A review of muscle structure, function and dysfunction
Looking where you’re going ... The reciprocal functional relationship between extraocular muscles and neck muscles: A review of muscle structure, function and dysfunction
By Erika Kruger with Liza Groenewald
1. Introduction
Comments by patients that their vision seems to temporarily improve after a thorough massage of the posterior neck area, prompted the authors to investigate this particular physiological connection. It is clear that there exists a strong relationship between eye muscles and neck muscles especially the muscles of the cranial area, the section directly below the skull at the back of the head. Neural activity in the one area influences the other, sometimes directly and sometimes very subtly.
In this article we will consider the nature of the relationship that exists between eye muscles and neck muscles, the structure and function of the extraocular and relevant neck muscles as well as the different ways in which this relationship can influence each other as well a how an imbalance in the relationship may lead to dysfunction.
We will divide the types of dysfunction that TM therapists encounter, as a result of the relationship between the eye and the cranial muscles, into two categories namely:
1) dysfunction of eye muscles causing postural asymmetry and
2) physical asymmetries that may lead to visual disturbances
The first category refers to a number of vision problems that may cause muscle contraction and skeletal misalignment especially in the neck and shoulder area. This, in turn may affect other parts of the body and entire postural patterns. Often the result is pain and discomfort. The second category refers to cases where incorrect postural habits exacerbate vision disturbances. It is clear from this differentiation that not all vision disturbances can be addressed by therapeutic massage and that it is therefore important that the TM therapist and the patient’s optometrist or ophthalmologist work together to establish the exact cause of the vision disturbance in order to put together an effective treatment plan. It does not however, fall within the ambit of this article to discuss specific therapeutic massage techniques for treating related muscle dysfunction.
2. Functional relationship between eyes and neck muscles
A literature search confirms that there exists a definite reciprocal functional relationship between the extraocular muscles and the neck muscles. The following studies corroborate this:
- According to Bexander, Mellor and Hodges (2005), the control of neck muscles is co-ordinated with the help of sensory organs including hearing, balance and vision. In a report on their investigation into the effect of eye position on neck muscle activity during cervical rotation, they reason that the interrelationship between eye position and neck muscles activity may affect the control of neck posture and movement.
- In their review of the anatomy and interactions of the postural and somatosensory reflexes, Morningstar, Pettibon, Schlappi and (2005) conclude that “visual and vestibular inputs, as well as joint and soft tissue mechanoreceptors, are major players in the regulation of static upright posture”.
- Hayman, Dutia and Donaldson (1993), in their report on experiments with pigeons, also point out that the head and eye movement together results in changes in direction of gaze. Their tests show that the position of the eye in the socket affects the electromyographic (EMG) activity in the neck muscles. They conclude that the test strongly suggests that extraocular afferent signals are involved in eye-head co-ordination.
- The opposite is also true - proprioceptive input from cervical muscles influences head-eye co-ordination as well as postural processes. In a study on the effects of changes in sensory inputs and postural control during quiet standing following muscular fatigue, Vuillerme, Pinsault and Vaillant (2005) show that cervical muscular fatigue resulted in increased centre-of-foot displacements in the absence of vision and that the availability of vision allowed the individuals to suppress this destabilizing effect. This was observed after 14 young and healthy adults were asked to sway as little as possible while (a) they were unable to see, (b) they were unable to see and they were standing on foam and (c) they could see. The experiment was executed in conditions of no fatigue and fatigue of the scapula elevator muscles. The researchers underscore the importance of cervical neuromuscular function on postural control during quiet standing and suggest a “reweighting of sensory cues in balance control following cervical muscular fatigue by increasing the reliance on somatosensory inputs from plantar soles and the ankles and visual information”.
- The role of proprioceptive input from the neck is confirmed by a study conducted at the University of Rome (Ivanenko, Grasso & Lacquantiti 2000) indicating that “continuous neck vibration evokes changes in the postural reference during quiet standing and in the walking speed during locomotion”. In addition the results suggest that proprioceptive input from the neck is integrated in the control of human posture and locomotion and is processed in the context of a viewer-centred reference frame.
