Ocular myasthenia revisited: Insights from pseudo-internuclear ophthalmoplegia

Neuromuscular junction disorders: mimics and chameleons

Neuromuscular junction (NMJ) disorders represent a heterogenous group of acquired and congenital disorders that present in variable and distinctive ways. The diagnosis is typically reached through a combination of clinical, serological, pharmacological and electrophysiological evaluation. While the diagnosis can be fairly straightforward in some cases, the overlap with other neurological disorders can make diagnosis challenging, particularly in pure ocular presentations and in seronegative patients. The over-reliance on serological tests and electrophysiological evaluation in isolation can lead to misdiagnosis. In this article, we provide an overview of the NMJ disorders, discuss red flags for the key differential diagnoses (mimics) and report the atypical ways in which NMJ disorders may present (chameleons).

Myasthenia gravis presenting as bilateral pseudointernuclear ophthalmoplegia in a patient with an incidental prolactinoma

Myasthenia gravis (MG) is a rare and potentially dangerous autoimmune condition, which affects the acetylcholine receptors at the neuromuscular junction of skeletal muscle. MG's diverse symptomatology may readily masquerade as other neurological conditions, posing a diagnostic challenge to clinicians. We describe a 24-year old man who presented to the emergency department with a new onset internuclear ophthalmoplegia. After a series of investigations, we eventually arrived at a diagnosis of MG with pseudointernuclear ophthalmoplegia with an incidentally detected prolactinoma. We explore the literature regarding the pathophysiology of pseudointernuclear ophthalmoplegia, the link between prolactin and autoimmunity and the association between prolactinoma and MG.

Myasthenia Gravis With Reversible Pyramidal Tract Damage and Pseudo Internuclear Ophthalmoplegia. A Case Report and Literature Review

Myasthenia gravis (MG) is a rare and treatable antibody-mediated autoimmune disease. Pseudo internuclear ophthalmoplegia (-INO) or pyramidal tract damage is rarely observed in MG, and there were no known cases of MG with both pseudo-INO and pyramidal tract damage. Here, we report a case of a 61-year-old female suffering from MG accompanied by pseudo-INO and pyramidal tract damage with a rapid progressive course. Her blood and cerebrospinal fluid (CSF) tests were normal, except for the presence of the anti-acetylcholine receptor antibody. CT and contrast enhancement of the chest showed a thymic involution. MRI and contrast enhancement images of the brain and whole spine were normal. Both the clinical response to the administration of neostigmine and the repetitive nerve stimulation test were positive. The motor evoked potentials at lower limb recordings were normal. According to her signs, symptoms, decrementing response on repetitive stimulation test, elevated anti-acetylcholine receptor antibody and positive response to neostigmine, the patient was diagnosed as having MG. After treatment with pyridostigmine, intravenous immunoglobulin, prednisone acetate tablets and methotrexate, all her symptoms disappeared, including pseudo-INO and pyramidal tract damage. To our best knowledge, this is the first report of a case of MG with both pseudo-INO and pyramidal tract damage. Based on our case and a review of the literature, we propose that pyramidal tract damage and pseudo-INO can be two signs of MG, and that MG can cause damage to other systems besides neuromuscular junctions.

Fixation stability of the upward gaze in patients with myasthenia gravis: an eye-tracker study

Objective

To quantify fixation stability of the upward gaze in patients with myasthenia gravis (MG) using an eye tracker.

Methods and analysis

In this study, 21 normal subjects, 5 patients with MG with diplopia, 5 patients with MG without diplopia and 6 patients with superior oblique (SO) palsy were included. Subjects fixated on a target in the upward direction for 1 min. The horizontal (X) and vertical (Y) eye positions were recorded using an eye tracker. Fixation stability was first quantified using the bivariate contour ellipse areas (BCEA) of fixation points as an index of whole stability. Then, the SDs of the X and Y eye positions (SDX and SDY, respectively) were quantified as indices of directional stability, with the data divided into three 20 s fractions to detect temporal fixation fluctuation.

Results

BCEAs were larger in patients with MG (both with and without diplopia) than normal subjects and patients with SO palsy, without significant differences among the three 20 s fractions. Compared with normal subjects, SDXs were larger only in patients with MG with diplopia; SDYs were larger in both patients with MG with and without diplopia. In addition, SDYs in patients with MG with diplopia were larger than those in patients with MG without diplopia and patients with SO palsy. Furthermore, a significant difference among the three 20 s fractions was detected for SDYs in patients with MG with diplopia.

