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P Apkarian

Publications and source records attributed to P Apkarian.

At least 19 recordsLinked to original sources
PubMed2003

Isolated absence of optic chiasm revealed by congenital nystagmus, MRI and VEPs.

Congenital nystagmus is a rare condition mainly characterised by rhythmic, conjugate, and horizontal oscillations of both eyes that persist in the vertical gaze. This disorder is usually noticed in the neonatal period and persists throughout life. It can be of sensory origin, associated with low visual acuity of various causes, or of motor origin, caused by a defect in the slow eye movement system. The former can be genetically determined. It can also be associated with several conditions, the most frequent being albinism. The achiasma syndrome has recently been recognized in two patients as an autosomal recessive inherited cause of congenital nystagmus. We report the case of the so far youngest reported baby having been diagnosed with the isolated achiasmatic condition, which presented with congenital nystagmus and see-saw nystagmus, and discuss its clinical findings and 18 months follow-up. The achiasmatic syndrome should be included in the differential diagnosis of congenital nystagmus, as all the described cases presented like that. Complete investigations should be performed to allow the best evolution and follow-up of these children.

Evoked Potentials, Visual↗
PubMed2001

See-saw nystagmus and congenital nystagmus identified in the non-decussating retinal-fugal fiber syndrome.

PURPOSE: The purpose of the present study was to accurately document the oculomotor misalignments and instabilities associated with the non-decussating retinal-fugal fiber syndrome, a rare inborn, isolated, achiasmatic condition. To date, the achiasmatic syndrome described has thus far been identified in three unrelated females. A comparable achiasmatic condition has also been identified in a canine breed. MATERIALS AND METHODS: The unique, inborn visual pathway malformation in the form of an isolated absence of the optic chiasm was confirmed by metabolic assay, the visual evoked potential (VEP) misrouting paradigm and neuroradiological evaluation, including magnetic resonance imaging (MRI). Horizontal and vertical, left and right eye, oculomotor scleral search coil recordings were measured, in one of the achiasmats, at 15 years of age. Comparable oculomotor recordings were implemented in three controls, including a normal age-matched control, an adult control and an adult with idiopathic congenital nystagmus (CN). In addition to central and eccentric target fixation with binocular and monocular viewing, binocular and monocular pursuit and optokinetic nystagmus (OKN) also were recorded. RESULTS: Following a binocular and monocular fixation paradigm, classic congenital nystagmus (CN) profiles were recorded in the achiasmat in the horizontal planes together with see-saw nystagmus (SSN) in the vertical planes; an alternating esotropia and vertical tropias also were documented. The CN showed interocular conjugacy and waveforms typically exhibited well-defined foveation periods. In general, both achiasmatic horizontal and vertical eye movements as well as corresponding interocular misalignments varied with stimu-lus and recording conditions. Moreover, horizontal eye movements showed sustained interocular conjugacy while vertical eye movements typically showed disconjugacy. Previous oculomotor studies in the achiasmatic canine breed revealed comparable oculomotor instabilities, albeit the latter were reported to present primarily with unyoked eye movements and uniocular saccades. In the present study, in addition to target fixation with binocular and monocular viewing, binocular and monocular pursuit and OKN responses also were recorded. For the achiasmat, high-gain pursuit results, as with eccentric fixation, emphasized a preference for fixation with the adducting eye. Also at eccentric gaze with monocular viewing, the eye under cover showed, relative to the fixating eye, a hypodeviation concomitant with adduction and a hyperdeviation concomitant with abduction. OKN profiles in the achiasmat with stimuli presented under both normal viewing and open-loop, retinal-field locked conditions proved rather complex; however, when an OKN response was identified, reversed OKN tracking was not apparent. CONCLUSIONS: In general, the oculomotor results reported herein demonstrate that the non-decussating retinal-fugal fiber syndrome is an afferent visual pathway disorder associated with vertical and horizontal misalignments concurrent with congenital nystagmus and also with the rarer see-saw nystagmus.

Adolescent↗
PubMed2000

Visual function and brain organization in non-decussating retinal-fugal fibre syndrome.

