INTRODUCTION
Multiple sclerosis is a common cause of disability in young adults (main productive age group)1. Although MS has been considered a primary demyelinating disease of the central nervous system (CNS), axonal loss is of critical importance because this pathological change may appear early in the disease course and may be potentially correlated with the neurological deficits2,3. Current investigational modalities such as non conventional magnetic resonance imaging (MRI) techniques used to measure axonal loss in MS, have remarkable limitations with insufficient resolution3,4.
The retinal nerve fibre layer (RNFL) is the only part of the CNS where axons can be directly visualized in-vivo. It contains the unmyelinated axons of the retinal ganglion cells which converge at optic disc forming the optic nerve fibre which becomes myelinated just behind the lamina cribrosa. Since the RNFL is composed only of unmyelinated axons, measuring the RNFL thickness represents a viable method of monitoring axonal loss in these patients with MS5,6.
The retinal nerve fibre layer is of particular interest in MS not only because optic neuritis may be an important event in establishing its diagnosis, but also because optic nerve dysfunction may occur subclinically in many other MS patients as evidenced by diminished contrast sensitivity and prolonged latency of visual evoked potential8-13.
Optical coherence tomography (OCT) is a recent non invasive, non contact, and reproducible objective technique that performs cross sectional imaging of internal tissue microstructure at a high resolution of ~2-10mm by measuring the echo-time delay of back scattered infrared light using an interferometer and a low coherence light source14,15. It can be used to quantitate accurately the thickness of the peripapillary nerve fibre layer and macula16. Because of its potential role in defining axonal loss and in assessing longitudinal changes in RNFL throughout the disease course17,18, the aim of this study is to identify the RNFL changes in MS patients with and without optic neuritis and to assess the correlation between such RNFL changes in these patients and the physical as well as the cognitive disability.
SUBJECTS AND METHODS
Thirty six right handed patients diagnosed to have definite MS according to Poser et al.19 criteria were selected to be enrolled in this study from neurology outpatient and inpatient Clinics of Neurology Department Zagazig University Hospital. Only MS patients with no history of optic neuritis or with history of single attack of acute unilateral optic neuritis (blurring of vision, color vision disturbance) with no recurrence and with good visual recovery were included in this study. The mean age of the patients was 31.2±6.8 (17-43 years), 23 female and 13 male with mean duration of education 12.2±4.7.
Additionally, thirty age, sex and educationally matched right handed healthy subjects without any known ophthalmic or systemic disease were selected as a control group. Subjects with aphasia, severe hearing or visual impairment, disturbed conscious level "e.g. delirium", concomitant other neurological or medical, metabolic disease known to affect cognition such as severe head trauma, CNS infection, brain tumors, hepatic failure, renal failure or hypothyroidism were excluded. Furthermore, patients with previous history of primary psychiatric disorder known to cause cognitive impairment e.g. major depression according to Hamilton depression scale20, history of drug intake known to affect mentality (e.g. antipsychotics or alcohol) or any other systemic or neurologic disease were also excluded from this study. Patients with any anterior segment eye diseases or any posterior segment disorders such as glaucoma, macular degeneration or other optic neuropathies or significant ametropia > 6 diopters were excluded.
All patients and controls included in this study were subjected to the following:
I. Full history with complete general and neurological examination focusing on age, sex handedness, education, special habits as well as onset, duration and course "relapsing remitting (RRMS), 2ry progressive (SPMS), primary progressive (PPMS), begin" of MS. Physical disability was rated using Kurtzke expanded disability status scale (EDSS) score21. Patients were divided into two groups according to EDSS with cutting off value 3.5. All cases underwent cerebral MRI after triple dose gadolinium diethylene-triaminepentra acetic acid using (0.5-TESLA general electric sigma contour system). Positive MRI brain images were defined according to the criteria of Fazekas et al.22, as the appearance of three or more ellipsoid demyelinating lesions whose diameter was 3 mm or more, within the brain of these lesions at least one lesion was evident to be within the eriventricular white matter or brain stem in a diameter of 6 mm or more.
