The experience of repeatedly noticing a smell of maple syrup or burnt sugar when no obvious source is present can be puzzling. The causes can be broadly categorized into three areas: external environmental factors, issues affecting the local nasal and nerve pathways, or more complex systemic and central nervous system conditions. Determining the source requires distinguishing between a true external odor, a phantom smell, and an internal bodily odor.
External Factors: Investigating Real Odor Sources
The most common cause of a maple syrup smell is an actual odorant in the environment. The distinctive aroma comes from sotolon, a potent chemical compound detectable even at very low concentrations. Sotolon is naturally present in maple syrup, but it is also found in other substances.
The spice fenugreek, widely used in cooking and as a dietary supplement, contains high levels of sotolon. Facilities processing fenugreek for flavorings, such as those used in artificial maple syrup, can release this compound into the air, causing a widespread, intermittent sweet smell noticed miles away. Sotolon is also present in aged rum, molasses, and certain types of wine, and its presence can sometimes be traced to industrial spills or food processing.
The sweet smell can also be linked to issues within a home. This includes certain types of mold or bacterial growth in heating, ventilation, and air conditioning (HVAC) systems. Some refrigerant leaks have also been reported to produce a faintly sweet, unusual odor. Checking immediate surroundings for new cleaning products, scented candles, or dietary supplements like fenugreek is the first step to ruling out an external source.
Peripheral Olfactory Issues: Phantom Smells Originating in the Sinuses and Nerves
If no external source is identified, the sensation may be phantosmia, or a phantom smell, perceived without any physical stimulus. When this originates in the nose and local nerve pathways, it is called peripheral phantosmia. The mechanism involves damage or irritation to the delicate olfactory neuroepithelium, the tissue lining the nasal cavity that contains the smell receptor cells.
Conditions like chronic sinusitis, severe upper respiratory infections, or nasal polyps cause significant inflammation and swelling in the nasal passages. This local irritation can cause olfactory neurons to fire spontaneously or send aberrant signals to the brain. The damaged nerves transmit a false signal that the brain interprets as a smell.
While phantom odors are often foul, they can occasionally be perceived as sweet or sickly-sweet, like burnt sugar. Phantosmia resulting from a respiratory infection or head trauma may resolve on its own as inflammation subsides and the olfactory neurons heal. A thorough examination of the nasal passages by an otolaryngologist is often necessary to diagnose these local causes.
Systemic Conditions: Metabolic Causes of Internal Odors
In rare but serious instances, the sweet smell is a real odor emanating from the body itself due to a metabolic disorder.
Maple Syrup Urine Disease (MSUD)
The most distinct cause is Maple Syrup Urine Disease (MSUD), a genetic disorder that prevents the body from properly breaking down three branched-chain amino acids (BCAAs): leucine, isoleucine, and valine. This inability leads to a toxic buildup of these compounds and their byproducts, including sotolon, which is excreted in sweat, urine, and earwax.
In infants with the severe form of MSUD, the sweet, syrupy smell appears within the first few days of life, often accompanied by lethargy, irritability, and feeding difficulties. Prompt diagnosis and strict dietary management are necessary, as untreated MSUD can rapidly lead to severe neurological damage and death. Milder variants may not manifest until childhood or adulthood, often triggered by stress, illness, or a high-protein diet.
Diabetic Ketoacidosis (DKA)
Another systemic condition producing a sweet body odor is diabetic ketoacidosis (DKA), a life-threatening complication of diabetes. DKA occurs when the body, lacking sufficient insulin, breaks down fat for fuel, producing an excess of acidic molecules called ketones. The primary ketone, acetone, is expelled through the breath and is described as smelling sweet or fruity, similar to nail polish remover. While DKA is usually not described as a maple syrup smell, any unusual sweet body odor, especially when accompanied by excessive thirst, frequent urination, or confusion, warrants immediate medical attention.
Central Nervous System Involvement and Medical Action
When a phantom smell is caused by abnormal electrical activity or structural issues in the brain, it is referred to as central phantosmia. The temporal lobe processes smell memories and perception, and disruption here can trigger olfactory hallucinations.
A common neurological cause is a focal aware seizure, often originating in the temporal lobe. The phantom smell serves as an olfactory aura, warning that a seizure is about to occur. These seizure-related smells are typically brief, lasting only seconds, and may be accompanied by feelings of déjà vu, fear, or a rising sensation in the abdomen.
A less common but more serious cause of central phantosmia is a tumor or lesion, particularly one located in the temporal lobe or near the olfactory bulb. The tumor’s presence can irritate or compress the neural tissue responsible for smell processing, leading to the perception of a persistent phantom odor.
If the sweet smell is recurrent, persistent, or accompanied by neurological symptoms such as confusion, vision changes, or involuntary movements, seeking medical evaluation is necessary. A healthcare provider will start with a detailed neurological and ear, nose, and throat (ENT) examination. Diagnosis of central causes often requires neuroimaging, such as a magnetic resonance imaging (MRI) or computed tomography (CT) scan of the brain, to look for structural abnormalities. A referral to a neurologist is often the next step to investigate a seizure disorder with an electroencephalogram (EEG).