Injury, Sympathetic Nerve
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Sympathetic axons are most frequently injured by trauma affecting major motor sensory nerves. Direct injury to the sympathetic chain may also occur, including iatrogenic or traumatic injuries, and usually consists of a Horner's syndrome or a dry limb. Major peripheral nerve injuries cause an absence of sympathetic innervation, which causes dry, warm, mottled, suffused, red skin secondary to an absence of sweating and vasoconstriction in the cutaneous territory of the nerve. Patients with sympathetic dysfunction can also have swelling of dependent portions of the limbs secondary to vaso-motor irregularities. Pain is a common finding and is usually quite severe, worse in the evening, and instigated by non-noxious stimuli. Chronic sympathetic denervation causes dystrophic changes in the affected limb. These include taut, hyperhydrotic, cyanotic skin; alopecia or hypertrichosis; and fingernail thickening, brittleness, and/or discoloration. For major motor-sensory nerve or severe brachial plexus injuries, the above findings are readily evident with observation of the extremity skin, especially in comparison to the normal limb. For more distal or focal nerve injuries, autonomic abnormalities may be subtle.
The sympathetic innervation originates with the preganglionic fibers of the first through the 12th anterior thoracic roots, gaining access to the sympathetic chain by way of the white rami; postganglionic fibers exit sympathetic ganglia joining the spinal and cranial nerves. Resection or injury of the second thoracic ganglion results in nearly complete sympathetic denervation of the upper extremity and has been considered to provide satisfactory sympatholysis for treatment of hyperhydrosis and causalgia.
Horner’s syndrome occurs when the sympathetic outflow to the orbit and face are damaged or disconnected. In brief, the sympathetic fibers originate in the hypothalamus and synapse in the intermediolateral column of cells between C8 and T2. The majority of the oculosympathetic fibers exit along the T1 spinal nerve, while sudo-motor fibers to the face exit with the T2-T4 spinal nerves. Of note, this is why patients with Horner’s syndrome from brachial plexus avulsion do not manifest hemi-facial anhydrosis; they only have anhydrosis along the affected superior orbital rim. The sympathetic fibers synapse in the superior cervical ganglion, which is located between the internal carotid artery and jugular vein at the skull base. Sudo-motor fibers to the face travel with the external carotid artery, which oculosympathetics follow the internal carotid artery to subsequently pass along the abducens and V1 division of the trigeminal nerve via the superior orbital fissure into the orbit. Damage to the oculosympathetic fibers anywhere along this pathway may cause Horner’s syndrome.
Clinical Presentation and Diagnosis
The physical examination of sympathetic injury begins with observation of the affected limb compared to the normal one. Vaso-motor, sudo-motor, and dystrophic changes should each be documented. Sweating on the volar surface of the hand may be evaluated by focusing on the skin with an ophthalmoscope. Small beads of sweat should be present with normal sympathetic function. Alternatively, a somewhat heavy metal spoon can be passed along the skin; if it passes easily then sweating is likely absent. Next, any joint abnormalities (e.g., Charcot joints) should be noted, including joint range of motion, crepitus, and misalignment. Both allodynia (pain from non-painful stimuli) and hyperalgesia (painful stimulus appears more painful than expected) should be evaluated with light touch, pinprick, and a cold tuning fork. The temperature of the affected limb should be compared to the normal limb, either subjectively or with a surface thermometer. When indicated (e.g., brachial plexus injuty), a pupillary examination in dim light (to exclude Horner’s syndome) should also be performed. Qualitative and quantitative laboratory tests may also be ordered to document sympathetic function, including:
- Sympathetic skin response (SSR) - This simple electrophysiological test evaluates the sympathetic response to a mildly noxious stimulus by recording the efferent electrical response to the sweat glands.
- Thermoregulatory sweat test (TST) - This qualitative test visually documents regions of sweating on the body.
- Quantitative sudo-motor axon reflex test (Q-SART) - This test is a very sensitive and reliable measure of post-ganglionic sympathetic (sudo-motor) function. This test is useful in peripheral nerve injury patients because a negative response only occurs when there is a postganglionic injury present.
- Skin wrinkling on water immersion - This deceivingly simple beside test documents a lack of wrinkling in the sympathetically denervated hand after immersion in water.
- Microneurography - A microelectrode can be placed directly into a major motor-sensory nerve to directly record sympathetic potentials without relying upon an effector organ (e.g., sweat gland), which is unique from other tests.
This classic syndrome was first described in detail by Johann Friedrich Horner in 1869 and consists of ipsilateral miosis, ptosis, and hemi-facial anhydrosis. Horner’s syndrome per se, may not not very bothersome to the patient, however, it may be the sign of significant pathology (e.g., neoplasm), and therefore its etiology should always be confirmed. The neurosurgeon can encounter Horner’s syndrome in a variety of circumstances, including T1 rootlet avulsion during brachial plexus injury, T1/T2 foraminal compression of the T1 spinal nerve secondary to an osteophyte or herniated disc, gunshot or stab injuries to the upper thorax and neck regions, cervical tumors including neurofibromas of the sympathetic chain, carotid dissections, skull base fractures and tumors, and disorders of the cavernous sinus.
Treatment and Outcome
Repair or decompression of injured peripheral nerves allow regeneration of sympathetic axons and subsequent recovery of function. The likelihood of recovery is uncertain and depends on a multitude of factors (e.g., chronicity of injury, type of repair, etc.). Because of their size and location, injuries to the sympathetic ganglia or chain is rarely indicated or performed. Receptor hypersensitivity is a common problem after significant sympathetic injury, including clammy hands, erythema, and allodynia. When sympathetic nerves regenerate, they may establish aberrant connections to sensory receptors, muscles, or other sympathetics receptors; this may lead to an over-response or abnormal response.