Conclusion

Patients with MG, especially those with diplopia, exhibit fixation instability in the upward gaze. Non-invasive quantification of fixation stability with an eye tracker is useful for precisely identifying MG-specific fatigue characteristics.

Trial registration number

UMIN000023468; pre-results.

Distance between intramuscular nerve and artery in the extraocular muscles: a preliminary immunohistochemical study using elderly human cadavers

PURPOSE

Extraocular muscles are quite different from skeletal muscles in muscle fiber type and nerve supply; the small motor unit may be the most well known. As the first step to understanding the nerve-artery relationship, in this study we measured the distance from the arteriole (25-50 μm in thickness) to the nerve terminal twigs in extraocular muscles.

MATERIALS AND METHODS

With the aid of immunohistochemistry for nerves and arteries, we examined the arteriole-nerve distance at 10-15 sites in each of 68 extraocular muscles obtained from ten elderly cadavers. The oblique sections were nearly tangential to the muscle plate and included both global and orbital aspects of the muscle.

RESULTS

In all muscles, the nerve twigs usually took a course parallel to muscle fibers, in contrast to most arterioles that crossed muscles. Possibly due to polyinnervation, an intramuscular nerve plexus was evident in four rectus and two oblique muscles. The arteriole-nerve distance usually ranged from 300 to 400 μm. However, individual differences were more than two times greater in each of seven muscles. Moreover, in each muscle the difference between sites sometimes reached 1 mm or more. The distance was generally shorter in the rectus and oblique muscles than in the levator palpebrae muscle, which reached statistical significance (p < 0.05).

CONCLUSIONS

The differences in arteriole-nerve distances between sites within each muscle, between muscles, and between individuals might lead to an individual biological rhythm of fatigue in oculomotor performance.

Life-threatening misdiagnosis of bulbar onset myasthenia gravis as a motor neuron disease: How much can one rely on exaggerated deep tendon reflexes

The autoimmune disease myasthenia gravis (MG), can mimic a variety of neurological disorders leading to a delay in diagnosis and treatment. On occasions, misdiagnosis of MG could lead to unnecessary therapeutic interventions. We report the case of a 50 year-old man, in whom MG was mistaken for motor neuron disease (MND). Subsequently, correct diagnosis and optimal management resulted in saving his life and significant improvement in his functional status. We discuss the importance of considering MG as one of the potential differential diagnoses among cases of new onset or recurrent unexplained bulbar symptoms, despite exaggerated deep tendon reflexes. Also, a literature review on the misdiagnosis of MG and the potential pitfalls in MG diagnosis are discussed.

Neuromuscular transmission failure in myasthenia gravis: decrement of safety factor and susceptibility of extraocular muscles

An appropriate density of acetylcholine receptors (AChRs) and Na(+) channels (NaChs) in the normal neuromuscular junction (NMJ) determines the magnitude of safety factor (SF) that guarantees fidelity of neuromuscular transmission. In myasthenia gravis (MG), an overall simplification of the postsynaptic folding secondary to NMJ destruction results in AChRs and NaChs depletion. Loss of AChRs and NaChs accounts, respectively, for 59% and 40% reduction of the SF at the endplate, which manifests as neuromuscular transmission failure. The extraocular muscles (EOM) have physiologically less developed postsynaptic folding, hence a lower baseline SF, which predisposes them to dysfunction in MG and development of fatigue during "high performance" eye movements, such as saccades. However, saccades in MG show stereotyped, conjugate initial components, similar to normal, which might reflect preserved neuromuscular transmission fidelity at the NMJ of the fast, pale global fibers, which have better developed postsynaptic folding than other extraocular fibers.

