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without structural heart disease have no increase in mortality rate. Even patients who remain
undiagnosed following extensive investigations have a good prognosis10,13. It is therefore of
paramount importance, from a prognostic and interventional point of view, to identify those
patients with syncope due to an underlying cardiac cause.

Drop attacks

Neurological causes of sudden collapse other than epilepsy and autonomic dysfunction
include intermittent obstructive hydrocephalus caused by, for example, a colloid cyst of the
third ventricle or a craniocervical junction abnormality such as an Arnold-Chiari
malformation. Colloid cysts present with syncope and sudden death, particularly with
changes in posture, are readily identified on neuroimaging and are amenable to neurosurgical
intervention41,42.

Diencephalic attacks, as sequelae of diffuse brain injury, are extremely rare and manifest as
autonomic dysfunction with diaphoresis, sinus tachycardia, collapse and intermittent
hypertension43.

Brainstem and spinal cord lesions or lower limb weakness of any cause may present with
unexplained falls without impairment of consciousness. There are usually fixed neurological
signs which will guide appropriate investigations and the episodes are rarely confused with
atonic or tonic seizures of epilepsy. Cataplexy usually occurs in association with the
narcoleptic tetrad of excessive daytime somnolence, hypnagogic hallucinations, and sleep
paralysis, although it may be the presenting feature. Further details regarding this condition
are found in Chapter 18 on epilepsy and sleep.

Idiopathic drop attacks are most commonly seen in middle-aged women. They take the form
of a sudden fall without loss of consciousness, and patients frequently remember hitting the
ground. Recovery is instantaneous but injury often occurs. Neurological, cardiac and
autonomic investigations are unrewarding.

It is likely that vertebrobasilar ischaemia is overdiagnosed and probably accounts for only a
small proportion of drop attacks. Typically, the attacks occur in the elderly, with evidence of
vascular disease and cervical spondylosis, both commonly occurring conditions which
frequently co-exist in the elderly population. Furthermore there is clinical overlap with other
more commonly occurring but benign conditions such as benign paroxysmal positional
vertigo. The attacks may be precipitated by head turning or neck extension resulting in
distortion of the vertebral arteries and haemodynamic ischaemia, although embolic events are
probably a more frequent cause. Drop attacks are accompanied by features of brainstem
ischaemia such as diplopia, vertigo and bilateral facial and limb sensory and motor deficits44.

Hyper- and hypokalaemic periodic paralyses (PP) are rare autosomal dominant disorders of
sodium and calcium ion channel dysfunction characterised by episodic flaccid weakness
secondary to abnormal sarcolemmal excitability and rapid changes in serum potassium levels.
Cranial musculature and respiratory muscles are usually spared. Attacks last from between
minutes in hyperkalaemic PP to hours and occasionally days in hypokalaemic PP.
Precipitants include fasting, alcohol, resting following exercise, stress (hyperkalaemic PP)
and a high carbohydrate meal, cold and exertion the previous day (hypokalaemic PP). Acute
treatment is directed at supportive care and normalisation of the serum potassium. Effective
prophylaxis of hypokalaemic PP, like many of the channelopathies, is with acetazolamide45.
Thyrotoxicosis is the commonest cause of secondary periodic paralysis.