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.
1.1 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
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
Posterior columns : uncrossed
Spinothalamic : crossed
2.1 Spinal reflexes
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.
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.
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.
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.
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.
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:
SENSORY: Damage to dorsal root or dorsal root entry zone produces sensory loss in a dermatomal distribution.
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).
Note that touch and pain are often not entirely lost in cord lesions because they are carried in bilateral and diffuse ascending systems.
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.
4.1 Infection / inflammation
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.
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.
4.5 Vitamin B12 Deficiency
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.
Cervical, Thoracic spine X Rays