Atonic seizures, often called “drop attacks,” are defined by the sudden loss of muscle tone. This abrupt relaxation of muscles in the head, trunk, or limbs causes the individual to instantly go limp, often resulting in a collapse. These events are very brief, lasting less than 15 seconds, and consciousness is usually recovered quickly. The primary concern is the high risk of severe injury, particularly to the head and face, from the sudden, uncontrolled fall. Understanding the underlying causes is necessary to manage the condition and implement safety measures, such as protective headgear.
Underlying Epilepsy Syndromes
Lennox-Gastaut Syndrome (LGS)
Atonic seizures often emerge as a feature of complex, severe epilepsy syndromes that typically begin in early childhood. One recognized condition is Lennox-Gastaut Syndrome (LGS), characterized by multiple seizure types, including tonic, atypical absence, and atonic seizures, alongside cognitive and developmental delays. LGS onset usually occurs between the ages of three and five years. The condition is difficult to treat, often requiring a combination of anti-seizure medications.
Myoclonic-Atonic Epilepsy (MAE)
Myoclonic-Atonic Epilepsy (MAE), formerly known as Doose syndrome, is another syndrome where atonic seizures are a defining characteristic. This condition typically starts between two and six years of age. Seizures often present as a myoclonic-atonic event, where a brief muscle jerk is immediately followed by the loss of muscle tone. While initially challenging to control, approximately two-thirds of children with MAE may achieve remission over time. These syndromes represent severe epileptic encephalopathies, meaning the seizure activity contributes to developmental stagnation or regression.
Genetic Mutations and Structural Brain Anomalies
Genetic Mutations
Gene mutations that affect the function of ion channels are frequently implicated, as these channels regulate ion flow across nerve cell membranes, which is essential for neuronal communication. For example, variants in the SCN1A gene, which codes for a subunit of a voltage-gated sodium channel, are linked to epilepsies that can include atonic seizures. These mutations destabilize brain networks by causing a loss of function in inhibitory neurons, making the brain overly excitable.
Structural Anomalies
Structural anomalies within the brain can also generate the abnormal electrical activity that results in atonic seizures. Malformations of cortical development, such as focal cortical dysplasia, are a common structural cause. These conditions involve areas where neurons have developed incorrectly, creating an inherent source of seizure activity. Lesions in the frontal lobes are frequently identified in cases of atonic seizures, sometimes presenting as subtle abnormalities requiring specialized imaging.
Acquired Brain Injuries and Metabolic Conditions
Acquired Brain Injuries
Atonic seizures can result from injuries and systemic conditions acquired later in life. Severe head trauma, brain infections like encephalitis, and lack of oxygen (hypoxia) can damage brain tissue. This damage creates a fixed lesion or scar that becomes a source of abnormal electrical discharges, leading to epilepsy. Hypoxic-ischemic encephalopathy, which is brain injury due to oxygen deprivation, is a notable acquired cause, often occurring around the time of birth.
Metabolic Conditions
Metabolic conditions, particularly mitochondrial disorders, are another group of acquired causes leading to seizure susceptibility. Mitochondria are the powerhouses of cells, and disorders that impair their function disrupt the production of ATP, the cell’s energy source. Because brain cells have a high energy demand, this metabolic dysfunction makes neurons unstable and susceptible to generating seizures, including the atonic type. If seizures are resistant to treatment and accompanied by other organ involvement, a metabolic cause may be suspected.
The Physiological Mechanism of Muscle Tone Loss
The abrupt “drop” during an atonic seizure results from a sudden, widespread inhibitory signal in the brain. Posture and muscle tone are normally maintained by a balance between excitatory and inhibitory signals originating in the brainstem and spinal cord. During the seizure, a brief burst of electrical activity spreads to brain networks responsible for inhibiting motor function. This rapid electrical discharge triggers the body’s “off” switch for muscle contraction. Evidence suggests that fast corticoreticular pathways, connecting the cerebral cortex to the brainstem reticular formation, play a role in this widespread inhibition, overwhelming the spinal cord centers that maintain muscle tone.