Misdiagnosis of Myasthenia Gravis and Subsequent Clinical Implication: A case report and review of literature

The autoimmune disease, myasthenia gravis (MG), can mimic a variety of neurological disorders leading to a delay in diagnosis and treatment. On occasions, misdiagnosis of MG could lead to unnecessary and potentially harmful therapeutic interventions. We report on a 12 year-old boy, in whom MG was mistaken for meningitic sequelae and subsequently for critical neuropathy/myopathy resulting in considerable morbidity for nearly a decade. Subsequent correct diagnosis and optimal management resulted in significant improvement in his functional status. We discuss the importance of considering MG as one of the potential differential diagnoses among cases of recurrent respiratory pump failure, or unexplained bulbar symptoms where documentary proof of the previous diagnoses including work-up for MG is lacking. We also review the literature on MG misdiagnosis and highlight the potential pitfalls in MG diagnosis.

Curtains

A 55-year-old immunocompetent man presented with headache, nausea, progressive bilateral upper lid ptosis, and diplopia. Examination showed bilateral asymmetric upper lid ptosis with limited adduction and elevation of both eyes. Cranial magnetic resonance imaging revealed enhancing intra-axial and extra-axial midbrain lesions. Blood and cerebrospinal fluid were positive for cryptococcal antigen and cerebrospinal fluid fungal cultures grew Cryptococcus neoformans. Treatment with liposomal amphotericin B and flucytosine resulted in complete resolution of his neurological deficits and lesions on neuroimaging. Patients with cryptococcal meningitis may rarely present with bilateral cranial nerve III dysfunction.

Infranuclear ocular motor disorders

This chapter covers the very large number of possible disorders that can affect the three ocular motor nerves, the neuromuscular junction, or the extraocular muscles. Conditions affecting the nerves are discussed under two major headings: those in which the site of damage can be anatomically localized (e.g., fascicular lesions and lesions occurring in the subarachnoid space, the cavernous sinus, the superior orbital fissure, or the orbit) and those in which the site of the lesion is either nonspecific or variable (e.g., vascular lesions, tumors, "ophthalmoplegic migraine," and congenital disorders). Specific comments on the diagnosis and management of disorders of each of the three nerves follow. Ocular motor synkineses (including Duane's retraction syndrome and aberrant regeneration) and disorders resulting in paroxysms of excess activity (e.g., neuromyotonia) are then covered, followed by myasthenia gravis and other disorders that affect the neuromuscular junction. A final section discusses disorders of the extraocular muscles themselves, including thyroid disease, orbital myositis, mitochondrial disease, and the muscular dystrophies.

Pseudo-internuclear ophthalmoplegia as a sign of overlapping myasthenia gravis in a patient with 'intractable' hypothyroidism

We describe a 52-year-old man with a history of increasing fatigability and gait disturbances that were first attributed to hypothyroidism. On examination, he had bilateral pseudo-internuclear ophthalmoplegia with weakness of adduction and abducting nystagmus. Convergence was also impaired and he showed proximal weakness of the limb. Intravenous edrophonium almost completely abolished the nystagmus and adducting muscle weakness, and improved the strength of proximal muscles groups. The clinical response to the administration of edrophonium, the presence of AChR binding antibodes and the repetitive nerve stimulation test findings indicated that the patient had pseudo-internuclear ophthalmoplegia as a manifestation of generalized myasthenia gravis. Whereas hypothyroidism was effectively controlled with levothyroxine, his subsequent response to thymectomy, prednisolone and cholinesterase inhibitors confirmed the diagnosis of generalized myasthenia gravis.

Changes in dynamic and kinematic properties of saccades in ocular myasthenia following intravenous immunoglobulin treatment

The dynamics and kinematics of saccades in a patient with severe ocular myasthenia were studied before and after treatment with intravenous immunoglobulin (IVIG). Before therapy, horizontal saccades were hypometric, but faster than similar-sized saccades made by normal subjects. During a 5-minute test period, saccades decreased in size (fatigue effect), but remained faster than those of controls. Listing's plane of the eye with greater ophthalmoplegia was increased in thickness. After IVIG treatment, the range of eye movements improved, but saccades remained faster than those of controls. Also, no fatigue was observed and the thickness of Listing's plane was reduced toward the normal range. Increased peak velocity, despite progressive hypometria due to fatigue, supports the hypothesis that the pale global extraocular muscle fibers are relatively spared in myasthenia. Involvement of other extraocular muscle-fiber types leads to limited range of eye movements and an increase in the thickness of Listing's plane.

