Brain Functional Connectivity Changes in Patients with Acute Eye Pain: A Resting-State Functional Magnetic Resonance Imaging (fMRI) Study

Background

Eye pain is the most common eye symptom in patients with ocular diseases, and many lesions of the eyeball and its adnexa can cause significant eye pain. Eye pain is not only related to symptoms of the eye, but can also be accompanied by headache [1]. Keratitis or corneal ulcers can cause severe eye pain. Studies have shown that the probability of human infection with infectious keratitis is 0.148% [2]. In addition, studies have shown that patients who experience keratectomy often have corneal pain symptoms [3]. The cornea is a structure with rich innervation in the human body, and its main sensory and nutrient nerves are the trigeminal ocular branches. Currently, the main method of treating keratitis or corneal ulcers is the use of antibiotics; however, there is no effective way to alleviate the eye pain associated with these conditions [4].

The direct cause of corneal pain is damage to the corneal nerves that triggers neural sensory networks, which are associated with cluster headaches [1]. Experimental studies in mice have shown that the pain caused by damaged corneal nerve fibers can activate neurons in the anterior cingulate cortex, rostroventral medulla, and insular cortex [5]. Research has also found that corneal pain can be related to local activation of the primary somatosensory cortex [6]. In addition, studies have confirmed that patients with chronic eye pain may have psychological or mental disorders [7,8], such as anxiety and depression [9].

Resting-state functional magnetic resonance imaging (rs-fMRI) was first developed in the 1990s and is becoming an increasingly important imaging technique because it can perform functional imaging of brain tissue [10]. FMRI has a wide range of applications. Blood oxygen level-dependent imaging (BOLD), diffusion tensor imaging, and magnetic resonance spectrum analysis can be used in fMRI studies. At present, rs-fMRI is widely used in a variety of diseases, such as epilepsy [11], Alzheimer’s disease [12], schizophrenia [13], and attention-deficit hyperactivity disorder [14]. However, the rs-fMRI method is a type of acquisition technology and cannot provide data related to the connectivity of the whole brain network. The pathophysiological basis underlying whole brain information processing remains unclear.

Voxel-wise degree centrality (DC) is a voxel-level graph-based network measurement tool. This technique takes each element as a node and then calculates the number of nodes connected to the rest of the nodes, indirectly reflecting the location and importance of the nodes or brain regions in the brain network. DC represents the direct connection sum of a given voxel to the rest of the entire brain voxel without the need for a priori nodes or regions of interest [15]. Therefore, the node with a high DC value can be regarded as the hub of information integration; that is to say, the node with high DC value is a node with substantial direct connection with other nodes. This technique is often used as a network measurement to explore the pathophysiological mechanisms of ocular diseases such as strabismus [16] and glaucoma [17]. The aim of this study was to explore the relationship between spontaneous brain activity and clinical characteristics in patients with acute eye pain and the differences in functional connection between healthy subjects and patients with eye pain.

Material and Methods

PARTICIPANTS:

In this study, we recruited 15 patients with acute eye pain (EP group) and 15 healthy controls (HC group) from The First Affiliated Hospital of Nanchang University. There were 10 men and 5 women in both groups. The groups were matched for age, sex, and education level.

The including criteria of all participants were as follows: (1) able to undergo MRI scanning; (2) no other systemic diseases; (3) no mental illness (anxiety, depression, bipolar disorder, or other mental disorders), and (4) no cerebral infarction.

Inclusion criteria for patients in the EP group were as follows: (1) acute condition with corneal ulcer or keratitis and (2) without other types of eye diseases. Exclusion criteria for the EP group were as follows: (1) long-term, chronic eye pain (>3 months); (2) eye pain caused by glaucoma or trauma; (3) with severe, related complications; (4) history of use of painkillers; (5) with pain. Inclusion criteria for the HC group were as follows: (1) no ocular diseases and (2) corrected visual acuity (>1.0).

The study was approved by the medical ethics committee of the First Affiliated Hospital of Nanchang University, and strictly followed the principles of the Declaration of Helsinki.

PARAMETERS FOR FMRI:

Participants in the study were scanned with a Trio 3-Tesla MRI scanner. To obtain the structure image, we used parameters to modulate the gradient echo sequence, as shown in Table 1.

FMRI DATA ANALYSIS:

We used DPARSFA 3.0 and SPM8 to process the data. The procedure for pre-processing included the following steps: (1) the first 10 volumes for each participant was discarded because of the participant’s adaptability to the scanning noise and the signal instability; (2) the translation (mm) and rotation (degree) of each participant were estimated to obtain the exercise time after head movement correction; (3) covariate effects of non-neuronal BOLD fluctuations (including cerebrospinal fluid and white matter signals) were rejected by using a linear regression process; (4) the 3×3×3 mm³ full-width images were smoothed to 6 mm, and we used the echo plane image template to standardize the fMRI images to satisfy the Montreal Neurological Institute (MNI) space criteria.

