SPINAL CORD

There are two distinct patterns of abnormality of disease of the spinal cord: longitudinal and segmental, corresponding to upper and lower motor neuron respectively. Usually both forms are present together. Longitudinal damage produces loss of function of the entire system (motor or sensory) from the level of injury down. Segmental damage produces signs limited to the level of a single segment or segments: there is segmental motor, sensory and autonomic loss leading to atrophy and weakness in a myotomal distribution, reflex loss, sensory loss in a dernatomal distribution and sympathetic and parasympathetic disturbance.



1. ANATOMY

1.1 External morphology

The spinal cord is a small structure, about the width of your thumb. Since the cord ends at the bottom of the first lumbar vertebra it is virtually limited to the cervical and thoracic areas. Note that owing to the small size of the cord it is usual for both sides of the body to be affected in cord disease.

There are a series of motor and sensory rootlets issuing from the cord: 8 Cervical, 12 Thoracic, 5 Lumbar and 5 Sacral. The ventral roots carry the myelinated axons to the muscles and muscle spindles and the myelinated axons of preganglionic "pathetic neurons (parasympathetic in the sacral region). Dorsal roots carry sensory input from skin, muscle spindle, joint and viscera in both myelinated and unmyelinated axons. The rootlets join together to form the mixed spinal nerves which exit from the spinal canal via the intervertebral foramen and then go on to supply the myotomes and dermatomes.

Note that since there are 8 cervical nerves and only 7 cervical vertebrae, each nerve exits above the corresponding vertebra; the eighth cervical spinal nerve passes below the C7 vertebra; below T1 all spinal nerves exit below the vertebrum of the same number ie L5 nerve passes below the pedicle of L5. Because the cord itself is shorter than the spinal column the roots of the lumbar and sacral segments travel downwards in the dural sac to reach their appropriate neural foramen: this is the cauda equina (looks like a horses tall).

The cord is surrounded by the meninges: The tough dura with subjacent arachnoid are separated from the cord by the subarachnoid space. The cerebrospinal fluid lies in the subarachnoid space. Like brain, pia is closely attached to the cord. The dural sheath terminates well below the cord, opposite the 2nd sacral vertebra.

1.2 Blood supply

Anterior spinal artery supplies the anterior two thirds of the cord (gray and white matter): its origin is from the vertebral arteries at the level of medulla and it is supplemented by branches from the intercostal arteries and in particular from an aortic branch at low thoracic level (Arteria radicularis magna = Artery of Adamkiewitz): there are watershed zones of poor supply in mid thoracic and lumbar segments.

Posterior spinal arteries lie on either side of the dorsal roots and supply the posterior columns and gray matter.

1.3 Internal Morphology

White Matter:

Descending tracts:
    From cortex : Corticospinal: fine finger movements
    From brainstem : Rubrospinal: movement of proximal muscles
    Vestibulospinal : produce activation of extensor muscles, kept in check by cortical control
Ascending tracts:
    Posterior columns : uncrossed
    Spinothalamic : crossed
    Spinocerebellar

Gray Matter:

  1. Columns of anterior horn cells comprising alpha motor neurons to muscle fibers and gamma motor neurons to muscles spindles.
  2. Intermediolateral cell column (T1 to L2) lies in the lateral horn comprising the cells of the preganglionic sympathetic outflow. Parasympathetic (S234) found in the anterior horn of the sacral segments.
  3. Nucleus dorsalis: receives input from spindle afferents and transmits this information to the cerebellum via the spinocerebellar pathway.
  4. Substantia Gelatinosa: caps the dorsal horn; contains Substance P and receives input from pain receptors.


2. SPINAL CORD FUNCTION

2.1 Spinal reflexes

         Phasic stretch
This is elicited by increase in muscle length which activates the muscle spindle. This information travels into the cord in the large myelinated 1a fibers and excites the corresponding anterior horn cell. 1a afferents also inhibit the corresponding antagonist muscle group. This sort of reflex evokes a dynamic response as a result of rapid change in muscle length: it will evoke a brisk muscular contraction. (Synonyms are myotactic reflex, stretch reflex, tendon-jerk reflex, deep tendon reflex). Note that the muscle spindle is extremely sensitive to vibration: striking a tendon with a tendon hammer sets up a vibration which travels to the spindle and evokes a signal in the 1a fibers.