Dynamics of primate oculomotor plant revealed by effects of abducens microstimulation

Despite their importance for deciphering oculomotor commands, the mechanics of the extraocular muscles and orbital tissues (oculomotor plant) are poorly understood. In particular, the significance of plant nonlinearities is uncertain. Here primate plant dynamics were investigated by measuring the eye movements produced by stimulating the abducens nucleus with brief pulse trains of varying frequency. Statistical analysis of these movements indicated that the effects of stimulation lasted about 40 ms after the final pulse, after which the eye returned passively toward its position before stimulation. Behavior during the passive phase could be approximated by a linear plant model, corresponding to Voigt elements in series, with properties independent of initial eye position. In contrast, behavior during the stimulation phase revealed a sigmoidal relation between stimulation frequency and estimated steady-state tetanic tension, together with a frequency-dependent rate of tension increase, that appeared very similar to the nonlinearities previously found for isometric-force production in primate lateral rectus muscle. These results suggest that the dynamics of the oculomotor plant have an approximately linear component related to steady-state viscoelasticity and a nonlinear component related to changes in muscle activation. The latter may in part account for the nonlinear relations observed between eye-movement parameters and single-unit firing patterns in the abducens nucleus. These findings point to the importance of recruitment as a simplifying factor for motor control with nonlinear plants.

Diagnosing disconjugate eye movements: phase-plane analysis of horizontal saccades

BACKGROUND

Saccades are fast eye movements that conjugately shift the point of fixation between distant features of interest in the visual environment. Several disorders, affecting sites from brainstem to extraocular muscle, may cause horizontal saccades to become disconjugate. Prior techniques for detection of saccadic disconjugacy, especially in internuclear ophthalmoparesis (INO), have compared only one point in abducting vs adducting saccades, such as peak velocity.

METHODS

We applied a phase-plane technique that compared each eye's velocity as a function of change in position (normalized displacement) in 22 patients with disease variously affecting the brainstem reticular formation, the abducens nucleus, the medial longitudinal fasciculus, the oculomotor nerve, the abducens nerve, the neuromuscular junction, or the extraocular muscles; 10 age-matched subjects served as controls.

RESULTS

We found three different patterns of disconjugacy throughout the course of horizontal saccades: early abnormal velocity disconjugacy during the first 10% of the displacement in patients with INO, oculomotor or abducens nerve palsy, and advanced extraocular muscle disease; late disconjugacy in patients with disease affecting the neuromuscular junction; and variable middle-course disconjugacy in patients with pontine lesions. When normal subjects made disconjugate saccades between two targets aligned on one eye, the initial part of the movement remained conjugate.

CONCLUSIONS

Along with conventional measures of saccades, such as peak velocity, phase planes provide a useful tool to determine the site, extent, and pathogenesis of disconjugacy. We hypothesize that the pale global extraocular muscle fibers, which drive the high-acceleration component of saccades, receive a neural command that ensures initial ocular conjugacy.

Dynamic properties of eye movements in mitochondrial chronic progressive external ophthalmoplegia

AIMS

To relate dynamic properties of eye movements to visual disability in a patient with chronic progressive external ophthalmoplegia (CPEO) due to mitochondrial DNA deletion.

METHODS

Visual function was evaluated by neuro-ophthalmological examination and visual function index (VF-14). Eye movements were measured using the magnetic search coil technique.

RESULTS

The patient denied diplopia or oscillopsia at rest or during locomotion. His range of eye movements was limited to <12 degrees horizontally, and <19 degrees vertically with smaller movements of the left eye. All classes of movements were impaired, but the eyes accelerated to over 1000 degrees /s/s at the onset of saccades and in response to brisk head rotations.

CONCLUSIONS

Diplopia and oscillopsia were suppressed, probably due to visual system adaptation during the slow disease progression. We postulate that relative sparing of pale global extraocular fibres accounted for preserved acceleration of saccades and vestibular eye movements.

Disorders of saccades

Saccades are rapid eye movements that assist vision by pointing the fovea of the retina, which contains the highest density of photoreceptors, at features of interest in the visual environment. A great deal is now known about the properties and neurobiology of saccades in both health and disease states. They have consequently become a valuable diagnostic and research tool. In this review, we describe the common saccadic disorders and their causes. We also highlight recent insights into the pathophysiologic mechanisms underlying these disorders and discuss how these insights have helped increase our understanding of the saccadic system as a whole.