Interruption of the sympathetic fibers results in miosis and ptosis (because of paralysis of the pupillary dilator muscle and of Müller muscle, respectively). The lesion may be central, between the hypothalamus and the points of exit of sympathetic fibers from the spinal cord (C8 to T3, mainly T2), or peripheral, in the cervical sympathetic chain, superior cervical ganglion, or along the carotid artery. A congenital form caused by perinatal injury, usually of the sympathetic chain in the neck is seen regularly (Fig. 13-10). A hereditary form of the Horner syndrome (autosomal dominant) is also known, usually but not always associated with a congenital absence of pigment in the affected iris (heterochromia iridis) (Hageman et al).
To the ophthalmic findings may be added loss of sweating on the same side of the face and redness of the conjunctiva. The entire complex is called the Horner syndrome, Bernard-Horner syndrome, or oculosympathetic palsy. The pupillary change may be subtle and may require covering the eyes or dimming the room lights to observe the lack of expected mydriasis on one side.
Most cases are caused by peripheral interruption of the sympathetic chain but the same effect may be produced by ipsilateral lesions of the sympathetic tract in the medulla or cervical cord. The pattern of sweating may be helpful in localizing the lesion in the following manner: With lesions at the level of the common carotid artery, loss of sweating involves the entire side of the face. With lesions distal to the carotid bifurcation, loss of sweating is not found or is confined to the medial aspect of the forehead and side of the nose (Morris et al). Retraction of the eyeball (enophthalmos), considered a component of the syndrome, is probably an illusion created by narrowing of the palpebral fissure.
Bilateral Horner syndrome is a rare occurrence; usually it is found in autonomic neuropathies and in high cervical cord transection. Although difficult to appreciate, bilateral miosis may be detected (using pupillometry or direct observation) by noting a lag in the redilation of the initially small pupils when light is withdrawn (Smith and Smith, 1999).
Stimulation or irritation of the sympathetic fibers, a rare phenomenon, has the opposite effect, that is, lid retraction, dilatation of the pupil, and apparent proptosis. Use is made of this phenomenon in the testing of the ciliospinal pupillary reflex, which is evoked by pinching the neck (afferent, C2, C3) and effecting pupillary enlargement through cervical efferent sympathetic fibers.
Extreme bilateral constriction of the pupils (miosis) is commonly observed with pontine lesions, presumably because of interruption of the pupillodilator fibers but the mechanism is not entirely clear. Narcotic ingestion is the most common cause of bilateral miosis in clinical practice except in the elderly, who often acquire small pupils, particularly if medication drops for glaucoma are being used.
Interruption of the parasympathetic fibers causes an abnormal dilatation of the pupils (mydriasis), often with loss of pupillary light reflex; in cases of coma, the “blown” pupil (Hutchinson pupil) is the result of a midbrain lesion or direct compression of the oculomotor nerve (see Chap. 16). Other signs of oculomotor palsy are usually conjoined.