Acute adult onset esotropia is a rare variety of esotropia that presents in late adulthood with sudden onset of a large angle esotropia usually preceded by intermittent diplopia in an otherwise previously orthophoric individual (1). Earlier investigators have conventionally classified acute adult esotropia into 3 broad categories based on the clinical features of the patients. Type 1 (described by Swan in 1947) follows fusional disruption secondary to monocular occlusion which has been reported to have caused both sudden onset esotropia as well as exotropia (2). Type 2 (reported by Francesschetti) occurs after some psychological stress or shock and features small degree of hypermetropia without an accommodative element in most of its patients (3); and type 3 (described by Bielchowsky in 1922) is reported to have occurred in patients with significant myopia especially in those who perform excess near work and are reluctant to wear myopic correction (4). Despite this traditional classification, a significant number of patients with acute onset esotropia have been reported with serious underlying neurological abnormalities like brain tumors (5), Arnold Chiari malformation (6-9), idiopathic intracranial hypertension (10) and cerebellar ataxia (11). That is the reason acute esotropia always warrants a detailed neurological examination but the need for radiological investigations to rule out any underlying neurological disorder has always remained an area of controversy and debate.Decompensated esophoria has also been reported as an independent cause of adult onset esotropia but only in very few published reports. Majority of the cases of acute adult onset esotropia have causes classified into one of the aforementioned types and not much attempts have been made to describe the clinical features of decompensated esophoria as an underlying cause of this condition. We present here a series of patients of decompensated esophoria with their clinical and magnetic resonance imagining (MRI) characteristics that developed acute onset large angle esotropia. ?MATERIALS AND METHODSIt was a retrospective chart review case series conducted at Department of Pediatric Ophthalmology & Adult Strabismus at Jules Stein Eye Institute after approval of its protocol from institutional review board of the Office for Protection of Human Subjects, University of California at Los Angeles, USA. We reviewed the clinical and surgical records of patients with decompensated esophoria presented at our institute from 2015 to 2017. All the patients underwent detailed history taking and ophthalmic examination that included assessment of unaided and best corrected visual acuity using Snellen’s charts, spherical equivalent of the refractive error, extraocular motility to see ductions and vergence in secondary and tertiary gazes, Titmus fly stereotest for testing stereopsis, and preoperative deviation measured in prism diopters at distance and near using alternate and prism cover tests.. We could also find preoperative convergence fusional amplitudes measured with base-out free prisms in 2 of the cases. All patients underwent high resolution surface coil MRI of the extraocular muscles to see any pathology in the extraocular and adjacent orbital tissues in the patients. Patients who were planned for surgery were given option for adjustable sutures for intraoperative or postoperative adjustment of ocular muscles to achieve postoperative target angle successfully. A cotton tipped swab was used to assess the cooperation and tolerability of the patient to the topical anesthesia. After instillation of 0.5% proparacaine hydrochloride in one of the eyes, the cotton tipped applicator was used to apply moderate pressure over the bulbar conjunctiva. If patient tolerated the pressure well, he was offered surgery under topical anesthesia with adjustable sutures. Otherwise surgery was planned under general anesthesia with adjustable sutures unless the patient was deemed least cooperative to allow for post-operative adjustments under topical anesthesia. Strabismus surgery under topical anesthesia was performed using 1% lidocaine hydrochloride with monitored anesthesia care provided by anesthesiologists. Small doses of intravenous propofol were given if the patient felt discomfort during surgery (12). Postoperative adjustments were performed with the patient in sitting position, with appropriate refractive glasses, and targets located centrally both at near and far. For patients undergoing bilateral surgery with adjustable sutures, the adjustment was performed in one of the two operated eyes. Amount of surgery performed was recorded for each case and its effect on ocular alignment was measured at various follow-up visits. Statistical analysis was performed using the statistical program SPSS 20.0 for Windows (SPSS Inc., Chicago, IL, USA). Mean ± S.D. was calculated for age, duration of ocular symptoms, visual acuity, spherical equivalent of the refraction, pre- and postoperative ocular alignment, and pre- and