II. Battery of seven neuropsychological tests for cognition were performed using Arabic version of the Wechsler Adult Intelligence Scale Revised (WAIS-R)23, Digit Span Test, Similarities Test, Vocabularies Test. Also, Verbal Fluency Test (VFT)24, Benton Visual Retention Test (BVRT)25, Story Recall Test26, Paced Auditory Serial Addition Test (PASAT)27 were used. The selected neuropsychological test battery consisted of measures of verbal intelligence, memory, abstract conceptual reasoning, attention, concentration, language, and visuospatial skills. A grading system developed by Camp et al.28 was applied to each patient test score, considering the number of standard deviations (SDs) below the control mean. In details, grade O was given when the patient scored at or above the control mean, grade 1 when the patient scored at or above 1 SD below the control mean, and grade 2 when the patient scored 2 or more SDs below the control mean. Patients were considered affected by impairment of a given cognitive domain when they obtained grade 2 score in at least one of the tests assessing the corresponding function. Patients who obtained grade 2 score in three or more tests were considered affected by overall cognitive impairment. Furthermore, the patients were divided according to Amato et al.29 into 2 subgroups based on the number of tests failed: mildly impaired (3-5 failed test) and moderately impaired (>5 failed tests).
III. Full ophthalmological examination was done in the Ophthalmology Department of Zagazig University Hospital including full history; best corrected Snellen's visual acuity, slit lamb biomicroscopy of the anterior and posterior segments, direct and indirect ophthalmolscopy and intraocular pressure measurement using applanation tonometry. Optical Coherence Tomography (OCT) images were acquired using Fourier-Domain Spectral OCT (Optovue RTVue 100 system, California, USA) (Fig. 1) with axial resolution of 5 mm, transverse resolution of 15 mm and scan rate 26,000 A-scan per second. All subjects were scanned with dilated pupil using 1% tropicamide, the Optovue OCT device and software were used to acquire three 3.45 mm diameter peripapillary circular scans centered on the optic disc for each eye. Throughout scanning the patients kept their eyes constantly fixed on an internal target provided by the equipment. One eye was randomly selected from each MS patient without ON or controls and the affected eye was selected from each MS patient with history of unilateral ON. The mean was used to express RNFL thickness as a single overall average value for the whole 360° scan (Fig. 2).
Statistical analysis:
Data were checked and entered for statistical analysis by using SPSS (version II). Data were expressed as mean ± standard deviation. Student "t" test, ANOVA test and post hoc test were used as appropriate. P<0.05 was considered statistically significant.
RESULTS
Multiple sclerosis disease duration ranged from 1-11 years with mean of 4.3±3.6 years. The course of MS was relapsing remitting in 24 patients (66.7%), secondary progressive 9 patients (25%) and primary progressive in 3 patients (8.3%).
Expanded disability status scale scores ranged from 1.5-7.5 with mean of 4.4±2.4 with EDSS >3.5 in 14 patients. Cognitive impairment was evident in 14 patients 39%. Ten patients had 3-5 failed tests "mild cognitive impairment" and four patients had >5 failed tests "moderate cognitive impairment". History of acute unilateral optic neuritis was present in 9 patients (25%) with history of blurring of vision, colour vision disturbance and temporal pallor of the optic disc.
Although the mean overall RNFL average thickness in MS patient with ON was significantly lower compared with those patients without ON, the mean RNFL average thickness was significantly lower in both MS patients with or without ON compared with the control (P<0.001) (Table 1) (Fig. 3 a & b).
The mean overall RNFL average thickness was significantly reduced in all types of MS course compared with the controls (P<0.01). However it is more significantly reduced in those patients with progressive course either primary or secondary. Regarding physical disability the mean overall RNFL average thickness was significantly lower in MS patients with more severe physical disability with EDSS > 3.5 (P<0.01) (Table 2).
