysis may gradually regain function. If the diplopia persists and is stable for more than 6 months, surgical correction is required. within 6 months, because the strabismus is unstable, surgical treatment is generally not considered, and trigeminal lenses or press fit trigeminal lenses can be worn. the advantage of fitting trigeminal lenses is that the diplopia is eliminated, and the disadvantage is that as the strabismus recovers on its own, trigeminal lenses are no longer applicable. Therefore, whether or not to fit a trigon depends on the patient’s financial situation, and quality of life requirements. In adults, if there is no abnormality of eye movement, it is also possible that the originally existing internal strabismus is transformed into a dominant strabismus due to the weakening of the fusion force (fusional separation), which leads to diplopia. In the case of diplopia with exotropia, if it is an actinic nerve palsy, the affected eye cannot turn inward and appears exotropic. It is often combined with a deficit in upward and downward eye rotation, as well as ptosis. In the absence of a history of trauma, a detailed examination of the cerebral system and imaging is necessary. There are also patients who have intermittent exotropia of their own, which is asymptomatic and unobtrusive in appearance due to good control at a young age, and have good binocular vision. As they age, the pooling control diminishes and when the uncontrolled manifestation of exotropia is present, diplopia develops, with horizontal diplopia predominating. In such patients, because of long-term control, the strabismus is usually not too small and there is little point in wearing glasses, requiring surgical treatment. Such patients will have a good outcome after surgery. One of the topics that such patients love to talk about in their post-operative follow-up is, “Doctor I feel fine now, but I am treating diplopia, not cosmetic at all, so why is it not reimbursed by health insurance? In fact, many patients with strabismus are trying to improve their binocular vision and should be covered by medical insurance. Vertical diplopia, where two objects are seen, one above the other, or diagonally above and below, is also common in clinical practice. Vertical diplopia can be either acquired or originally had vertical strabismus, which is compensated by skewed head or vertical fusion, and when it is lost, diplopia is produced. It is meaningful to identify whether there was a previous strabismus or not, because if there was a previous strabismus and now it is out of compensation and diplopia occurs, there is no need for neurology or other systemic in-depth investigations, and treatment for diplopia alone is sufficient. Conversely, a newly acquired vertical strabismus with no previous problems requires a detailed examination in the brain department, including MRI. How to identify it? The easiest way is to look at old photos from the past. If the photos from childhood are crooked, it mostly indicates that vertical strabismus was present as a child. You can also measure whether the facial development is symmetrical, because a long-term crooked head will lead to facial development asymmetry, by measuring the distance from the corner of the mouth to the outer corner of the eye, if the distance between the two sides is greater than 5 millimeters, it means that the face is asymmetrical. Of course, when it comes to the doctor, there are other ways to identify and analyze the cause of diplopia. Rotational diplopia, which is rare clinically, is characterized by looking at things, especially at linear objects, such as the sides of cabinets and the floor line, in a crooked manner. Some patients with vertical strabismus, looking downward, may also find the skew, but such patients often have their rotational diplopia eliminated with them after correction of the vertical strabismus. Those whose true rotation cannot be eliminated are often patients who have been in a car accident and have a history of coma. These patients can have good results after surgical treatment. Only some patients may require 2 surgeries. In addition, patients with thyroid-related eye disease, i.e., those with hyperthyroidism or hypothyroidism, are prone to diplopia. This is because thyroid disease is actually a class of autoimmune diseases in which the body attacks two of its own targets, one being the thyroid and the other being the eye socket. So, if it attacks the thyroid, it is called hyperthyroidism, and if it attacks the orbit, it is called TAO and is prone to diplopia. Of course, some patients only attack one of them, but it is a disease. Myasthenia gravis is also a systemic disease that needs to be ruled out. In patients with diplopia due to these systemic diseases, diplopia can also be improved by wearing tricorders or strabismus correction surgery, depending on the case, so it is not a generalization. Other patients with diplopia are post-strabismus correction patients, and these patients need to be evaluated for binocular vision, which can mostly recover on its own if it is paradoxical diplopia. If it is a residual strabismus or persistent diplopia from overcorrection, it needs to be treated promptly. Especially in children, we know that children under 4 years of age are prone to amblyopia if the exotropia is followed by an internal strabismus. Older children will complain of diplopia, and after appropriate conservative treatment, if there is no improvement, surgical treatment is required. If the eye position does not improve and the symptoms of diplopia disappear, it is not necessarily a good sign, because it is possible that the oblique eye has established monocular suppression, which is a sign of binocular vision loss, so therapeutic intervention should be performed before that.