Functional neuroimaging, psychophysical and electrophysiological investigations were performed in a patient with non-decussating retinal-fugal fibre syndrome, an inborn achiasmatic state in which the retinal projections of each eye map entirely to the ipsilateral primary visual cortex. Functional magnetic resonance imaging (fMRI) studies showed that for monocularly presented simple visual stimuli, only the ipsilateral striate cortex was activated. Within each hemisphere's striate cortex, the representation of the two hemifields overlapped extensively. Despite this gross miswiring, visual functions that require precise geometrical information (such as vernier acuity) were normal, and there was no evidence for the confounding of visual information between the overlapping ipsi-lateral and contralateral representations. Contrast sensitivity and velocity judgments were abnormal, but their dependence on the orientation and velocity of the targets suggests that this deficit was due to ocular instabilities, rather than the miswiring per se. There were no asymmetries in performance observed in visual search, visual naming or illusory contour perception. fMRI analysis of the latter two tasks under monocular viewing conditions indicated extensive bilateral activation of striate and prestriate areas. Thus, the remarkably normal visual behavior achieved by this patient is a result of both the plasticity of visual pathways, and efficient transfer of information between the hemispheres.

Adolescent↗
PubMed1999

Motoric response inhibition in finger movement and saccadic eye movement: a comparative study.

OBJECTIVE: To study cortical potentials associated with suppression of intended motoric actions. METHODS: Electro-encephalographic activity was recorded in a Go/NoGo reaction time paradigm. Subjects viewed computer-generated pacing stimuli, which provided information concerning the time at which an imperative Go/NoGo signal occurred. A motoric response was required following Go stimuli while motoric response inhibition was required following NoGo stimuli. To examine whether previously reported 'Go/NoGo effects' on event related potential (ERP) components may be generalized across movement modalities, the present experimental paradigm was performed with either finger movement or saccadic eye movement as required motoric response. RESULTS: For both movement modalities, comparable differences in the morphology, amplitude and scalp topography of ERP components were observed between Go trials, with proper movement execution, and NoGo trials, with complete suppression of motoric activity. In addition, for either movement modality a similar 'error related negativity' (ERN) was found for NoGo trials in which motoric activity was present. CONCLUSIONS: The results of the present study suggest that cortical activity underlying the Go/NoGo differences in ERP components represent general cortical processing associated with detection and/or suppression of inappropriate response behaviour, independent of movement modality.

Adult↗
PubMed1998

Inter-hemispheric lateralization of event related potentials; motoric versus non-motoric cortical activity.

To study hemispheric lateralization of cortical potentials associated with motoric and non-motoric function, cortical activity was recorded accompanying either finger extension or saccadic eye movements in a contingent negative variation (CNV) paradigm. Subjects viewed computer-generated pacing stimuli, presented in the left visual hemi-field, and were instructed to either initiate or inhibit a motor response following an imperative signal. Motoric lateralization was assessed by means of the lateralized readiness potential (LRP). In addition, a measure complementary to the LRP was introduced to investigate non-motoric lateralization (NML). Contralateral inter-hemispheric lateralization was evident in the LRP preceding finger movement, but was absent prior to eye movements. However, pre-saccadic cortical response profiles did exhibit a right hemispheric, non-motoric lateralization (NML) during stimulus presentation. Comparable non-motoric lateralization was found for finger extension. Results of the present study suggest that non-motoric lateralization may be a contributing factor to the frequently reported inter-hemispheric asymmetry preceding self-initiated saccadic eye movements. Results of the present study also suggest that the latter may be related to a covert shift of visuospatial attention toward the saccadic target. Associated shifts of attention are suppressed in a CNV paradigm, where attentional focus is primarily on the CNV stimulus during the pre-saccade period.

Adult↗
PubMed1996

Selective broad-band spatial frequency loss in contrast sensitivity functions. Comparison with a model based on optical transfer functions.