        Static/Tonic response
Sustained contraction of the muscle activates a different group of spindle fibers and evokes a signal in Group 11 afferent fibers to the anterior horn cells: This gives a continuous background of excitatory input and is responsible for clinical muscle tone.

        Clasp knife response
If a spastic muscle is stretched beyond a certain point there is a sudden decrease in resistance: this is in part due to firing from the Golgi tendon organs which lie in series with groups of muscle fibers. The tendon organ monitors muscle contraction and inhibits the corresponding anterior horn cells (opposite effect to the spindle). Together the spindle and tendon organ provide the CNS with all the information needed to calibrate movement: length of the muscle fibers and force exerted on muscle.

        Flexion reflex
This refers to various flexes which depend on a number of interneurons to spread the incoming sensory input widely to the motor neuron pool (ie they are polysynaptic): thus painful stimuli cause a withdrawal response and pressure on the foot when walking causes flexion of the leg. Activation of flexor motor neurons is typically widespread so that there is withdrawal at the ankle, knee and hip.

2.2 Control of Sphincters

Urine is held in the bladder as a result of the physiological sphincter of the detrusor muscle and the external striated muscle sphincter. As the bladder fills sensory fibers (Parasympathetic; S234; Pelvic Nerve) carry sensation to the reflex micturition centre in the spinal cord: this input ascends to the brain.

Emptying of both bladder and rectum is brought about by motor impulses also carried in parasympathetic fibers. In the bladder there is detrusor contraction and opening of the physiological internal sphincter at the time of micturition.

Sympathetic activity acts to inhibit parasympathetic activity and increases urine storage.



3. SPINAL CORD DISEASE

3.1 Symptoms

1. Motor - Weakness

This is the usual reason for the patient seeking help. If of gradual onset the complaints are usually of dragging the leg or stiffness of the limbs. Patients may also complain about flexor spasms and may present with urgency, urge incontinence or urinary retention with overflow. Impotence and ejaculatory impairment or difficulty attaining orgasm are frequent complaints if asked for.

2. Sensory

Sensory complaints are often slight, other than numbness; one should enquire about Lhermitte's sign: a shock-like sensation down the back when flexing the neck (as a result of focal demyelination in cord from a variety of causes). At times a band-like sensation ("like a belt squeezing me") is described: suggestive of segmental spine disturbance at that level.

3.2 Signs and segmental locatisation

Focal disease may present with a combination of segmental and longitudinal symptoms and signs, if there is involvement of ascending/descending pathways along with segmental structures. Diffuse spinal cord disease may involve a single system (ie motor) or more than one system

Segmental / Local Signs:

MOTOR: Damage to anterior root or anterior horn cell produces atrophy, flaccid weakness and fasciculations in a myotomal distribution.

SENSORY: Damage to dorsal root or dorsal root entry zone produces sensory loss in a dermatomal distribution.

Damage to either motor or sensory pathways will give reflex loss. Often root irritation will give rise to pain: this is made worse by movements that increase pressure in the subarachnoid space (coughing, sneezing) and by movement.

Signs of vertebral involvement: local pain and tenderness to percussion.

Note that the nerve roots have the same segmental derivation as the mixed spinal nerves which they form: clinicians talk about "root disturbance" or "radiculopathies" when often they are referring to disease of the mixed spinal nerves: (it amounts to the same thing).

Longitudinal Signs:

MOTOR:

  1. Spasticity
  2. Hyperreflexia: there is spread of reflexes eg. vibration induced by the tendon hammer is conducted through the bone to other muscle spindles and activates them: an example is the crossed adductor reflex.
  3. The normal supraspinal inhibition of antagonist muscles which forms part of the reflex arc is lost; as a result clonus occurs following tendon stretch. Note that patients may complain of this themselves, particularly after putting their foot hard on the brake-pedal of a car.
  4. Normal flexion reflexes are altered: damage to corticospinal tract gives rise to an extensor plantar (= Babinski) response; also superficial cutanueous reflexes (Abdominal and Cremasteric) disappear.
  5. Weakness in an upper motor neuron distribution (eg weakness of the arm extensors and the flexors of the leg).
SENSORY:
Typical sign is one of a sensory level below which sensation is diminished. Note that a sensory level is often several segments below the motor level; also note that the dermatomes over the upper chest abruptly alter from thoracic T2/3 to Cervical C4: this is caused by the development of the arm which drags out the C5 to T2 segments as it forms. Since diaphragm is supplied by C345, confusion of a C4 level with a T3 level can be a life-threatening error.