postoperative stereopsis. DISCUSSIONAcute adult onset esotropia has conventionally been classified into 3 types depending on clinical features and underlying etiology. Type I (Swan type) occurs following occlusion of one eye or monocular vision loss secondary to some disease or trauma (13). Type II (Francesschetti type) is characterized by presence of hypermetropia and is seen soon after some psychological stress or shock. Type III (Bielschowsky type) is seen in patients with uncorrected myopia of -5.00 D or more (14). One of the hypothesized mechanisms for this type of esotropia is increased muscle tone of medical rectus muscles that develops as a result of their excessive use in convergence to see near objects by uncorrected myopes (4). Another reported feature of these patients is that they have large angle esotropic deviation at far but are nearly orthophoric at near (4). Some of the authorities even excluded such patients from the aforementioned classification and labeled them as having divergence insufficiency or divergence paralysis esotropia (15, 16).The patients in our case series do not fit in any of the aforementioned types of acute onset esotropia. None of our patients had history of any ocular injury, monocular vision loss or occlusion ruling out type 1 (Swan type). Similarly, none of the patients had hypermetropia and none gave history of any recent emotional trauma or psychological cause to classify them in type II (Francesschetti type). Lastly, two of our patients had myopia more than -5.00 D but none of them had abstained from putting on their refractive correction in the past. This feature distinguished them from type II (Bielschowsky type). Presence of almost comitant esotropic deviation for both near and far excluded the possibility of divergence paralysis esotropia in our patients (17). Since all the neurological examination and radiological investigations came out to be normal, the possibility of a CNS lesion was also excluded. Young age of the patients, normal ocular examination findings, presence of only esotropia without hypotropia and unremarkable MRI results of the extraocular muscles and orbital tissues helped us eliminate the remote possibility of heavy eye and sagging eye syndrome in our patients (18, 19).We present here a series of patients who presented with history of gradually progressive intermittent horizontal, binocular diplopia that suddenly converted to comitant large angle esotropia. These patients did not fit in any of the traditional subtypes of the acute adult comitant esotropia and we considered the benign possibility of classifying them as having simple esophoria that decompensated over time to acute adult esotropia. Lyons et al presented a case series of 10 patients and reported that instead of the Swan, Francesschetti or Bielschowsky types of comitant deviation, decompensation of a pre-existing phoria or monofixation syndrome were the commonest etiological causes of acute adult comitant esotropia (20). Spierer et al presented a case series of 10 adult patients collected over 7 years who had significant myopia and developed large angle esotropia (21). He managed his patients surgically and was able to restore binocularity in most of them. He proposed to label these patients as having ‘acute concomitant esotropia of adulthood’. We believe that his report also encompasses the patients who had esophoria that decompensated over time and eventually resulted in large angle esotropia. Since not all of our patients were myopic, we consider our patient series to be different from his report. Another aspect that needs attention here is the response to conventional surgery in these patients. We performed bilateral medical rectus recessions in all of our patients following the surgical dosage recommended by Park et al. But despite optimal surgical dosing, 5 (71.4%) of our patients had residual esophoria of 2-14 PD. This implies that slight overcorrections may be aimed in such patients and it is advisable to use adjustable sutures in all such cases to make appropriate postoperative adjustments to achieve target angle successfully. As far mechanism of decompensation is concerned, we propose that patients with decompensated esophoria slowly deteriorate and present with esotropia when their enhanced divergence fusional amplitudes no longer suffice. This contrasts with acute adult comitant esotropia developing suddenly without history of phoria or intermittent diplopia. This makes decompensated esophoria progressing to esotropia a separate, distinct and generally benign diagnosis. Strabismologists should always keep the possibility of decompensated esophoria in their minds before classifying the adult onset esotropia into one of its conventional types. We believe that clinical features and descriptive findings should be used to describe a clinical condition when the underlying pathophysiology is unknown; but once pathophysiology has been established, that should be used to describe the condition instead of conventional names.