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Horizontal binocular saccadic failure: an early isolated sign of Demyelination

Southampton Eye Unit, Southampton General Hospital, UK

Correspondence: Aysha Salam
Mail Point 104, Department of Ophthalmology, Southampton General Hospital, Southampton, SO16 6YD E-mail: aysha_salam@yahoo.com

Date of acceptance: 21st September 2006

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Intro

Abnormal eye movements have been shown to provide a sensitive and reliable measure of brainstem and cerebellar involvement in patients with multiple sclerosis (MS). The optimisation of eye movements for visual demands is carried out by a constant array of neural impulses generated by a network of neurons in the brainstem and cerebellum [1]

The commonly described ocular motor syndromes in patients with MS are Internuclear Ophthalmoplegia (INO), abnormalities of saccades and smooth pursuit, spontaneous or positional nystagmus, as well as a part of isolated cranial nerve palsies.[2]

The stereotyped nature of saccadic eye movements (SEM) favours their reproducibility and hence measurement of saccadic parameters have been found to be preferable to smooth pursuit which can vary as a function of effort and attention [2] Saccades are described as the most sensitive clinical tests of the dynamic aspects of eye movements. We present an interesting case of a female patient who presented with asymmetric features of bilateral saccadic failure in the absence of any other systemic symptoms and was later on diagnosed as having a Clinically Isolated Syndrome of inflammatory demyelination. (CISs)

Case Report

A 48-year-old lady presented to the eye Casualty clinic with one-week history of difficulty in focusing at objects to her sides on changing fixation. It was especially pronounced on looking to the objects in her right field of vision. She had difficulty reading as well as driving especially when approaching roundabouts. She described her present condition as a relative slowing of her reaction time with failure to grasp visual information and thus being an unsafe driver.

She denied any symptoms of diplopia or blurred vision. She was fit and well in herself and her medical history was unremarkable. Her general Medical and neurological examinations were completely unremarkable. Ophthalmic examination showed visual acuities of 6/6 in either eye. Anterior segments and fundi were healthy, with no afferent pupillary defect.

Ocular motility showed orthophoria for distance and near vision. However, despite full smooth pursuit movements, there was marked slowing of her horizontal saccades to the left along with a virtual absence of saccades to the right. Her vertical saccades were normal. There were no signs of abduction nystagmus or an INO. There was no evidence of an extraocular muscle palsy. (See Video)

Her 120-point screening visual fields were full and an Ishihara plate colour testing was normal (21/21 OU). Her cerebrospinal Fluid (CSF) analysis was normal with absence of oligoclonal bands, no evidence of elevated immunoglobulin G (IgG) synthesis rate or IgG index. There were no malignant cells demonstrated. The CSF VDRL was negative. Pattern reversal visual evoked potentials (VEPs) showed normal conduction velocities MRI brain scan showed an abnormal signal within the pons along the floor of the fourth ventricle. The signal change was symmetrical about the midline and was not associated with any discernable mass effect. (Fig 1)

Comment:

Saccades are rapid conjugate eye movements aimed to redirect the fovea to a new target and achieve fixation. They are generated by burst cells which reside within the ipsilateral parapontine reticular formation. The neural signal for a saccade is a pulse of activity, which activates the ipsilateral abducens nucleus thus causing the ipsilateral eye to abduct. The axons of the abducens interneurons cross the midline and travel in the medial longitudinal fasciculus to stimulate the oculomotor nucleus concerned with the medical rectus function causing the contralateral eye to adduct.[3] Multiple sclerosis (MS) is currently diagnosed on the clinical demonstration of CNS white matter disease with multiple lesions occurring at different anatomical sites. Currently, the investigative procedures used in diagnosis of MS include MRI and CT scan, cerebrospinal fluid (CSF) examination and VEPs.[4]

Analysis of saccadic eye movements has been used to detect clinically silent lesions in MS since it offers the possibility of studying more than one neurophysiologic system. It also helps in carrying out a qualitative estimation of the CNS involvement as ocular motor syndromes often form a part of the CIS of inflammatory demyelination.[5, 6] Tedeschi et al. in their study involving 57 MS patients showed horizontal saccades to have a sensitivity of 76.3% in detecting clinically silent lesions due to MS, which was comparable to a sensitivity of 78.2% found by an MRI.[5] The frequent involvement of oculomotor system may be attributed to the close proximity of the oculomotor apparatus to the brainstem periventricular zones which include the fourth ventricle and the cerebral aqueduct of Sylvius.[6]

We describe an unusual pattern of CIS in a woman with the solitary finding of horizontal saccadic failure. The quantitative analysis of her eye movements in the absence of any other systemic or neurologic features proved to be a useful diagnostic tool for identifying a clinically silent CNS lesion. The MRI corroborated the clinical localization by revealing a demyelinating lesion in dorsomedial pontine tegmentum.

Her response to steroids was gratifying with relative normalization of SEM, which in our case proved to be of primary diagnostic significance. Although, the SEM findings were confirmed by higher investigations i.e, MRI, they can be especially invaluable in studying the function of the brain stem and medial longitudinal fasciculus that may not be very well visualized by the MRI and thus play an important role in identifying clinically silent CNS lesions in MS patients. Acknowledgement: We acknowledge this case report to Dr A Ditchfield who kindly helped in reading the MRI scan.

References

• Serra A, Derwebskus J, Leigh JR. Role of eye movement examination and subjective visual vertical in clinical evaluation of multiple sclerosis. J Neurol;2003;250:569-575
• Flipse J.P, Straathof C.S.M, Steen VJ, Van Leeuwen AF, Doorn PAV, Meche FGA, Collewijin H. Binocular saccadic eye movments in multiple sclerosis. Journal of Neurological Sciences. 1997;148::53-65
• Slamovitis TL, Hedges TR, Kupersmith MJ, Newman NJ, Sadun AA, Sedwick LA. Basic and Clinical Science Course. In: Neuro-Ophthalmology.Sect 5. American Academy of Ophthalmology, San Francisco, USA. 1996-1997;104-107
• Farlow MR, Markand ON, Edwards MK, et al. Multiple Sclerosis: Magnetic resonance imaging, evoked potentials, and spiral fluid electrophoresis. Neurolology 1986;36:828-31
• Tedeschi G, Allocca S,Costanzo AD, Dino A, Bonavita V. Roles of saccadic analysis in the diagnosis of multiple sclerosis in the era of magnetic resonance imaging. Journal of Neurology, Neurosurgery and Psychiatry 1989;52:967 -969
• Frohman EM, Frohman TC. Horizontal monocular saccadic failure: an unusual clinically isolated syndrome progressing to multiple sclerosis. Mult Scler. 2003 Feb;9(1):55-8.