Dose Finding Study of Recombinant Human Alpha-mannosidase for the Treatment of Patients With Alpha-mannosidosis

Inclusion Criteria: 1. The patient must have a confirmed diagnosis of alpha-Mannosidosis as defined by alpha-mannosidase activity \< 10% of normal activity in blood leukocytes 2. The patient must have an age at the time of screening ≥ 5 year and ≤ 21 years 3. The patient must have physical ability to perform 6-minutes walk test (6MWT), 3 minute-stair climb test (3MSCT) and pulmonary lung function test (spirometry, body plethysmography). 4. The patient must have the ability to mentally cooperate in the cognitive and motor function tests 5. The patient must have the ability to hear and follow a request. Hearing aids can be worn. 6. Patient or patient's legally authorized guardian(s) must provide signed, informed consent prior to performing any study-related activities (trial-related activities are any procedures that would not have been performed during normal management of the subject) 7. The patient and his/her guardian(s) must have the ability to comply with the protocol Exclusion Criteria: 1. The patient cannot walk without support. 2. Presence of known chromosomal abnormality and syndromes affecting psychomotor development, other than alpha-Mannosidosis 3. History of bone marrow transplantation 4. Presence of known clinically significant cardiovascular, hepatic, pulmonary or renal disease or other medical conditions that, in the opinion of the Investigator, would preclude participation in the trial 5. Presence of an ECHO with abnormalities within half a year that, in the opinion of the Investigator, would preclude participation in the trial 6. Any other medical condition or serious intercurrent illness, or extenuating circumstance that, in the opinion of the investigator, would preclude participation in the trial 7. Pregnancy 8. Psychosis within the last 3 months