Note that touch and pain are often not entirely lost in cord lesions because they are carried in bilateral and diffuse ascending systems.

AUTONOMIC:
Typically there is development of an upper motor neuron bladder with signs of urgency, urge incontinence and a small volume bladder. There may also be faecal incontinence and alterations of sweating and blood pressure.



4. CAUSES OF SPINAL CORD DISEASE

4.1 Infection / inflammation

Extradural:
  1. Epidural Abscess: fever, pain, segmental + longitudinal signs
  2. TB spine
Meningeal:
  1. Arachnoiditis (scar tissue formation choking off blood vessels): follows TB, meningitis, bilharzia, surgery
  2. Infiltration: syphilis (generally meningovascular and causes cord ischaemia)
Cord:
  1. Transverse Myelitis: an acute, usually ascending inflammation of cord, caused by multiple sclerosis, viral infections or SLE.
  2. HIV and HTLV 1
  3. Tabes dorsalis (syphilis)
4.2 Tumours Extradural:
  1. Metastases: bronchus, breast and prostate
Meningeal:
  1. Infiltration by carcinoma or leukaemia
  2. Tumours arising from the: dura: meningioma
Nerve sheath: neurofibroma   Cord:
  1. Gliomas, Ependymoma
  2. Paraneoplastic necrotising myelopathy
4.3 Spondylosis
    Cervical spondylosis

    This is associated with disc protrusion and osteophyte formation of the vertebral bodies with resultant compression of both roots and the cord; a component of vascular insufficiency with cord ischaemia is common. Commonest are C56 and C67 discs, which compress the sixth and seventh cervical roots respectively.

    Lumbar spondylosis

    There is no cord compression, but if the disc is large several roots may be comrpessed. Commonest are L45 and L5S1 discs which compress the fifth lumbar and first sacral roots respectively. 

    Disc lesions cause pain, muscle spasm and scoliosis with segmental loss of motor and sensory function in a myotomal and dermatomal distribution, eg C67 disc produces wasting and weakness of triceps, absent triceps reflex and sensory loss in C7 dermatome.

4.4 Radiation Myelopathy

4.5 Vitamin B12 Deficiency

4.6 Vascular causes
  1. Anterior spinal artery syndrome from thrombosis or embolism will affect spinothalamic and corticospinal tracts and spare posterior columns.
  2. Spinal Arterio Venious Malformations
4.7 Syringomyelia This is often associated with an Arnold-Chiari malformation in which the cerebellar tonsils or lower brainstem protrude into the upper cervical spinal canal. Classic features of a syrinx (or any intrinsic cord lesion) are:
  1. Segmental Signs: Wasting and weakness, typically of C8T1 muscles, because of anterior horn cell damage.
  2. Spinothalamic loss in several dermatomes, because pain and temperature fibers are damaged as they cross the central part of the cord: this may produce a "cape-like" sensory loss over the upper body.
  3. Autonomic disturbance.
  4. Longitudinal Signs: Spasticity from compression of the corticospinal tracts by the gradually increasing size of the central cord cavity.
The posterior columns are normal. (The sensory loss in this case is called dissociated because there is a dissociation between pain and temperature (lost) on one hand and joint position sense (preserved) on the other).
Note that a classic (but uncommon) sign of intrinsic cord lesions is "sacral sparing": this is because the sacral segments of the spinothalamic tract are closest to the outside of the cord and therefore are not affected until late by a gradually increasing central cord lesion.


INVESTIGATION:

ACUTE CORD COMPRESSION IS AN EMERGENCY: pressure on the cord gives rise to symptoms and then obstructs venous drainage which results in cessation of arterial flow and infarction: a patient with an infarcted cord will never walk again.

If there is evidence of spinal cord disease obtain plain films of the cervical and thoracic spine.
DO NOT do a lumbar puncture if you think there is acute cord compression: the cord itself or tumor may herniate down and make matters worse. To put this in a different way: if a patient presents with weakness of the legs and has a sensory level, a lumbar puncture is contra-indicated until an imaging study has excluded a mass lesion.

Commonest causes of a spinal cord lesion with a normal myelogram or MRI are syphilis and transverse myelitis.

Obtain an urgent myelogram or MRI scan and contact the neurosurgeons beforehand to warn them of possible need of their services.

If chronic problem: Vitamin B12, VDRL.
CSF
Cervical, Thoracic spine X Rays
Myelogram, MRI.