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Convulsive movements and transient focal hypermotor episodes

Convulsive limb movements commonly accompany episodes with transient loss of awareness
and are most commonly due to epilepsy, syncope or dissociative seizures. Transient, episodic
limb movements without loss of awareness are also frequently misdiagnosed as epilepsy.
There is often a degree of overlap with myoclonus as the clinical manifestation of a variety
of pathophysiological processes embracing the subspecialty fields of both epilepsy and
movement disorders. Epileptic myoclonus, which is cortical in origin, can be confused with
other hyperkinetic movement disorders, including myoclonus originating from subcortical
structures, brainstem, spinal cord or peripheral nerves, tics, chorea, dystonia and tremor.
Definitive localisation of the myoclonic focus requires electrophysiology, specifically a time-
locked back-averaged EEG. Careful neurological examination is also often helpful in this
regard, for example, in identifying spinal cord pathology or evidence of a cortical process.

Cortical myoclonus arises from a hyperexcitable focus within the sensory-motor cortex, and
involves an arm, leg or the face. In general, it is typically arrhythmic, although in the setting
of epilepsia partialis continua jerks may appear rhythmic. Cortical myoclonus is triggered by
action or intention, and is often stimulus-sensitive. Subcortical myoclonus refers to
myoclonus without a preceding cortical discharge and arises from structures such as the
thalamus, and is usually, although not exclusively, stimulus-insensitive. In practice, it is
frequently difficult to differentiate cortical from subcortical myoclonus on clinical grounds,
and neurophysiological investigation is required. Neuroimaging may also be helpful in this
regard. Myoclonus arising from the brainstem (startle, palatal and reticular reflex
myoclonus), spinal cord (segmental and propriospinal myoclonus) and peripheral nerves are
usually recognised and differentiated from epilepsy without difficulty.

Among the hyperkinetic movement disorders, tremor is the entity most often confused with
myoclonus and convulsive limb movements. Tremor is habitually rhythmic and oscillatory,
and significantly slower than myoclonus; however, occasionally tremor may be jerky and
irregular, mimicking clonic jerks to the degree that electrophysiological investigation is
required to differentiate between them.

Like myoclonus, tics are also brief; however, they are typically preceded by an urge to
perform the movement and can usually be temporarily suppressed, features not seen in
myoclonus or simple partial seizures. Tics are usually stereotyped, repetitive and often
complex, involving multiple different noncontiguous muscle groups.

Chorea, a brief involuntary ‘dance-like’ movement, is usually easy to distinguish from
myoclonus and epilepsy due to the characteristic flowing movements. Dystonia is an
involuntary movement disorder characterised by repetitive, sustained movements that
typically produce twisting postures. Dystonia rarely mimics myoclonus although it may be
confused with epileptic tonic spasms or the dystonic posturing seen in partial seizures of
frontal or temporal lobe origin. Many patients with dystonia possess a manoeuvre that
attenuates the dystonia, termed a ‘geste antagoniste’.

Paroxysmal dyskinesias are a genetically and clinically heterogeneous group of rare
movement disorders characterised by episodic dystonic or choreiform movements.
Paroxysmal kinesigenic dyskinesia (PKD) is the most common type, although the precise
prevalence is unknown. This condition is characterised by brief attacks of unilateral or
bilateral limb dystonia or chorea, lasting less than one minute and with preserved
consciousness, triggered by initiation of voluntary movements. An ‘aura’, such as an unusual
cephalic or epigastric sensation, may precede the attacks, further adding to the diagnostic
confusion46. Sporadic cases occur; however, PKD is considered to be an autosomal dominant
condition with variable penetrance, linked to the pericentromeric region of chromosome