Paralytic Strabismus: A Review of 13 Years at a Tertiary Care Center in Western-Central India

Purpose: To describe the incidence, aetiologies and follow-up of patients with paralytic strabismus. Methods: This retrospective study included 193 strabismic patients with isolated III, IV or VI cranial nerve palsy, cases of the strabismus from the Department of Ophthalmology of the tertiary care center in western-central India between January 2007 and December 2020. The present study analyzed the injured cranial nerve, the affected eye, sex distribution, etiology and follow-up. Results: About 5.7% of the cases were congenital and 16.1% were of pediatric age. The Trochlear nerve was the most frequently affected (56.5%) in these cases. Incidence was higher in males (58.1%) than females. Trauma was the common cause of III (7.1%), IV (8.3%) and VI (50%) nerve palsy. The disease management modalities included surgical, medical and spontaneous resolution. Conclusions: The sixth cranial nerve was affected most frequently. However, the most common underlying cause was yet undetermined.


Introduction
Paralysis and paretic strabismus are characterized by weakness of one or more oculomotor muscles innervated by the nerves, resulting in an uncommon deviation that can require clinical and surgical treatment or spontaneous regression [1].
Paralysis or paresis of the cranial nerves III, IV and VI have been analyzed previously and these studies are classic hallmarks [2][3][4][5]. Cranial nerve VI has been identified as the most affected region.
The present study aimed to elucidate the epidemiological and clinical profiles of paralytic strabismus and clinical outcome on follow-up with or without management.

Methods
After clearance from the hospital ethical committee (DN/2020/Dec/21) This retrospective study analyses the incidence, etiology and evolution of paralytic or paretic strabismus in patients seen in the Strabismus Department of Ophthalmology, India, the data of patients with paralytic paresis or paralysis were obtained from the electronic medical records from 2007 to 2018.
The aetiological factors of the cases were classified into four groups: trauma, vascular anomaly (including systemic diseases such as systemic hypertension, DM and cerebral aneurysm), undetermined (no cause was found after the diagnostic investigation) and others. This grouping facilitated the comparison between the current and previous data since most groups adopt this categorization method. In addition, congenital cases constituted an independent group. Evolution was also divided into five groups: regression spontaneous, surgery (patient undergoing surgery), without follow-up (patient follow-up was lost), still in follow-up and discharge after cure or on request.
Data related to sex, the affected nerve, the affected eye, the etiology and the evolution of acquired paresis and isolated paralysis were collected in the pre-tested format, exported to Excel and analyzed using SPSS 22 (no conflict of interest). Descriptive and cross-tabulation statistics were used to analyze the data. P<0.05 indicated statistical significance.

Results
The current cohort of 193/66 (34.2%) females and 127 (65.8%) males patients with isolated paresis or paralysis of the muscles innervated by the cranial nerves III, IV or VI, correspond to 4.7% of the patients in the Ocular Motility Section ( Table 1). The age of onset of the clinical manifestations of the group with paralytic strabismus or acquired paretic was between 1 and 85 years. The congenital cases were studied separately; 11/193 (5.7%) patients were congenital The right eye was involved in 82/193 (42.5%), the left eye in 73 (37.8%) and both eyes in 38 (19.7%) cases. Our cohort included 2 cases of myasthenia gravis, suggesting a myogenic cause.
Among the other causes of cranial nerve III palsy, the central nervous system infection was detected in 1 case.
The comparison between the deviation in primary position at the time of presentation and the end revealed significant differences (p<001), but no difference was observed while comparing traumatic and non-traumatic cases (P=0.435).

Discussion
The proportion of gender in the current cohort was similar to that described in the literature. This slight predominance of the male sex could be attributed to the frequency of traumatic causes in this sample [1].
Among the acquired isolated paralysis and paralysis, the most affected cranial nerve was VI (56.9%), followed by III (24.4%) and IV (1.01%). These data coincide with those in the literature, wherein nerve VI was most affected with a frequency of 40.9%, 51.1%, 41.9% and 55.0% [2][3][4][5]. In a private clinic, which is a reference in strabismus, the frequency of cranial nerve IV involvement was 62.2%; this might be because the clinic is a differentiated reference service [6].
The data related to the affected eye showed a discrepancy between our sample and those in the literature. The right eye was involved in 82/193 (42.5%) cases, the left eye in 73 (37.8%) cases and both eyes in 38 (19.7%) cases, while Gustavo, et al., reported 49.5% impairment of the right eye, 41.9% of the left eye and 8.6% of bilateral impairment [1]. Regarding paresis or acquired paralysis of cranial nerve III, 8.3% was traumatic etiology and 59.1% was due to undetermined causes. Some studies showed that 45.5% were congenital cases [7,8]. Another study found a high frequency of indeterminate cases (23.7%), followed by vascular causes (19.8%) [6]. A previous study also reported a frequency of 40.5% of undetermined cause and 29.2% of vascular etiology [8]. These differences could be attributed to the demographic properties of the catchment area. The low occurrence of vascular etiology (17.5%) is probably due to these patients' lack of diagnosis or referral to our department [1].
The cases of paresis or acquired paralysis of the cranial nerve IV incidence is lesser in our cohort (1.01%) compared to the higher rate (52.4%) reported by Gustavo, et al., wherein 30.5% cases were classified as the undetermined cause. Literature analysis also revealed that trauma (25.7%) and vascular injury (15.7%) were the most frequent aetiological factors, while 28.3% were undetermined causes [1,7]. The current study reported 5.7% congenital cases and 16.1% cases in the pediatric age group. Other studies reported 43.5% congenital cases, 38.8% undetermined causes and 7.6% cases of traumatic origin [9,10]. Another study found 39.5% congenital cases, 34.0% traumatic causes and 23.2% cases of undetermined origin [11]. Some surveys conducted on children demonstrated that 57.3% of the cases were of unknown cause and 13.6% were detected during follow-up [12,13]. Previous studies also observed that spontaneous regression occurred in 9.3% cases and the frequency of these cases was 48.3% and 66.6%, respectively [4,9].
In the studies described above, vascular etiology cases had the highest rate of spontaneous regression, while this group presented a low frequency in our cohort. In addition, the loss of patients to follow-up, which is common in public hospitals, may have caused a decrease in the frequency of this regression.
Jason, et al., reported a similar outcome irrespective of the etiology, however, the number of surgeries may be more than that in the current study [14]. Kiyoung, et al., also reported that vascular causes display maximum recovery rate and a short recovery time [15].

Conclusion
Paralytic strabismus is caused by several reasons. First, disease management is either medical or surgical. Vascular causes show the highest rate of recovery and the shortest recovery period compared to other factors. Cranial nerve VI is the most commonly involved region, followed by III in this disease.