- The proprioceptive input from the cervical muscles is facilitated by high spindle content as well as high spindle density when compared with other muscles. This is confirmed by a quantitative study of muscle spindles in suboccipital muscles of human foetuses conducted by Kulkarni Chandy and Babu (2001). They show that the superior oblique capitis has 190 spindles per gram of muscle, the inferior oblique capitis 242 spindles and the rectus capitis posterior 98 spindles. These results substantiate other studies confirming that in humans, the highest spindle density is in the hand, foot and neck muscles, thus small muscles doing fine motor tasks.1 The authors conclude that the “possible role of these small muscles as movement sensors of craniovertebral joints need to be evaluated further by electrophysiological and biomechanical studies.” According to (Kulkarni et al (2001), when there is an interference with sensory input from the upper cervical region it may manifest as disturbances of gait, effects similar to unilateral labyrinthecomy (the surgical removal of the labyrinth of the ear to prevent vertigo), dizziness and ataxia or loss of control over body movements after whiplash as well as altered cervical vascularity and altered sympathetic tone.
A short discussion of individual muscles involved in the reciprocal relationship between the eyes and the neck, follows. It is assumed that the reader is familiar with the attachment sites, relationships and actions of each muscle. It is therefore not discussed in detail.
3. Musculature of the eye and neck area
3.1 Muscles of the eye
Eye movement is controlled by six extraocular muscles, each one about 40mm long. These muscles move the eyeball up, down and from side-to- side. It is attached to the sclera or white outer part of the eye by a strong layer of collagen fibres (tendons). This complex system of connective tissue links the muscles in order to stabilize and align the eyeballs (Graham & Stidham 2005). Behind the eye, the muscles penetrate the Tenon capsule around globe and attaches to the optic nerve which connects each eye to the brain.
Another muscle related to pain in the ocular region that should be considered is the orbicularis oculi as well as temporalis and frontalis especially when dysfunction of the sternocleidomastoid (SCM) is present (Simons,Travell & Simons 1999:210).
3.2 Deep neck muscles
As pointed out earlier, muscles in the cervical region provide dynamic stabilization, mobility but also proprioceptive feedback vital for balance and fine postural control (Hendrickson 2003:163). Literature identifies a number of neck muscles that are directly related to activity in the extraocular muscles. This list includes the deep posterior neck muscles namely the cervicals, the semispinalis and, as discussed in under the previous heading, the suboccipital muscles. Other muscles pertinent to this review are the superficial posterior and anterior neck muscles namely the trapezius, SCM and spelenii.
3.2.1 Suboccipital muscles
The group of muscles, positioned deep to the semispinalis and erector spinae muscles, is known as the suboccipitals. It consists of the rectus capitis posterior minor and major, obliquus capitis superior and obliquus capitis inferior. The function of these postural muscles is usually considered to be head extension and rotation of the joints between the skull and C1 and C2 vertebrae aimed at fine-tuning movements that position the head (Kapit & Elson 1993:41, Hendrickson 2003:169). Kulkarni et al (2001) however, disagree for a number of reasons. The muscles are to small too effect these movements, it is at a mechanical disadvantage being positioned too close to craniovertebral joints and it has high proprioceptive content suggesting that this group of muscles rather act as sensors of joint position and movement of craniovertebral joints. Also, the small number of tendon organs in the suboccipital muscles indicates that they are incapable of sensing contractile tensions. Thus, the authors conclude, the suboccipital muscles sense length and thus movement.
McPartland and Brodeur (1999 in Chaitow & Delany 2000:207) agree that the importance of the suboccipital muscles lie in its “role as proprioceptive monitors of the cervical spine and head. EMG tests have shown that these muscles remain in a sustained contraction for long periods during movement as well as standing and sitting postures” (Kapit & Elson 1993:41).