DEGREE CENTRALITY:

Each voxel was utilized as a node, and the related voxels were utilized as edges to generate a voxel functional network for each participant. Preprocessed functional images were used for voxel correlation analysis in the condition of DPARSFA’s default brain barrier (with a voxel size of 3×3×3 mm3 and a resolution of 61×73×61 in the MIN-152 standard space). In addition, the Pearson correlation between the time courses of any pair of voxels was calculated, resulting in a 70831×70831 correlation matrix. Based on the individual voxel functional network, we calculated the significant super-threshold correlation number for each participant as a DC value. The relevant calculation equation was Zi=DCi-meanall/stdall, where Zi referred to the z-score of the i-th voxel, DCi referred to the DC value of the i-th voxel, and stdall referred to the standard deviation of DC values of all voxels in brain hood [18].

DATA ANALYSIS:

SPSS version 24.0 was used to perform independent sample t tests to show differences in clinical performance between the EP and HC groups. We used the SPM8 toolkit in GLM analysis to investigate the difference in DC values between the EP group and the HC group. Pearson correlation analysis was also used to explore the relationship between mean DC value and clinical manifestations. In addition, we used the hospital anxiety and depression scale (HADS), a self-assessment scale used to analyze the severity of anxiety and depression, and correlation analysis to explore the relationship between DC in different brain regions and anxiety and depression. P<0.05 was considered statistically significant and modified by the Gaussian random field theory. Our general statistical approach was as follows:

Results

DEMOGRAPHICS AND VISUAL MEASUREMENTS:

Comparison of clinical data between the EP group and the HC group revealed no significant differences in terms of age (P=0.796) or weight (P=0.845) (Table 2).

DC VALUES OF DIFFERENT BRAIN REGIONS:

The DC value of the left limbic lobe in the EP group was significantly lower than that of the HC group. The DC values of the left cerebellum posterior lobe, right cerebellum posterior lobe, left inferior temporal gyrus, left inferior parietal lobule, left precuneus were significantly elevated in the EP group (Figure 1A, Table 3). The mean change in DC of patients in the EP group and HC group are shown in Figure 1B.

ROC CURVE ANALYSIS:

The DC values in different regions of the brain were significantly different in the EP and HC groups, showing that DC values could be useful as markers for distinguishing between patients with eye pain and healthy controls. Results of the area under the curve (AUC) analyses were as follows: the left cerebellum posterior lobe (AUC 0.767); right cerebellum posterior lobe (AUC 0.770); left precuneus (AUC 0.749); left inferior temporal gyrus (AUC 0.796) (EP>HC) (Figure 2A), and left limbic lobe (AUC 0.749) (EP <HC) (Figure 2B).

CORRELATION ANALYSIS:

The visual analog scale (VAS) value of the eyes in the EP group was negatively correlated with the left limbic lobe signal value (r=−0.789; P=0.001) and positively correlated with the left inferior parietal lobule signal value (r=0.561; P=0.046). All participants completed the hads. We used SPSS version 24.0 to analyze the correlation between the values of HADS scores and DC. The results showed that the HADS scores and the DC value of left limbic lobe were negatively correlated (r=−0.837; P=0.001), as shown in Figure 3.

Discussion

ANALYSIS OF THE REDUCTION IN DC VALUE IN PATIENTS WITH EP:

The limbic lobe refers to the inner side of the cerebral hemisphere and the periorbital structure of the brainstem junction and the corpus callosum, consisting of the subcallosal area, endplate gyrus, cingulate gyrus, hippocampal convolution, hippocampus, and the dentate gyrus [22]. In a previous study, Broca studied the brains of primates and found that the limbic lobe was associated with olfaction; however, more recent studies have found that this lobe carries out many more functions than first thought and represents an important center for regulating visceral activity [23]. Because the limbic lobe is closely related to the insular lobe, temporal pole, orbital gyrus of the cerebral cortex, and the amygdala under the cortex in terms of structure and function, some authors have referred to these structures as the limbic system [24]. The limbic system is related to memory and emotional output. Its main functions are that the hippocampus, as part of the Papez circuit, participates in memory [25] and the amygdala plays roles in emotional response, memory, and drive. The main clinical manifestations of lesions in the limbic system are epilepsy, confusion, and cognitive impairment [26]. In our present study, we found that the DC value of the left limbic lobe (LLL) of patients in the EP group was significantly lower than that of the HC group (see spot no. 6 in Figure 4). We suggest that EP may cause dysfunction of the LLL, which may explain the emotional irritability, instability, and headache of patients with eye pain. In addition, according to the correlation analysis shown in Figure 3C, we can hypothesize that LLL is related to anxiety and depression.

We also found that the VAS value of the eyes of the EP group showed a negative correlation with the signal value of the LLL (r=−0.789; P=0.001) and that the VAS value reflected the degree of pain in patients with eye pain. We further speculated that the reduced signal value of the LLL may reflect that the eyes of the patients in the EP group were damaged.