PURPOSE: Contrast sensitivity functions (CSFs) were measured under various optical conditions in healthy observers together with CSFs from selected patients. Threshold increases across the spatial frequency range were compared with predictions of a theoretical optical model based on modulation transfer functions. METHODS: Contrast thresholds for various spatial frequencies were determined with a computer-automated method of ascending limits in a control group and a group of patients with various visual pathway diseases ranging from retinal disorders, such as diabetic retinopathy, to neural disorders, such as multiple sclerosis. For normal control subjects, simulated contrast sensitivity losses also were effected by manipulating pupil diameter and dioptric blur. Modulation transfer functions of the eye's optics in polychromatic light were calculated. The wave aberration function included standard spherical aberration, coma, and small amounts of irregular aberrations. RESULTS: Experimentally, slight dioptric blur (e.g., 0.4 to 0.75 D) introduced increased CSF thresholds within either a narrow or broad bandwidth. For the latter, decreased CSF sensitivity occurred across a spatial frequency range as broad as 1 log unit, from low spatial frequencies (2 cyc/deg), and for pupil sizes equal to or larger than 3 mm. Predictions based on an optical model are qualitatively and quantitatively in agreement with these findings. Contrast sensitivity losses of the patients were neither specific nor selective to the pathologic condition at hand. Furthermore, various CSF losses optically induced in the control subjects were indistinguishable from nonoptically induced pathologic CSF profiles. CONCLUSIONS: Selective broad-band contrast sensitivity loss may be optically induced by slight refractive error. As a result, selective contrast sensitivity loss at lower and intermediate spatial frequencies concurrent in patients with various pathologic, neuro-ophthalmologic conditions cannot be a priori attributed to neural factors without carefully controlled and well-defined optical variables.

Adult↗
PubMed1995

Non-decussating retinal-fugal fibre syndrome. An inborn achiasmatic malformation associated with visuotopic misrouting, visual evoked potential ipsilateral asymmetry and nystagmus.

We report a newly identified syndrome in which nasal retinal fibres fail to decussate due to the inborn absence of an optic chiasm. Visual evoked potential (VEP) assessment and neuro-opththalmic evaluation in two unrelated, non-albino children revealed the unusual visual pathway anomaly in the form of misrouted retinal-fugal projections. Monocular VEP responses across the occiput, regardless of stimulus mode (full- or partial-field pattern onset, pattern reversal, luminance flash or high temporal frequency luminance flicker) showed unequivocal evidence of pathological VEP ipsilateral asymmetry. Marked attenuation of primary visual evoked responses from the occiput contralateral to the eye of stimulation, indicative of aberrant contralateral retinal-fugal projections, was confirmed by MRI which depicted the remarkable achiasmatic condition. MRIs and neurological evaluation also confirmed the absence of accompanying congenital or acquired brain malformations or anomalies. Ophthalmic evaluation revealed that both achiasmatic children had reduced distance acuity for age, alternating esotropia, torticollis, head tremor and ocular motor instability; visual fields were normal. Eye movements were also monitored and indicated congenital nystagmus waveforms in the horizontal plane; see-saw nystagmus was observed in the the horizontal plane; see-saw nystagmus was observed in the vertical and torsional planes. The age range of the two children during evaluation and follow-up, over a 6-year period, was about 4-15 years. Comparisons of VEP responses from age-matched normal, albino and idiopathic congenital nystagmus controls, recorded under the same VEP test conditions, were also performed. In contrast to the achiasmatic ipsilateral inter-ocular asymmetry, the albinos showed the expected monocular VEP topography pattern of contralateral asymmetry. Also as expected, VEP profiles from the normal controls and those with congenital nystagmus, evinced no aberrant asymmetry patterns. In general, the results indicate that the VEP misrouting protocol is indispensable for the non-invasive electrophysiological detection and differential diagnosis of optic pathway mutations and may well identify individuals with purported idiopathic congenital nystagmus or albinism that are, in fact, achiasmatic.

Adolescent↗
PubMed1994

A unique achiasmatic anomaly detected in non-albinos with misrouted retinal-fugal projections.

In mammals with binocular vision, projections of retinal axons to primary retino-recipient nuclei establish a strict visuotopic and eye-segregated arrangement. Normal primate visual pathway organization is characterized by orderly hemiretina separation in which nasal-retinal axons cross at the optic chiasm and project to primary contralateral subcortical and cortical structures while temporal-retinal fibres project ipsilaterally to corresponding visual structures. We report here, in two unrelated children, an unusual visual pathway malformation in which nasal-retinal cortical projections, unable to decussate due to the inborn absence of an optic chiasm, erroneously route ipsilaterally to visual projection targets. We have termed this newly documented achiasmatic condition the non-decussating retinal-fugal fibre syndrome (Apkarian et al., Invest. Ophthalmol. Vis. Sci., 34, Suppl., 711, 1993).

