The body’s cooling system is designed for symmetry, so excessive sweating in only one armpit, known as unilateral axillary hyperhidrosis, is unusual. Sweating is the primary mechanism for regulating core body temperature, controlled by signals that originate centrally to ensure a balanced, whole-body response. When one armpit sweats significantly more than the other, it signals a disruption somewhere along the communication pathway controlling the sweat glands on that specific side. This asymmetry suggests a localized or systemic issue interfering with the normal, balanced function of the nervous system.
The Sympathetic System and Normal Sweat Regulation
Sweat production is controlled by the sympathetic nervous system (SNS), the branch of the autonomic nervous system responsible for the “fight or flight” response. Signals to activate the eccrine sweat glands, found abundantly in the armpits, are sent from the central nervous system down the sympathetic nerve pathways. These signals travel from the brain and spinal cord, utilizing acetylcholine as the primary neurotransmitter to stimulate the glands. Because the central thermoregulatory center in the hypothalamus coordinates this process, the output is typically delivered bilaterally to maintain thermal homeostasis. The symmetrical nature of this control means that any lack of uniformity points to a specific point of disruption in the signaling cascade.
Causes Related to Localized Nerve Irritation or Injury
The causes of asymmetrical sweating are often temporary and related to a localized disruption of the peripheral nerves or the gland itself. Minor physical trauma or temporary nerve compression, such as prolonged pressure on the shoulder or arm, can irritate the sympathetic nerve fibers that supply the axilla. This peripheral irritation can cause a transient overactivity in the nerves of only one armpit, leading to a temporary increase in sweat production.
A more defined example of localized irritation is Thoracic Outlet Syndrome (TOS), where the nerves or blood vessels between the collarbone and first rib are compressed. This compression sometimes results in unilateral hyperhidrosis in the hand or axilla. Localized issues restricted to the skin, such as a minor infection or a temporary obstruction of the sweat duct, can also cause an imbalance in how the two sides function. These causes do not involve a central nervous system issue.
Systemic and Neurological Conditions Causing Asymmetry
When the sweating asymmetry is persistent, it often indicates a disruption much higher up the nervous system pathway, affecting the signal before it reaches the armpit. These causes involve lesions or damage to the central or major peripheral nervous system structures that coordinate the sympathetic outflow.
Central Disruption
One classic example of a central disruption is a lesion in the brainstem or hypothalamus, which houses the body’s primary thermoregulatory center. A stroke or a tumor in these areas can damage the descending sympathetic fibers, causing a failure of the sweat signal on one side of the body.
Horner’s Syndrome
Damage to the sympathetic pathway in the neck or chest can manifest as Horner’s Syndrome. This condition is characterized by a constricted pupil, a drooping eyelid, and decreased sweating (anhidrosis) on the affected side of the face. Disruption of the sympathetic chain can sometimes lead to complex sweating patterns, including hyperhidrosis in the contralateral armpit or an asymmetrical pattern in the axilla.
Other Neuropathies
The phenomenon of contralateral hyperhidrosis, where the side opposite the neurological lesion sweats excessively, can occur after a brain injury like a stroke. Other forms of autonomic neuropathy, often seen with conditions like diabetes, can also selectively damage the small nerve fibers in a non-symmetrical pattern, leading to an inconsistent sweat response between the two armpits.
When to Seek Medical Evaluation
While some cases of unilateral sweating are transient and benign, the symptom should prompt a medical consultation to rule out a serious underlying condition. You should seek immediate attention if the asymmetrical sweating is accompanied by other sudden neurological symptoms.
These warning signs include:
- A sudden, severe headache
- Double vision
- Persistent weakness
- Difficulty speaking or a drooping of the face
- Unexplained weight loss, fever, chest pain, or a rapid or pounding heartbeat
These accompanying signs suggest the asymmetrical sweating may be a manifestation of a systemic disease or a central nervous system event.