ANALYSIS OF THE INCREASED DC IN PATIENTS WITH EYE PAIN:

The posterior part of the cerebellum can receive signals from the cerebral cortex to participate in motor coordination [27], manifesting in the formation and proficiency of delicate movements. Injury to the posterior cerebellum causes a reduction in muscle tension and coordination of voluntary movements [28], which manifests mainly in the accuracy and coordination of movement. Studies have shown that the cerebellum is responsible for performing accurate eye movements [29] and dealing with visual movements. Purkinje cells transmit the only output of the cerebellar cortex to the deep cerebellar nucleus [30], which can cause ataxia when the Purkinje cells are injured. We found that the DC in the cerebellar posterior lobe was increased in patients in the EP group (see spots no. 4 and 5 in Figure 4). This suggests that there may be dysfunction of the posterior cerebellar lobe in patients with eye pain. We suggest that the enhancement of cerebellar functional connections may be related to eye pain-related sensory inputs. Recurrent headache, dizziness, and nausea may therefore be related to an abnormal posterior cerebellar lobe.

The inferior temporal gyrus is located below the temporal transverse gyrus. This area participates in visual processing and may also be related to facial perception and data identification. The occipitotemporal gyrus is related to the recognition of visual objects [31]. The inferior temporal cortex includes the middle and inferior temporal gyrus and can process visual stimuli and participate in memory and recognition of objects [32]. That is to say, this region recognizes the color and shape of an object and stores the information and memory related to the object, so as to be able to identify the object. Prosopagnosia is a disease in which patients cannot recognize and distinguish faces. A previous study reported the case of a patient who was able to identify objects normally but could not recognize faces; this patient was subsequently found to have a lesion on the right fusiform gyrus [33]. In other words, part of the inferior temporal gyrus had lesions, a main causes of the symptoms of such patients [34]. We think the reason eye pain can increase the activity of the left inferior temporal gyrus may the compensatory mechanism of brain visual abnormality in patients with eye pain (spot no. 3 in Figure 4).

The inferior parietal lobe (IPL) is located below the interparietal fissure of the parietal lobe and includes the angular gyrus and supramarginal gyrus. The IPL is involved in the interpretation of sensory information and the perception of facial stimuli [35]. It is also related to semantic processing [36] and mathematical operations [37]. Gerstmann syndrome can be caused by IPL lesions, particularly in the dominant hemisphere; the symptoms of this condition are left and right confusion, finger agnosia, dyslexia, dyscalculia, and contralateral hemianopia [38]. It has been reported that most patients with schizophrenia exhibit an abnormal structure of the IPL [39], such as reduced gray matter volume and thinned cortex [40], and that these patients have most of the above symptoms. In addition, apraxia can be caused by lesions of the supramarginal gyrus and angular gyrus [41], which manifests in the absence of paralysis and deep sensory disturbances, whereby the limbs have an operational barrier [42]. In addition, the left inferior parietal lobe (LIPL) contributes to the recognition of visual words [43]. In our present study, we found that the DC value of the LIPL in patients in the EP group was significantly elevated, suggesting that eye pain may cause LIPL dysfunction (spot no. 2 in Figure 4).

We also observed that the VAS value of the eyes in the EP group showed a positive correlation with the signal value of the left inferior parietal lobule (r=0.561; P=0.046). We suspect that when the degree of pain was higher, the signal value of the brain region was also, but this hypothesis needs to be tested further in future studies. We also speculate that an increased signal value of the LIPL may reflect damage to the eyes of patients.

The precuneus is in the medial part of the parietal lobe. This is an important area for visual spatial integration, episodic memory recovery, and self-awareness [44]. The impact of alterations of different brain regions is shown in Table 4. Neuroimaging studies have shown that the structure and function of the precuneus are abnormal in patients with obsessive-compulsive disorder [45], while other studies have reported a positive correlation between the gray matter of the left precuneus and harmful symptoms [46]. Studies have also shown that auditory verbal hallucinations in patients with schizophrenia can be associated with changes in the structure and function of the left precuneus [47]. Other studies have shown that the posterior cingulate and precuneus are the initial locations for the development of Alzheimer’s disease [48]. In addition, the precuneus is the judgment center of verbal politeness [49] and can assess inconsistencies between tactile and visual texture information [50]. A summary of previous studies is presented in Table 5. In the present study, we found a significant elevation in DC value of the left precuneus of patients in the EP group, suggesting that eye pain may lead to over activation of left precuneus activity (spot no. 1 in Figure 4). Therefore, we speculate that patients with eye pain will have hyperfunction of visual spatial integration, and dizziness, vomiting, and obsessive-compulsive personality, among other symptoms. Our team has made significant progress in this area in previous research [51–57].

LIMITATIONS:

It should be noted that there are still many limitations in this study. The sample size of this study is small, and there may be deviation; therefore, we need to use a larger sample size to explore these concepts in future studies. Also, because the subjects were all from the same hospital, there may be collection bias.

Conclusions

In conclusion, this study showed that patients with eye pain had abnormal spontaneous activity in many brain regions compared with healthy control participants, which provided insight into the neurological variation in patients with eye pain and facilitated the interpretation of the potential mechanisms underlying eye pain. Patients with eye pain should not only pay attention to physical therapy; psychological therapy should also be involved as early as possible. We hope that our results can be beneficial to the treatment of patients with eye pain.