Welcome back to the continuing series investigating the functional neurological approach to patient management. We are continuing from last month’s article, where a variety of definitions and explanations concerning examination techniques were outlined. We will start our examination process of the neuraxis with the pupils.
Examination of the pupils
Pupil size reflects a balance in tone between the sympathetic and parasympathetic nervous systems. You can get a reasonable measure of the actual sympathetic tone in the patient by measuring the resting pupil size in darkness. The sympathetic tone is represented by the degree of dilation of the pupil and demonstrates the degree of resting constriction in vascular smooth muscle in most parts of the body. Vestibular, cerebellar and cortical influences on both sympathetic and parasympathetic tone should also be considered.
Various components of the pupil light reflex are subserved by each component of the autonomic nervous system. The time to activation (TTA), amplitude of constriction, smoothness and maintenance of constriction, time to fatigue (TTF) and time to redilation of the pupil response need to be measured and recorded in each pupil. These are all aspects of the pupil light reflex that have been researched and correlated with the central integrative state of the various contributing components of the nervous system.
Pupil constriction pathways
Accommodation is the constriction of the pupil that occurs during convergence of the eyes for close focusing.
The Edinger-Westphal nucleus is activated by the adjacent oculomotor nucleus, which activates the medial rectus muscle more powerfully than the light reflex. There is also contraction of the ciliary muscle to aid close focusing, which is referred to as the near response.
Parasympathetic fibres lie superficially on the oculomotor nerve and they relay in the ciliary ganglion of the orbit, which lies on the branch to the inferior oblique muscle. They begin in dorsal position and rotate to a medial and then inferior position as they enter the orbit. Blood supply to the pupil fibres is different to the main trunk of the nerve. The pupil fibres receive their blood supply from the overlying pia mater, therefore the pupil fibres are usually spared in an oculomotor nerve trunk infarction.
An afferent pathway lesion results in a Marcus-Gunn pupil. The swinging light test will reveal that the affected pupil will not react to light as well as the other pupil, but it may constrict normally in response to stimulation of the opposite pupil during testing of the consensual light reflex. This occurs in multiple sclerosis, and diabetes conditions that affect the optic nerve due to demyelination or vascular lesions. You might also expect this to occur when there is an increase in sympathetic tone to the pupil on the side of relative afferent defect. This could distinguish a high firing intermediolateral (IML) cell column from transneural degeneration (TND) in the mesencephalon.
The Wernicke pupil reaction refers to differential summation, depending on whether you are shining the light into the nasal or temporal aspects of the retina (i.e., intact or ablated fields). This may be observed in an optic tract lesion. Supposedly, the resting size of the pupil is uninterrupted, due to the consensual light reflex.
The nasal half of the retina is significantly more sensitive to light than the temporal half of the retina, and the direct responses are significantly larger than the consensual response. With temporal retina stimulation, the direct and consensual reflexes are nearly the same. Direct and consensual pupil reactions, when stimulating the temporal retina, are nearly equal. This may suggest an input of temporal retina to both sides of the pretectum. Such a crossing of temporal fibres may take place in the chiasm.
The net effect of the pupillary light reaction, which involves shining light into the monocular zone from the temporal hemi-field of one eye, leads to greater constriction of the pupil on that side.
Parinaud Syndrome results when damage to decussating fibres of the light reflex at the level of the superior colliculus is present. This results in semi-dilated pupils fixed to light, plus loss of upward gaze.
The Argyll Robertson pupil is most commonly seen in neurosyphilis. Common signs are bilateral ptosis, an increased frontalis tone, as well as a pupil that is irregular, small, and fixed to light, but constricts with accommodation. This type of pupil can not be dilated by atropine.
Differential diagnosis of this particular pupillary dysfunction includes senile miosis, pilocarpine or β-blocker drops for glaucoma. This pattern of findings is reversed in encephalitis lethargica.
Holmes-Adie pupil or tonic pupil occurs due to degeneration of the nerve fibres in the ciliary ganglion and is thought to be produced by a combination of slow inhibition of the sympathetic and partial reinnervation by parasympathetic fibres.
This condition can also be associated with loss of patella reflex, decreased sweating, blurred vision for near work and eye pain in bright light.
Horner’s Syndrome.
Disruption of the sympathetic chain at any point from the hypothalamic or supraspinal projections to the oculomotor nerve can result in a spectrum of symptoms referred to as Horner’s syndrome. The classic findings in this syndrome include ptosis, miosis and anhidrosis, but a number of other abnormalities may also be present. Ptosis or drooping of the upper eyelid is caused by the interruption of the sympathetic nerve supply to the muscles of the upper eyelid. Miosis or decreased pupil size is a result of the decreased action of the dilator muscles of the iris due to decreased sympathetic input. This results in the constrictor muscles acting in a relatively unopposed fashion, resulting in pupil constriction. A Horner’s pupil will still constrict when light is shined on the pupil, although careful observation is sometimes required to detect the reduced amount of constriction that occurs. Innervation to superior and inferior tarsus muscles is carried in CN III. Vasomotor fibres are carried in the nasociliary branch of CN V and make no synapses in the ciliary ganglion after branching off from the carotid tree.
Pupillodilator fibres are carried in the long ciliary branches of the nasociliary nerve. This syndrome is characterised by the following signs and symptoms:
• Ptosis / apparent enophthalmos
• Small pupil
• Anhydrosis (forehead or forequarter of body)
• Blood shot eye (loss of vasoconstrictor activity)
• Heterochromia
Horner’s syndrome can occur due to lesions at various peripheral and central sites. Some of these sites may be in the spinal cord, hemispheric lesions of the brain, in the brain stem, nerve root lesions, the carotid artery, the jugular foramen, the orbit or the cavernous sinus.
Depending on the location of the lesion, other cranial nerves may be involved, such as III, IV, VI and VI when it is near the cavernous sinus or superior orbital fissure, and IX, X and XII when the lesion is at the base of the skull.
In the spinal cord, the mixed signs associated with syringomyelia may be present because of the widening of the central canal. This widening of the central canal would cause a loss of segmental reflexes.
When T1 nerve root involvement exists, Horner’s Syndrome may be present with weakness of finger abduction and adduction, wasting of the intrinsic hand muscles, loss of pain sensation in the medial aspect of the arm and armpit and deep pain in the armpit. This is rarely due to spinal degeneration and serious causes such as Pancoast’s tumor should be considered. Referral for MRI, chest X-rays and / or CT scan should then be considered.
Different lesion levels affect sweating differently. Central lesions may affect sweating over the entire forequarter, due to involvement of the descending pathways from the hypothalamus. Lower neck lesions may affect sweating over the face only, due to involvement of sympathetic efferents in the arterial plexus (carotid / vertebral).
Lesions above the superior cervical ganglion may not affect sweating at all, or it may be restricted to the forehead.
Randy Beck, B.Sc., D.C., Ph.D., is a graduate of Canadian Memorial Chiropractic College. He has completed postgraduate studies in Psychology, Immunology and Neurology. He is presently involved in a number of international research projects and is co-authoring a textbook on Functional Neurology. He was formerly the Dean of Chiropractic and Basic Sciences and Director of Research at the New Zealand College of Chiropractic. Presently, he practices Chiropractic Functional Neurology at the Papakura Neurology Center and The Maungakiekie Clinic located in Auckland, New Zealand.
References
Beck, R. W. Functional Neurology for Manual Therapists. Elsevier, UK. 2007 (in Press).