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As seen in horizontal saccadic palsy, the impairment of the contralateral side gaze is caused by the disrupted pathways coming from the PPRF, while the "half" impairment is from the signal passing through the medial longitudinal fascicles not being able to reach its target. One-and-a-Half syndrome is normally associated with horizontal gaze.
Horizontal gaze palsies can be caused by a lesion affecting any structure in these pathways. Lesions to abducens nucleus or PPRF typically create an ipsilateral gaze palsy, while lesions to MLF typically cause Internuclear ophthalmoplegia , a type of horizontal gaze palsy in which the affected eye cannot adduct in conjugation with the ...
A conjugate eye movement is a movement of both eyes in the same direction to maintain binocular gaze (also referred to as “yoked” eye movement). This is in contrast to vergence eye movement, where binocular gaze is maintained by moving eyes in opposite directions, such as going “cross eyed” to view an object moving towards the face.
With the abducens nucleus it makes up the horizontal gaze centre. [1] It is situated in the pons adjacent to the abducens nucleus . [ 2 ] It projects to the ipsilateral abducens (cranial nerve VI) nucleus, and contralateral oculomotor (cranial nerve III) nucleus [ note 1 ] to mediate conjugate horizontal gaze and saccades .
The conjugate gaze is the motion of both eyes in the same direction at the same time, and conjugate gaze palsy refers to an impairment of this function. The conjugate gaze is controlled by four different mechanisms: [4] the saccadic system that allows for voluntary direction of the gaze; the pursuit system that allows the subject to follow a ...
More formally, it is characterized by "a conjugate horizontal gaze palsy in one direction and an internuclear ophthalmoplegia in the other". [1] [2] Nystagmus is also present when the eye on the opposite side of the lesion is abducted. Convergence is classically spared as cranial nerve III (oculomotor nerve) and its nucleus is spared bilaterally.
The paramedian pontine reticular formation (PMPRF) is involved in coordinating horizontal conjugate eye movements and saccades. To do so, besides projecting to the ibsilateral abducens nucleus, the PMPRF projects fibers through the MLF to the contralateral oculomotor nucleus (specifically, those of its motor neurons that innervate the medial rectus muscle).
Some of the frontopontine fibers participate in a pathway contributing to horizontal conjugate gaze: [1]: 404 Visual cortex → frontal eye fields (of the middle frontal gyrus) → frontopontine fibers → (contralateral) paramedian pontine reticular formation → (ipsilateral) abducens nucleus and (contralateral) oculomotor nucleus