Neuro-ophthalmology questions of the week: NOI13 Diplopia 9.4 – Internuclear & Supranuclear Lesions

Questions:
38. Where are eye movements initiated?
39. Which of the following can cause a patient to be unable look to the left?
      A. Right FEF, B. Left FEF, C. Right PPRF, D. Left PPRF
40. Where are smooth pursuit eye movements generated?
41. What is the function of inputs to the visual system from the vestibular nuclear complexes?
42. What is the cause of skew deviation?

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Questions with answers:
38. Where are saccadic eye movements initiated?
Saccadic eye movements are initiated in the frontal lobe and parietal lobe eye fields.

39. Which of the following can cause a patient to be unable look to the left?
      A. Right FEF, B. Left FEF, C. Right PPRF, D. Left PPRF
      If a patient cannot look to the left this can be due to A. Right FEF and/or D. Left PPRF dysfunction.
Saccadic eye movements (fast conjugate eye movements to a fixed target) are initiated in the frontal and parietal lobes (frontal and parietal eye fields).
● The horizontal saccadic pathway is a crossed pathway. Pathways from the frontal and parietal eye fields descend via the superior colliculus into the brainstem and cross at the level of the midbrain-pontine junction to synapse on the contralateral PPRF.
● The PPRF projects to the ipsilateral sixth nerve nucleus. From the sixth nerve nucleus, axons of abducens motor neurons travel to the ipsilateral lateral rectus muscle, whereas axons of sixth nerve interneurons cross over and ascend in the contralateral MLF to the medial rectus subnucleus of the third nerve.
● This internuclear connection between the sixth nerve nucleus and the contralateral third nerve nucleus via the MLF is responsible for conjugate horizontal gaze.
● Each frontal eye field, therefore, generates a conjugate movement of the eyes toward the contralateral side of the body.
● Brainstem pathways for vertical saccades involve the riMLF, the posterior commissure (PC) and nucleus of the PC, and the INC

40. Where are smooth pursuit eye movements generated?
Smooth pursuit eye movements are generated in higher cortical centers, especially the parieto–occipital–temporal junction.

41. What is the function of inputs to the visual system from the vestibular nuclear complexes?
Conjugate gaze in both the vertical and horizontal planes is stabilized through inputs from the vestibular nuclei.
From each vestibular nuclear complex, axons subserving horizontal gaze-holding send an excitatory connection to the contralateral sixth nerve nucleus and an inhibitory projection to the ipsilateral abducens nucleus; motor neurons from the sixth nerve nucleus innervate the lateral rectus, whereas interneurons cross over to ascend in the MLF to the third nerve nucleus. Stimulatory input from each vestibular nucleus, therefore, produces conjugate horizontal gaze toward the contralateral side of the body.

42. What is the cause of skew deviation?
Skew deviation is caused by abnormal prenuclear vestibular input to the ocularmotor nuclei.
Inputs from the vestibular nuclei also influence vertical gaze-holding through inputs (some via the MLFs) to the contralateral fourth nerve nucleus, third nerve nucleus, INC, and riMLF. The maintenance of ocular alignment in the vertical plane is controlled by the existence of balanced inputs from the vestibular nuclei to the fourth nerve nucleus (which innervates the contralateral superior oblique muscle), the superior rectus subnucleus (innervates the contralateral superior rectus), and the inferior oblique and inferior rectus subnuclei (innervate the ipsilateral inferior oblique and inferior rectus). An imbalance between these inputs to the various subnuclei results in skew deviation.

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The information below is from Neuro-ophthalmology Illustrated-2nd Edition. Biousse V and Newman NJ. 2012. Theme


Control of the Saccade System
Saccadic eye movements (fast conjugate eye movements to a fixed target) are initiated in the frontal and parietal lobes (frontal and parietal eye fields).
● The horizontal saccadic pathway is a crossed pathway. Pathways from the frontal and parietal eye fields descend via the superior colliculus into the brainstem and cross at the level of the midbrain-pontine junction to synapse on the contralateral PPRF.
● The PPRF projects to the ipsilateral sixth nerve nucleus. From the sixth nerve nucleus, axons of abducens motor neurons travel to the ipsilateral lateral rectus muscle, whereas axons of sixth nerve interneurons cross over and ascend in the contralateral MLF to the medial rectus subnucleus of the third nerve.
● This internuclear connection between the sixth nerve nucleus and the contralateral third nerve nucleus via the MLF is responsible for conjugate horizontal gaze.
● Each frontal eye field, therefore, generates a conjugate movement of the eyes toward the contralateral side of the body.
● Brainstem pathways for vertical saccades involve the riMLF, the posterior commissure (PC) and nucleus of the PC, and the INC.

Slow Saccades
Differential diagnosis of slow saccades includes the following:
● Lesions in the pons and the cerebellum
  ○ Spinocerebellar ataxias
  ○ Lesions of the PPRF
  ○ Internuclear ophthalmoplegia
  ○ Paraneoplastic syndrome
● Lesions in the midbrain
  ○ Progressive supranuclear palsy
  ○ Whipple disease
● Lesions in the basal ganglia
  ○ Parkinson syndromes
  ○ Huntington disease
  ○ Creutzfeldt-Jakob disease
● Miscellaneous
  ○ Medications (anticonvulsants, benzodiazepines)
  ○ Wilson disease
  ○ Lipid storage diseases

Control of the Pursuit System
Smooth pursuit eye movements (conjugate maintenance of fixation of the eyes while following a moving target) are generated in higher cortical centers, especially the parieto-occipital-temporal junction.
Inputs are sent from each parieto–occipital–temporal junction to the superior colliculus (SC), which mediate control of horizontal and vertical pursuit eye movements.
Descending horizontal gaze pursuit fibers synapse on the sixth nerve nucleus directly without synapsing in the PPRF.
Unlike the saccadic system, in which each hemisphere (frontal eye fields and other centers) produces conjugate horizontal eye movements toward the contralateral direction because of crossing of the saccadic pathways in the brainstem, the pursuit system is designed such that each hemisphere controls conjugate pursuit eye movements to the ipsilateral visual space.

Vestibulo-ocular System
Conjugate gaze in both the vertical and horizontal planes is stabilized through inputs from the vestibular nuclei.
● From each vestibular nuclear complex, axons subserving horizontal gaze-holding sends an excitatory connection to the contralateral sixth nerve nucleus and an inhibitory projection to the ipsilateral abducens nucleus; motor neurons from the sixth nerve nucleus innervate the lateral rectus, whereas interneurons cross over to ascend in the MLF to the third nerve nucleus. Stimulatory input from each vestibular nucleus, therefore, produces conjugate horizontal gaze toward the contralateral side of the body.
● Inputs from the vestibular nuclei also influence vertical gaze-holding through inputs (some via the MLFs) to the contralateral fourth nerve nucleus, third nerve nucleus, INC, and riMLF. The maintenance of ocular alignment in the vertical plane is controlled by the existence of balanced inputs from the vestibular nuclei to the fourth nerve nucleus (which innervates the contralateral superior oblique muscle), the superior rectus subnucleus (innervates the contralateral superior rectus), and the inferior oblique and inferior rectus subnuclei (innervate the ipsilateral inferior oblique and inferior rectus). An imbalance between these inputs to the various subnuclei results in skew deviation.

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