3.2.2 Semispinalis
The semispinalis muscles runs from the mid-thorax to the posterior skull and is divided into three sections namely semispinalis capitis, cervicis and thoracis. It extends the neck and head bending it backwards when acting bilaterally and to the side when acting unilaterally. Its role as lateral flexor and rotator (Hendrickson 2003:172) is however disputed (Simons et al 1998 in Chaitow et al 2000:196). As these are postural muscles, they tend to shorten when stressed leading to cervicogenic headaches and headaches extending around the head to eye area, and possibly, restricted flexion and rotation (Chaitow et al 2000: 196).
3.3 Superficial neck muscles
The superficial neck muscles assist in holding the head up against gravity. They also extend the head and neck. The posterior muscles support the back of the body in extension while in front the SCM , which attaches to the clavicle and the sternum and the skull behind the ear, pulls up front of body (Myers 2003: 114)
3.3.1 Trapezius
The trapezius elevates the shoulders, bends the neck and head laterally to the same side and aids in extreme rotation to opposite side. It also acts as a neck stabilizer, as synergist for the SCM during head and neck motion and as an antagonist to the levator scapulae and serratus anterior in scapular rotation. The upper trapezius works continually to keep the head and neck vertical and eyes level (Simons et al 1999:189). Trigger points in the upper trapezius refer pain to side of head, temple and back of the orbit, the ear and the jaw (Simons et al 1999:184)
3.3.2 Sternocleidomastoid
Together the two SCM muscles assist in flexion of the neck, pulling the head forward and check-reining hyperextension. As noted earlier Myers (2003:111) concludes that the SCM, rather than pulling the head forward, pulls the rib cage up. With the trapezius, the SCM muscles stabilize the head when chewing. It also plays a role in inspiration and swallowing while contributing to spatial orientation, weight perception and motor co-ordination (Simons et al 1999: 206).
Homolaterally, one SCM rotates the face towards the opposite side, tilts it upwards and with the upper trapezius, it side-bends the head and neck. Working with the scalenes and trapezius, the SCM compensates for a head tilt caused by a shoulder tilt. It acts as a synergist to homolateral upper trapezius during lateral bending of the head and neck. It also check-reins in lateral bending to the opposite side. Both SCMs assist with check-reining in hyperextension of head and neck. The sternal division acts as an antagonist to opposite muscle in rotation.
Neither of the two SCM divisions refers pain to the neck (Simons et al 1999:206) but both refer pain to the face and cranium. Facial pain from SCM trigger points is often diagnosed as ‘tension headache’, atypical facial neuralgia or myofascial pain-dysfunction syndrome. The lower end of the sternal division of the SCM refers pain to the sternum while the midlevel refers pain homo-laterally across the cheek into the maxilla and into the orbital area as well as the auditory canal. The upper end refers pain to the occipital ridge behind but not close to the ear and to the vertex.
Specific eye symptoms related to trigger points in SCM are excessive lacrimation or tearing, reddening of eye, drooping of upper eye lid or ‘ptosis’ with normal pupillary size, tilting the head backwards to look up due to inability to raise upper eyelid. Visual disturbances include blurry vision, dimming of perceived light intensity, even double vision, severe nasal congestion and maxillary sinus congestion (Simons et al 1999:203,206, Chaitow et al 2000:214). Mechanical stress factors that affect the SCM are:
- mechanical overload such as overhead work,
- overuse in sport e.g. rugby, horse riding,
- injury such as whiplash,
- chronic postural stress e.g. compensatory neck positions due to deformity / injury,
- structural inadequacy e.g. short leg,
- functional scoliosis,
- shoulder-girdle tilting to level eyes,
- a chronic cough, sinus infection and dental abscess (Simons et al 1999:207).
3.2.3 Splenii muscles
The splenius capitus and splenius cervicus are postural muscles that shorten when stressed. Its function is to extend and rotate the head. When the face is rotated and chin is pointing upwards, both capitus and cervicus are engaged. Tests conducted on the splenii muscles in particular have also confirmed tonic coupling between these muscles and horizontal eye movements (Andre-Deshays, Berthoz & Revel 1988). Trigger points in this muscle manifests as pain in the vertex (top of head) as well as blurred vision (Simons et al 1999: 296).
