Supplementary Motor Area Seizures with Asymmetric Tonic Posturing

The supplementary motor area (SMA) is located at the mesial side of the frontal lobes adjacent and anterior to the primary motor cortex (Brodman area 6), superior to the cingulate gyrus. Unilateral as well as bilateral tonic posturing as an ictal phenomen is thought to originate in this area. Seizures originating in the SMA are explosive in onset. The patient may suddenly involuntarily posture an arm or leg, with preserved consciousness and at times with preserved motor function of the ipsilateral side. The patient may try to catch or hold down his tonic extremity with his uninvolved arm. If neck muscles are involved there will be head turning toward the extended arm with the patient assuming a typical "fencing" posture (Figure 2.5). Seizures are brief, often nocturnal, and may occur in clusters (28-30).

The tonic posturing of an arm or leg or both occurs contralateral to the side of SMA-seizure origin. Penfield, in his stimulation experiments, identified a somatotopic representation within the supplementary motor area comparable to the primary motor cortex (31). This was confirmed in later studies (30,32). The face representation is located most anterior, the leg representation posterior, and the arm representation in between the face and the leg area. Bilateral tonic extension could be due to a rapid involvement of the contralateral side, but stimulation experiments have shown that both extremities are represented in the each SMA area (32). Head turning usually occurs to the contralateral to the side of seizure origin (33,34).

Sometimes, seizures originating in the SMA are preceded by an aura of a somatosensory sensation that

TABLE 2.1

Differential Diagnosis of Frontal Lobe Complex Partial Seizures with Hypermotor Automatisms

Frontal Lobe Complex

Partial Seizures

with Hypermotor

Nonepileptic

Automatisms

Psychogenic Seizures

Tourette's Syndrome

Duration

Brief (<60 sec)

Variable

Brief

Frequency

Frequent, in clusters

Variable, commonly clusters

Variable, can be continuous

Time of occurrence

Nocturnal preponderance, often out of sleep

Out of wakefulness

When awake, disappears during sleep, worsens with attention

Motor manifestations

Bizarre but stereotyped

Variable: shaking, jerking common

Simple and complex motor tics, verbal tics, variable location

Speech

Violent vocalization or speech arrest

Slurred speech

Echolalia, echopraxia

Consciousness

Preserved

Confusion and altered consciousness common, waxing and waning

Preserved

Associated features

Secondary generalization

Pain syndromes,

Obsessive compulsive

possible, clonic or tonic

psychiatric disease

disorder

features

especially PTSD

Age of onset

Childhood and adolescence

Adulthood >> childhood

Childhood

Spontaneous

Uncommon

Possible

Common

remission

EEG changes

Common but may lack

Normal

Normal

MRI imaging

Possibly epileptogenic lesion

Normal

Normal

can be bilateral, widespread, and proximal, indicating that there is also somatosensory representation within the SMA (30,31).

SMA seizures are often mistaken for nonepileptic events as the patient may have bilateral motor activity with preserved consciousness and no EEG changes. However, familiarity with these seizures makes them readily recognizable as epileptic events.

SMA epilepsy is observed with SMA lesions such as tumors or cortical dysplasias.

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Responses

  • rose
    What is bilateral motor posturing?
    7 years ago
  • bertha
    What are supplementary motor seizures?
    7 years ago
  • ren koertig
    What is posturing when having a sezure?
    6 years ago
  • diamanda
    What is supplementary motor seizures in children?
    6 years ago

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