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Title

Think metabolic! Biotidinase deficiency mimicking neuromyelitis optica

Purpose	Neuromyelitis optica spectrum disorders (NMOSDs) and biotidinase deficiency (BD) both involve optic atrophy and spinal cord lesions. However, BD starts in early childhood and only very rarely presents in adulthood.
Methods	A full ophthalmic and neurological workup
Results	A 19-year-old male patient was referred for further investigation of profound, painless, binocular visual loss evolving over 2 months, exacerbated by physical activity. Best-corrected visual acuity was 0.16 in the right eye and 0.1 in the left eye. Fundoscopy revealed a pale and atrophic temporal side of the optic disc confirmed by nerve fibre layer loss on OCT. Otherwise, he was in general good health. Brain MRI directed towards a NMOSD diagnosis. However spinal cord MRI, chest CT, cerebrospinal fluid examination and biochemical tests for other auto-immune, infectious, toxic and genetic causes (LHON) were all found to be negative. During the workup he developed spastic paraparesis. Urgent treatment with methylprednisolone followed by plasmapheresis did not relieve symptoms. Further exhaustive biochemical screening showed a reduced biotidinase serum activity (<5%). Biotin substitution was started with slow visual improvement up to 0.8 but normal gait and reflexes.
Conclusion	Adult-onset BD may mimic inflammatory disorders such as the NMOSD or other genetic disorders such as spastic paraplegia type 7 or LHON. When confronted with patients with a similar clinical course, the absence of laboratory confirmation of the former disorders should guide towards low-cost enzymatic screening.
Conflict of interest	No

Authors 1

Last name	IMSCHOOT
Initials	J
Department	UZ Gent, Dpt of Ophthalmology
City	Gent

Authors 2

Last name	Van Iseghem
Initials	V
Department	UZ Gent, Dpt of Neurology

Authors 3

Last name	Sprengers
Initials	M
Department	UZ Gent, Dpt of Neurology

Authors 4

Last name	Sindic
Initials	CJM
Department	UCL
City	Louvain

Authors 5

Last name	Willekens
Initials	B

Authors 6

Last name	Hemelsoet
Initials	D
Department	UZ Gent, Dpt of Neurology

Authors 7

Last name	Laureys
Initials	G
Department	UZ Gent, Dpt of Neurology

Authors 8

Last name	De Zaeytijd
Initials	J
Department	UZ Gent, Dpt of Ophthalmology
City	Gent