The autonomic nervous system

Anatomy and physiology

The autonomic nervous system controls important functions that are not under voluntary control, that is autonomous. The extent of this autonomy varies; initiation of swallowing or micturition is voluntary and subsequent execution is automatic, but there is no voluntary component to gastrointestinal motility. As well as descending control from the CNS, some autonomic structures may function independently, and under humoral control. For example, the heart has an intrinsic pacemaker in the sinoatrial node, which may be modulated by circulating adrenaline (epinephrine) and by sympathetic and parasympathetic nerve inputs.

Causes of autonomic dysfunction

Lesions at all levels of the nervous system may affect autonomic function (Fig. 1) and the site of the lesion determines the pattern of involvement as well as associated deficits. Most autonomic disturbances are acquired but there are some rare inherited causes, for example certain inherited neuropathies. Some common neuropathies particularly affect autonomic function, especially those due to diabetes mellitus or alcohol. The autonomic nervous system is also affected by many widely used medications, especially antihypertensive agents.

Fig. 1 Patterns of autonomic disturbance with different lesion sites.

Primary autonomic failure may occur as a pure syndrome or as part of multisystem atrophy, in association with degeneration in other neurological systems, including the cerebellum, extrapyramidal system and pyramidal tracts. This condition usually presents in middle age and the order in which these neurological systems are affected varies between patients. Commonly, first presentation is with Parkinsonism, which may be difficult to distinguish from idiopathic Parkinson’s disease and may partially respond to levodopa. Subsequently, sphincter disturbance and giddiness secondary to postural hypotension develop, giving a clue to the diagnosis. Central respiratory disturbance, for example acute stridor, may occur. The prognosis is for gradual deterioration, with survival 5–10 years.

Clinical manifestations of autonomic disturbance

Autonomic disturbance may affect just one system, for example cardiovascular, or it may be more generalized. Clinically important consequences affect respiration, swallowing, cardiovascular, bowel, bladder and sexual function (Table 1). Rarely, failure of sweating can result in dangerous hyperpyrexia. There are three main categories of autonomic dysfunction:

In organs with both SNS and PSNS input, the net effect will depend on the balance of pathology affecting the two systems. For example, Guillain–Barré syndrome may cause tachydysrhythmia (unopposed SNS) or bradydysrhythmia (unopposed PSNS). The bladder relies on coordination of the SNS and PSNS and spinal cord malfunction in multiple sclerosis particularly causes attempted emptying against a closed sphincter, as well as retention or incontinence. This ‘dyssynergia’ causes urinary urgency, frequency and incomplete voiding.

Investigation of autonomic failure

The first step is confirmation of autonomic dysfunction. Simple clinical tests are described in Table 2. More sophisticated autonomic responses can be measured in a specialized laboratory. These may be supplemented by tests for cause (Table 3), neuroimaging for CNS disease and nerve conduction studies for peripheral disease.

Table 2 Clinical tests of autonomic failure