Adolescent↗
PubMed1994

Electrodiagnosis in paediatric ophthalmogenetics.

In the present overview, practical application of the visual evoked potential (VEP) in paediatric neuro-ophthalmology is described across a wide range of ophthalmogenetic disorders, including albinism, Pelizaeus-Merzbacher disease and spastic paraplegia. The VEP approach is based on a four parameter subdivision of the electrophysiological response which includes, (1) amplitude (microV), (2) latency (ms), (3) waveform (component specificity), and (4) topography (potential distribution across the electrode array). In the case studies presented, evoked potential measures provide clinically useful and even at times invaluable insights concerning the presence, extent and type of visual pathway compromise. The four parameter subdivision along with age matched normative standards provides a sensitive and reliable means to facilitate detection and diagnosis of sensory anomalies in the developing visual system. The non-invasive electrophysiological assessment of visual function and its maturational course also provides early identification of affected family members as well as an objective measure of various treatment regimes which, particularly for the pre-verbal and non-verbal child, is critical for effective patient care.

Child↗
PubMed1994

VEP in albinism.

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Adolescent↗
PubMed1993

Visual evoked potential characteristics and early diagnosis of Pelizaeus-Merzbacher disease.

OBJECTIVE: Early diagnosis of Pelizaeus-Merzbacher disease; assessment of disease progression. DESIGN: Pediatric neuro-ophthalmology evaluation of visual function with pattern and luminance visual evoked potentials; behavioral state assessment; electrophysiological diagnostic test; baseline estimates at the age of 11 weeks; 1-year follow-up. SETTING: University hospital electrodiagnostic vision research laboratory. PATIENTS CASE STUDY: X-linked Pelizaeus-Merzbacher disease; leukodystrophy; patient tested at the age of 11 weeks and at the age of 58 weeks for follow-up; five age-matched normal controls. INTERVENTIONS: Physical therapy, medication, evaluation. MAIN OUTCOME: Early diagnosis; diagnostic confirmation; objective monitor of disease progression. RESULTS: Abnormal spatial and temporal vision and abnormal visual pathway maturation; visual evoked potentials of proband with Pelizaeus-Merzbacher disease were grossly abnormal, reflecting myelination disorder. CONCLUSIONS: Visual evoked potential pediatric electrodiagnosis yields reliable measures of visual function and visual system maturation in leukodystrophy.

Behavior↗
PubMed1993

Refinement of the localization of the X-linked ocular albinism gene.

Although physical and genetic mapping studies assigned the X-linked ocular albinism gene to Xp22.3, the exact gene order in this region is still unclear. We present additional genetic mapping data concerning X-linked ocular albinism that suggests the consensus order Xpter-STS-DXS237-KAL-(OA1, DXS143)-DXS85-DXS16-Xcen.

Albinism, Ocular↗
PubMed1993

Contrast sensitivity function in Graves' ophthalmopathy and dysthyroid optic neuropathy.

Contrast sensitivity function was measured by a computer automated method on 38 eyes with dysthyroid optic neuropathy and 34 eyes with Graves' ophthalmopathy only. The results were compared with 74 healthy control eyes. Disturbances of contrast sensitivity functions were found in both groups when compared with controls. The eyes affected with dysthyroid optic neuropathy showed pronounced loss of contrast sensitivity in the low frequency range, which facilitates differentiation between the two groups.

Adult↗
PubMed1993

Visual evoked response asymmetry only in the albino member of a family with congenital nystagmus.

