Inclusion Criteria:

• At minimum, 4 years of age at Visit 1.
• Previously molecularly and/or enzymatically-confirmed PMM2-CDG Affected with ataxia evidenced by Mini International Cooperative Ataxia Rating Scale (Mini-ICARS) score > 0 at baseline.
• Parent or legal guardian available to provide consent on behalf of minor subjects or adult subjects who are unable to give informed consent due to developmental disabilities. Willingness of subject or legal guardian to provide consent.
• If female and > 10 years old, willing to practice abstinence or using an effective contraceptive regimen that is constituted by two acceptable effective methods of contraception, including a barrier method such as a condom or occlusive cap (diaphragm or cervical/vault cap) with spermicidal foam/gel/film/cream/suppository and an established non-barrier method such as oral, injected, or implanted hormonal methods, an intrauterine device, (IUD), or intrauterine system for the entire duration of the treatment period and for at least 28 days after receiving the last study drug dose. Sterilized or infertile subjects (defined as having undergone surgical sterilization, ie, bilateral tubectomy, hysterectomy and bilateral ovariectomy or as being postmenopausal, defined as at least 12 months of amenorrhea prior to enrollment) will be exempted from the requirements to use contraception in this study.

Exclusion Criteria:

• Hepatic impairment defined as AST/ALT > 5 x ULN in the last 12 months.
• Renal impairment defined as serum creatinine 1.5-1.9 x ULN for age OR ≥ 26.5 umol/l increase from ULN for age.
• Hypersensitivity to acetazolamide.
• Hypersensitivity to any of the components of the placebo.
• History of treatment with experimental drug within 28 days of Visit 1.
• Currently taking Mecamylamine, Sodium Phosphates, Salicylates, Mefloquine, Methenamine and other Carbonic Anhydrase Inhibitors.
• Participation in another therapeutic trial - the subject will not be permitted to participate in any other drug trial during the blinded phase and during the 28 days prior to Visit 1.
• Pregnant (positive urine beta-HCG test) or nursing (by history) women.