Splenius cervicis comes into play during side-bending, extension and rotation of the head. The pain pattern that develops as a result of trigger points in this muscle is pain in the neck, cranium and behind the eye. As a result a patient may complain of blurred vision (Chaitow et al 2000:197). As patients often describe this eye pain as the eyeball feeling like it wants to explode, glaucoma and other eye pathologies have to be ruled out first (Chaitow et al 2000:198).
Postural stress that influences the splenius include:
- postures that overload extension and rotation of head and neck;
- sitting with neck extended to compensate for strong thoracic kyphosis;
- bad sleeping habits e.g. head and neck in crooked position and
- adaptations to wearing contact lenses and trifocal lenses. (Simons et al 1999: 299).
In the next section we consider the interrelationship between dysfunction in the eye muscles and dysfunction in the neck muscles.
4. Category 1: Dysfunction of eye muscles causing postural asymmetry
Asymmetrical contraction or weakness in one of the eight eye muscles may lead to a variety of vision irregularities. The extraocular muscles are highly innervated and according to Curl (1994) the proprioceptor population is comparable to the neck muscles reaching up to 500 spindles per gram of muscle tissue. Just like any other kind of muscle, extraoccular muscles may develop contraction of muscle fibres and even trigger points leading to double and/or blurry vision and even impaired proprioception (Buttner-Ennever, Horn 2002 in Starlanyl 2003). Eye conditions could cause but also be exacerbated by neck muscle dysfunction include myopia or nearsightedness and strabismus or poor alignment of eyes.
4.1 Myopia
Myopia may be caused by strabismus as well as chronic isometric muscle tension in the upper neck, jaw and throat. Distorted postures such as a forward-head posture (FHP) may also result in nearsightedness (Simons et al 1999: 216). A study by Valentine and Fabozzo (1993) on the interaction between specific neck muscles and the extraocular muscles of the myopic eye, indicate a clear difference in tonic activity of neck muscles of the myopic and the normal group. The aim was to demonstrate a possible alteration of the posture of the head due to abnormal tonus of the trapezius and SCM in people presenting with myopia. They conclude that “in the correction of visual defects, attention should be paid to postural adjustment of the neck by means of a series of programmed exercises directed towards the trapezius and SCM.”
4.2 Strabismus
Strabismus can affect postural alignment and may cause headaches. It also leads to increased irritability of the scalp and the neck muscles and a FHP (See Section 5.2).
4.3 Ocular torticollis
Asymmetrical contraction or weakness in one of the eight eye muscles not only effects vision irregularities but may also result in persistent eye tilting or ocular torticollis (Spokane Eye Clinic). Ocular torticollis is caused by visual problems such as strabismus, visual field defects and nystagmus (rapid jerking movement of eye). Patients develop ocular torticollis when they turn their heads in an attempt to improve their vision and to prevent double vision. This may in turn lead to tight neck muscles, facial asymmetry and even spinal problems (Spokane Eye Clinic).
Also the muscle that surrounds the eye socket (obicularis oculi) may develop trigger points. This will refer pain to the vicinity of the nose and the cheek and the area above the eye. When reading, letters may appear jittery.
Poor ergonomics as well as the incorrect use of prescribed interventions and aids for vision problems can contribute to musculature and postural imbalances. Sustained flexion as a result of lenses of glasses with too short a focal length or improperly adjusted frames results in neck muscle dysfunction and posterior cervical trigger point activity. Another problem that can lead to tight neck muscles is forward flexion of the head and neck. As a result the suboccipital extensors’ checkreining function for the SCM becomes overloaded. This may be caused by maladjusted frames, uncorrected nearsightedness, lenses with too short a focal length, the use of trifocal lenses, inverted bifocals and bending the head to avoid glare from strong light source.
It should be explained to the patient that the forward bending of the neck can be prevented by avoiding or adjusting the use of trifocals, using lenses with adequate focal length as well as rearranging furniture and lighting to eliminate glare on the inside and outside of lenses when reading or working on the computer. The patient should rather place copy material directly in front of them on a vertical stand to avoid turning the neck. Self-stretches should also be prescribed.
