"Staircase" saccadic intrusions plus transient yoking and neural integrator failure associated with cerebellar hypoplasia: a model simulation.
We present hypothesized ocular motor mechanisms of unique "staircase-like" sequences of saccadic intrusions in one direction that we have named, "staircase saccadic intrusions (SSI)," square-wave jerks/oscillations (SWJ/SWO), and transient failures of yoking and neural integrators in a patient with severe hypotonia, ataxic speech, motor and language developmental delays, and torticollis (Joubert syndrome). Brain magnetic resonance imaging showed hypoplasia of the cerebellar vermis and inferior cerebellar peduncles, abnormal superior cerebellar peduncles with deepening of the interpeduncular fossa, and enlargement of the fourth ventricle. During far and near fixation and smooth pursuit (rightward markedly better than leftward), the subject exhibited conjugate SSI (rightward more than leftward, with intersaccadic intervals equivalent to the normal 250 msec visual latency), SWJ, SWO, and uniocular, convergent and divergent saccades (including double saccades). Simulations using a behavioral ocular motor system model identified hypothetical mechanisms for SWJ, SWO, and SSI and ruled out the loss of efference copy as the cause. SSI may result from simultaneous dysfunctions: 1) a transient loss of accurate retinal-error information and/or sampled, reconstructed error; plus 2) a constant sampled, reconstructed retinal error that drives saccades.
Rucker
Janet C
JC
The Daroff-Dell'Osso Ocular Motility Laboratory, Louis Stokes Cleveland Veterans Affairs Medicla Center and Case Medical School, OH 44106, USA.
Dell'Osso
Louis F
LF
Garbutt
Siobhan
S
Jacobs
Jonathan B
JB
D002524:Cerebellar Ataxia / Q000503:physiopathology*; D002531:Cerebellum / Q000002:abnormalities*; D002648:Child; D003941:Diagnostic Techniques, Ophthalmological; D006801:Humans; D008607:Intellectual Disability / Q000503:physiopathology*; D008279:Magnetic Resonance Imaging; D008297:Male; D008954:Models, Biological; D009123:Muscle Hypotonia / Q000503:physiopathology*; D015835:Ocular Motility Disorders / Q000503:physiopathology*; D009801:Oculomotor Muscles / Q000503:physiopathology*; D012438:Saccades / Q000502:physiology*; D013577:Syndrome