The sensation of a sneeze building up only to vanish at the last moment is a common and often frustrating physiological experience. This phenomenon, where the body’s protective reflex is initiated but not fully executed, is a momentary lapse in a complex neurological circuit. Understanding why this process stalls requires looking closely at the sneeze’s biological mechanism and the ways the brain can interrupt this involuntary command.
The Anatomy of a Sneeze
The sneeze, formally known as a sternutation, is a rapid, forceful expulsion of air intended to clear irritants from the nasal passages. This reflex begins when nerve endings in the nasal lining are activated, typically stimulated by dust, pollen, or foreign particles. These sensory signals travel along the trigeminal nerve, the largest cranial nerve, which is responsible for sensation in the face.
The signal is relayed to the sneeze center, a specific region located in the medulla of the brainstem. Once a sufficient stimulus threshold is reached, this center triggers a cascade of involuntary muscle contractions. This motor response involves a deep inhalation, followed by the simultaneous tightening of the chest and abdominal muscles, and the closing of the eyes and throat. The sudden opening of the glottis then releases the pressurized air, mucus, and irritants through the nose and mouth at speeds that can reach up to 100 miles per hour.
Causes of Sneeze Inhibition
The feeling of a blocked sneeze occurs when the initial sensory signal reaches the brainstem, but the motor response is aborted before the expulsion phase. One primary cause is a shift in sensory focus, where the brain prioritizes a new input, overriding the sneeze command. This neurological interruption can happen when a person is suddenly distracted or experiences a change in environment.
The required reflex threshold may also drop before the full action is executed. The sneeze center requires a sustained and intense signal to trigger the full motor sequence. If the irritant is removed or the nerve stimulation fades quickly, the reflex stalls.
A change in light source, known as the photic sneeze reflex, can either trigger or interrupt a sneeze. This is thought to be an overlap between the optic nerve and the trigeminal nerve pathways in the brainstem. The brain’s semi-voluntary ability to interfere with the reflex can also play a role, as a momentary conscious effort or sudden thought can dampen the autonomous process, causing the urge to vanish.
How to Encourage a Sneeze
When a sneeze is partially triggered but blocked, re-stimulating the trigeminal nerve can often complete the reflex. One safe and effective method is gently massaging the bridge of the nose with firm, downward pressure, which can reactivate the nerve endings. Stimulating the roof of the mouth with the tongue can also work, as the trigeminal nerve has branches that run through this area.
Another direct approach involves using a clean, rolled-up tissue corner to lightly tickle the inside of the nostril. This mechanical stimulation re-introduces the irritant signal directly to the nasal nerve endings, often pushing the reflex past its stalled point. For individuals with the photic reflex, quickly looking toward a bright light source, such as a window, can provide the necessary sensory shift to trigger the sneeze. Taking a deep breath of cold air or sniffing a strong scent like a spice can also provide the necessary sensory jolt.
Potential Health Consequences of Forced Suppression
While the involuntary inhibition of a sneeze is generally harmless, actively suppressing a sneeze once the motor sequence has begun can carry physical risks. A full sneeze generates considerable pressure, with air sometimes moving at high speeds. Pinching the nose and closing the mouth forces this pressure to redirect internally.
This sudden pressure buildup can travel through the Eustachian tubes connecting the nose to the middle ear, potentially causing damage to the eardrums. Though rare, extreme pressure has been linked to the rupture of small blood vessels in the eyes or nose, which appear as temporary red spots. In extremely rare, documented cases, forcefully stopping a sneeze has caused air to be trapped in the chest cavity or resulted in a tear in the throat tissue. Doctors advise allowing the sneeze to be released naturally to avoid these internal pressure complications.