Can the Characteristic Odor in MSUD Come from Sweat?
Yes, the characteristic maple syrup or burned sugar odor in Maple Syrup Urine Disease (MSUD) is present in sweat, along with urine and cerumen (earwax), as the volatile branched-chain ketoacids are excreted through multiple body fluids.
Sources of the Characteristic Odor
The pathognomonic sweet odor in MSUD originates from the accumulation of branched-chain ketoacids (BCKAs), which are volatile compounds that can be detected in multiple body secretions 1:
- Sweat and body odor: The burned sugar or maple syrup smell emanates from the patient's body and skin, as documented in multiple case reports where clinicians noted the odor "from the body" in addition to urine 2
- Urine: The most commonly recognized source, where the odor is typically most concentrated 1, 2
- Cerumen (earwax): Another characteristic location where the sweet odor can be detected 1
Clinical Recognition and Diagnostic Value
The odor serves as an important clinical clue, particularly in the neonatal period 2:
- Early presentation: The peculiar burned sugar smell from both body and urine was noted in infants as young as 10-20 days of age, often accompanying other symptoms like poor feeding, vomiting, and convulsions 2
- Metabolite responsible: One of the branched-chain amino acids (isoleucine) produces a metabolite that causes this distinctive maple syrup smell 3
- Detection timing: The odor becomes apparent when branched-chain amino acids (leucine, isoleucine, valine) and their corresponding ketoacids accumulate in plasma, urine, and cerebrospinal fluid 4
Biochemical Mechanism
The odor results from deficient activity of the branched-chain α-ketoacid dehydrogenase (BCKAD) enzymatic complex 5:
- Metabolic block: BCAAs are normally transaminated to BCKAs, but the enzyme required to metabolize the BCKAs is deficient, resulting in elevation of both BCAAs and BCKAs 3
- Volatile compounds: These accumulated BCKAs are volatile and lipophilic, allowing them to be excreted through sweat glands and other secretory pathways 1
Clinical Pitfall to Avoid
Do not rely solely on detecting the odor in urine—examine the patient's body and skin directly, as the odor may be more readily apparent from sweat, particularly in the early neonatal period when diagnostic suspicion is critical 2. The presence of this odor from any body fluid (sweat, urine, or cerumen) should prompt immediate plasma amino acid analysis and metabolic evaluation 1.