Our study aimed to investigate if the RNFL thickness and GCL thickness are affected in patients with a chronic migraine that may improve the understanding of the etiology and pathophysiology of migraine.
In our study, the average RNFL thickness and the RNFL thickness of all quadrants (superior, inferior, nasal, and temporal) were significantly thinner in the chronic migraine patients, either with or without aura, than the healthy controls. The diminished RNFL thickness means a reduction in the number of axons in migraine patients. Our results come in agreement with different previous studies [18,19,20]. Other studies have demonstrated only RNFL thinning in a specific quadrant; Colak and colleagues  reported RNFL thinning in the superior and inferior quadrants, while Martinez and colleagues  reported significant decreased RNFL thickness in the temporal quadrant only. Some studies have observed that RNFL thickness was thin in the nasal quadrant only [23, 24] and others have reported RNFL thinning in the superior quadrant [25, 26]. The selective RNFL involvement was attributed to the differences in the vulnerability of the axons to retinal ischemia .
On the other hand, Simsek and coworkers  reported that no significant difference in the average RNFL thickness or any of the quadrants in migraine patients, with or without aura, and healthy controls except for the nasal quadrant of the right eye, which had a significantly higher value. Also, two previous studies found [16, 28] no statistically significant differences in the retinal thickness between migraine patients and healthy control were found. Finding no significant difference in RNFL may be due to short mean disease duration or low numbers of migraine attacks in those studies .
Regarding the GCL thickness, in our study, we found no statistically significant difference between the GCL thickness in migraine patients and healthy control. This goes in agreement with a few earlier reports [21, 29]. However, several studies showed the opposite of what we found [16, 19, 30]; Abdellatif and Fouad  reported that the superior and inferior GCL thicknesses were significantly diminished in patients with migraine, either MwA or MwoA, compared to healthy controls. Reggio and colleagues  hypothesized that the alteration in the blood supply to the anterior optic nerve head results in an oligemic-hypoxic insult that contributes to ganglion cell damage .
We further analyzed the patients with migraine into subgroups: MwA and MwoA; in which we found a significant thinning of the average, inferior, and nasal quadrants of RNFL thickness in patients with MwA than patients with MwoA.
Our study results are in agreement with a study achieved by Ao and coworkers, who reported a significant reduction in the RNFL thickness of the nasal and inferior quadrants in patients with MwA compared to MwoA . Also, the study of Tunç and colleagues showed a significant reduction in the RNFL thickness of the average and superior quadrants between MwA and MwoA . The study of Ekinci and coworkers found non-significant more thinning of the RNFL in patients with MwA than patients with MwoA .
During the aura of migraine, the posterior part of the cerebral hemisphere shows cerebral hypoperfusion which can explain the more RNFL thinning in MwA compared to MwoA . However, some studies found no significant difference in the RNFL thickness between patients with MwA and MwoA [14, 19, 31].
Regarding the GCL thickness between the migraine subgroups; MwA and MwoA, our study found that the GCL thickness was thinner in patients with MwA than in patients with MwoA. There was a statistically significant difference in the average and superior half GCL thickness between patients with MwA and patients with MwoA. Our results were in agreement with previous studies [19, 26, 30]. On the other hand, it was found no significant difference in the GCL thickness between patients with MwA and patients with MwoA [20, 21].
As migraine patients experience headaches almost on the one side, we studied the ipsilateral involvement of the RNFL and we found no significant difference in RNFL thickness between the eyes on the side of the headache and the eyes on the contralateral side. Our finding goes in concordant with the study of Gunes and colleagues  which had investigated the association between laterality of migraine and RNFL thickness headache side, and they found more thinning of RNFL on the same side of the headache and the asymmetry was not statistically significant . Such thinning of RNFL on the headache side could be attributed to the chronic course of migraine with periodic reduction of the blood flow on the ipsilateral hemisphere during the attacks that could lead to permanent cerebral and retinal damage .
In agreement with previous studies, our results showed no significant correlation between the average RNFL or the average GCL thicknesses and the duration of the migraine, the attack frequency, or the severity of the migraine [14, 16, 26, 27].
Theoretically, migraine with long disease duration, higher frequency of attacks, or severe attacks might cause more damage to the RNFL and GCL thickness. In such a manner, Abdellatif and Fouad found that the duration of migraine was negatively correlated with superior and inferior RNFL and GCL, while the severity of migraine showed a significant negative correlation with inferior and temporal RNFL and the superior and inferior GCL . Also, Martinez and colleagues reported a similar negative correlation between the severity and duration of migraine and RNFL thickness .
The disparity between the results of the current study and previous studies may be attributed to differences in methodology and sample size, mean age of the population, ethnic variations, and the absence of standardized migraine characteristics, including length of migraine history, severity, and frequency of attacks.