PURPOSE: To examine the purported relationship between visual fiber misrouting and congenital nystagmus (CN) by studying a family containing members with either hereditary CN alone or in conjunction with albinism. METHODS: Eight relatives in three generations of a family with two genetic disorders (congenital nystagmus [CN] and albinism) underwent complete ophthalmologic examination and visual evoked potential (VEP) assessment of visual pathway organization using a luminance flash and checkerboard pattern onset/offset stimulus paradigm. Age-matched controls patients (albino, CN, or normal) corresponding to the three affected family members underwent the same procedure. RESULTS: The standard VEP albino misrouting test did not reveal any signs of abnormality in all family members tested except for the clinically diagnosed 8-month-old albino proband patient. Visual evoked potential assessment from the albino patient evinced contralateral asymmetry characteristic of aberrant temporal retinostriate projections. CONCLUSIONS: Although CN and albinism share some of the same clinical symptoms, these findings indicate that the disorders may be inherited and manifested independently in members of one family. Furthermore, the functional and/or structural factors responsible for CN cannot be readily ascribed to VEP retinal-cortical misprojections recorded in albinism.

Adult↗
PubMed1992

Detection and maturation of VEP albino asymmetry: an overview and a longitudinal study from birth to 54 weeks.

The genetic anomaly in albinism prevents adequate melanin metabolism within the fetal eye cup and stalk. This results in severe disruption of pre- and postnatal retinal development and the condition of abnormal temporal retinal projections. The obligate misrouting of retinal-geniculate-cortical projections in albinism can be detected in the topographical representation across the occiput of the visual evoked potential (VEP). Age-dependent misrouting detection methods are described which yield 100% detection rates with zero false positives across the life span. By combining appropriate state-defined neonatal recording procedures with the albino infant VEP test paradigm, the presence of aberrant optic pathway projections was observed in a 5-day-old full-term infant. Maximum asymmetry was observed within a long-latency window of the response which shifted during the postpartum period to shorter latencies. Longitudinal studies show two specific latency regions of significant VEP asymmetry. The first occurs within 40-70 ms after stimulus onset and remains constant across the age range. The second, more robust, cluster of asymmetry occurs within a longer latency window and shows an age-related shift towards shorter latencies. The decreasing latency of this asymmetry is concomitant with normal maturational changes of the evoked response. These results show that VEP misrouting can be extended to reliable albino diagnosis within the neonatal period and to the assessment of visual maturation.

Aging↗
PubMed1992

A practical approach to albino diagnosis. VEP misrouting across the age span.

In addition to the genetic heterogeneity in albinism, widespread clinical heterogeneity frequently impedes albino detection and differential diagnosis. Further, several auxiliary ocular and/or cutaneous manifestations of this inherited error of pigmentary metabolism are neither pre-requisite nor specific to the albino condition. However, one feature that is specific to albinism regardless of genotype or phenotype is a unique pattern of abnormal visual pathway organization. With an appropriate test paradigm, the albino visual pathway can be revealed by the non-invasive recording of the visual evoked potential (VEP) distribution across the occiput which shows contralateral hemispheric asymmetry following full field monocular stimulation. As described in this report, the VEP albino misrouting detection test has been refined to yield extraordinarily high sensitivity and selectivity across the age span from the neonate to the elderly. As the VEP profile undergoes maturational changes, these changes have been taken into account in the development of an albino age-range VEP test recipe which includes the pattern onset paradigm for older albinos and a luminance flash paradigm for the albino infant. The age appropriate optic pathway misrouting test provides reliable albino detection and definitive differential diagnosis. Further, as the albino VEP signature of contralateral asymmetry is also age specific, the VEP misrouting test can be extended to the objective assessment of visual pathway maturation.

Adolescent↗
PubMed1991

Effects of behavioural state on visual processing in neonates.

Assessment of visual capacity in full-term neonates as a function of behavioural state was investigated with the luminance flash visual evoked potential (VEP). Infant state (quiet sleep, active sleep, quiet wakefulness, active wakefulness) was determined by behavioural observation and polygraphic recording of EEG, eye movements, electrocardiogram, electromyogram and respiration. VEPs were recorded across the sleep/waking cycle only under carefully defined state conditions. The results indicate that state dramatically effects the amplitude, latency and waveform of the transient luminance flash VEP with the most significant differences between sleep and wakefulness. The results further suggest that controlled state recording reduces intra and intersubject VEP variability.

Apgar Score↗