The Mean RNFL average thickness was significantly lower in MS patients with impaired cognition compared with those patients with intact cognition (P<0.05). Furthermore mean RNFL average thickness was significantly reduced in MS patients with more severe cognitive impairment with >5 failed tests compared with those patients with mild cognitive impairment (3-5) failed tests (P<0.05) (Table 3).
DISCUSSION
Multiple sclerosis is a chronic neurodegenerative disease with inflammatory demyelination of the CNS. The social impact of M.S disability is substantial because it may result in loss of employment with social isolation and dependence on care providers1,6,30. Although MS is a primary demyelinating disease of CNS, axonal loss is critical pathological event that may appear early in the disease course2,7. Because of the potential correlation between the neurological deficits in MS and axonal loss, in-vivo diagnosis and monitoring of axonal loss is of vital importance3,5. However, there are limited abilities to achieve definite diagnostic criteria of this axonal loss in- vivo using the indirect non conventional MRI techniques including MRI spectroscopy with inadequate resolution and high cost4. This growing need to have an easy, accurate, reproducible, more direct and economic technique for in-vivo studying of axonal degeneration in MS patient14,15 was the stimulus of this work which aimed at exploring the OCT measured retinal nerve fibre layer thickness changes (as a viable biomarker of axonal loss) in MS patients and to assess its correlation with the physical disability as well as cognitive impairment in such patients.
The present study demonstrated a significant reduction of the mean overall RNFL average thickness in MS patients with and without optic neuritis (P<0.001) when compared with normal controls. These results are in agreement with that of Trip et al.10, who also reported a significant reduction of RNFL average thickness in MS patients with and without ON 33% & 27% respectively (P<0.001) when compared with their controls. This axonal degeneration in the RNFL of patients with MS could also be detected before any visual signs or symptoms of ON and has been associated with electrophysiological abnormalities namely prolonged latency of VEP (visual evoked potential)12,13. Such axonal loss is not any evident in early MS but also appear to progress slowly even among those patients with clinical recovery or with no symptoms8,17. All these data may support the potential role of OCT measured RNFL thickness to become a mode for studying axonal loss in MS patients5,9.
Regarding the correlation between mean overall RNFL average thickness and MS course as well as physical disabilities the mean over all RNFL average thickness was significantly reduced in all types of MS course compared with the controls (P<0.01). However, it is more significantly reduced in those patients with progressive course either primary or secondary. Furthermore, mean overall RNFL average thickness was significantly lowered in MS patients with more severe physical disability with EDSS >3.5 (P<0.01) in this study. These findings are in accordance with that of Toledo et al.31, who mentioned that OCT measured RNFL thickness seems to be a sensitive technique to identify changes associated with MS evolution. On the other hand, a significant negative correlation has been also found between N-acetyl asparate (NAA) intensity signals using MR spectroscopy and functional disability using EDSS in MS patients3,4. All these data help to clarify the correlation between the axonal loss in MS patients and the severity of physical disability5,31.
Cognitive impairment is a significant source of disability in MS patients affecting about 30-70% of them and may occur early in the disease course1,32. In this study the incidence of cognitive impairment in MS patients was 39% and the mean overall RNFL average thickness was significantly lower in MS patients with impaired cognition compared with those patients with intact cognition (P<0.05). Also, the mean overall RNFL average thickness reduction was significantly correlated with more severe cognitive impairment with >5 failed tests (P<0.05). These findings are comparable to the results of Toledo et al.31, who also reported a significant correlation between RNFL atrophy and cognitive impairment in MS patients.
Conclusion
This study not only augments the role of OCT as a useful tool in the diagnosis and monitoring axonal loss of RNFL in MS patients with and without optic neuritis but also supports it as a sensitive biomarker for the correlation with physical and cognitive deficits evolving during MS progress. Future directions towards the use of retinal OCT in longitudinal clinical trials to examine the possible neuroprotective and other disease modifying therapies for such MS patients is recommended.
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