5. Category 2: Physical asymmetries that may lead to visual disturbances
A number of postural patterns can lead to visual disturbances. Physical asymmetry affects the eyes as it puts extra strain on these organs to interpret visual signals. Especially in the developmental years the child may attempt to adapt to postural deviations through visual adaptations. This may lead to permanent visual dysfunction even after the postural asymmetry has been resolved. The kinds of vision problems that may develop are strabismus, deceased clarity of sight in one eye (amblyopia), astigmatism and anisometropia or significantly different prescriptions for the two eyes (Kennedy, Krieger Institute 2005). The most obvious ones that we will discuss are: 1) torticollis, 2) forward head posture (FHP) and 3) leg-length difference whether as a result of trauma, surgery or soft tissue contraction.
5.1 Torticollis
Torticollis is an umbrella term for the condition where the head is pulled to one side, leading to abnormal head posture. A number of different types of torticollis have been identified: congenital, infant, spasmodic, wryneck as well as ocular torticollis (See Section 4.3) (Werner 2005:155-6). Torticollis may be caused by a spasm on one side of the neck. It could be a constant or intermittent spasm that causes pain in the neck, the shoulders and the back. The simple stiff neck or wryneck, with which one wakes up one morning or that develops when the head is suddenly turned, is caused by irritation of the intertransverse ligament at C7, cervical misalignment or trigger points in the splenius cervicis muscle. It is often the result of bad sleeping position or trauma. Torticollis may lead to visual problems as physical asymmetry makes it difficult to use signals from both eyes at the same time.
5.2 Forward head posture
Body asymmetry may also be caused by poor posture and incorrect ergonomics such as slouching and the forward-head posture. As with other postural compensation patterns, the aim of the FHP is to maintain the eyes and ears in a level position. The result is a crowded suboccipital space as the suboccipital group of muscles becomes notably shortened. This leads to a kyphotic thoracic curve (Hendrickson 2003:124,165) as well as increased lordosis. This can lead to hypomobilty and degeneration of facet joints (Cailliet 1991 in Chaitow et al 2000:163). It also leads to nerve root pressure, increased weight bearing on cartilage, degeneration, fatigue, decreased blood flow (compression of vertebral artery) and even early senility.The typical muscular imbalances that a therapist will encounter in the FHP are:
• weak cervical extensors – these are tight and long with abnormal medial torsion;
• suboccipitals that are tight and short;
• attachments on skull becoming thick and fibrotic;
• short and tight SCM pulling the head down;
• contracted upper trapezius;
• a weak lower trapezius and
• weak abdominals
This postural pattern is exacerbated by eyes maintained in horizontal plane for a long time as in driving and sitting in front of a computers screen; long periods of cervical flexion and lazy posture while sitting.
5.3 Leg-length difference
A third postural pattern that may cause visual disturbances is a leg-length difference. A discrepancy in the leg-length may be caused by functional scoliosis or lateral curvature of the spine, muscle imbalances in the pelvic girdle, legs and feet. This leads to additional muscle imbalances in the paraspinal muscles as well as the shoulder girdle. The upper trapezius for example is continually active in keeping the head and neck vertical and the eyes level (Simons et al 1999: 189). A leg-length discrepancy may also cause the dropping of one shoulder to maintain a normal head position and eye level. Neck muscles remain stretched long on the dropped shoulder-side and contracted on the side that is lifted. In addition the cervical spine may become extended causing a forward-head posture, rounded shoulders and a head-back posture. As explained before the patient may also attempt to adapt to postural asymmetry through visual adaptations.
6. Conclusion
From this brief review it is clear that a physiological explanation exists for patients’ experiencing an improvement in vision after a therapeutic massage treatment. Although it is not possible for therapeutic massage to affect visual dysfunction directly, the therapist can treat muscoloskeletal misalignment especially in the neck and shoulder area when caused by problems with the eyes. It also falls within the scope of therapeutic massage to treat and prevent physical asymmetries that may lead to visual disturbances. An important aspect of the treatment will have to be patient education about the nature of the relationship that exists between eye muscles and neck muscles as well as good posture and ergonomics.
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