Spinal shock was first defined by Whytt in 1750 as a loss of sensation accompanied by motor paralysis

Paralysis

Paralysis is loss of muscle function for one or more muscles. Paralysis can be accompanied by a loss of feeling in the affected area if there is sensory damage as well as motor. A study conducted by the Christopher & Dana Reeve Foundation, suggests that about 1 in 50 people have been diagnosed...

 with initial loss but gradual recovery of reflexes, following a spinal cord injury (SCI) – most often a complete transection. Reflexes in the spinal cord caudal to the SCI are depressed (hyporeflexia) or absent (areflexia), while those rostral to the SCI remain unaffected. Note that the 'shock' in spinal shock does not refer to circulatory collapse.

Phases of spinal shock

|Phase>

Time

Physical exam finding

>- |1

0-1d

Areflexia/Hyporeflexia

>- |2

1-3d

Initial reflex return

>- |3

1-4w

Hyperreflexia (initial)

>- |4

1-12m

Hyperreflexia, