Objective: To describe eye movements in a cohort of patients affected with Sialidosis type 1.

Background: Sialidosis, or Mucolipidosis type 1, is a rare autosomal recessive lysosomal storage disorder caused by mutations in NEU1, which encodes for neuraminidase. It usually presents in the second or third decade and it is characterized by myoclonic epilepsy, ataxia, hyperreflexia, vision loss and the characteristic “cherry red spot macula”. As in Sialidosis type 2, patients can have a variety of eye movement abnormalities. To date, a detailed analysis of eye movements in Sialidosis type 1 has not been reported.

Method: We characterized clinical features in three patients with Sialidosis type 1 and NEU1gene mutations. Eye movements were also assessed objectively with video-oculography (VOG) (EyeLink 1000+).

Results: Case 1 is a 19 y.o. woman with prominent lower limb myoclonus, dysmetria, and psychiatric features. The Eye movement exam was suggestive of square wave jerks and macrosaccadic oscillations, saccadic pursuit and ocular dysmetria. VOG revealed marked difficulty with target following due to excess saccadic intrusions and oscillations, dynamic overshoots of saccades (saccadic hypermetria), and double saccadic pulses. Case 2 is a 40 y.o. man with significant upper limb myoclonus. The eye movement exam was suggestive of subtle downbeat and gaze-evoked nystagmus, saccadic hypermetria, an abnormal trajectory of vertical saccades, and saccadic pursuit. VOG data were too unreliable to be useful. Case 3 is a 31 y.o. man with ataxia, upper limb myoclonus, and distal lower limb neuropathy. Eye movement exam and VOG showed hypometric saccades and saccadic pursuit, as well as mild right sixth nerve dysfunction and convergence insufficiency. All patients had vision loss and a cherry red spot macula.

Conclusion: Patients affected with Sialidosis type 1 (“cherry red spot myoclonus”) may present with variable phenotypes, including eye movement impairment. Despite phenotypic differences,, the majority of eye movement deficits localize to the cerebellum. Eye movements can be invaluable in localization and in diagnosing rare conditions and can be explored as possible biomarkers in the setting of clinical trials.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/looking-cherry-red-spot-myoclonus-in-the-eyes/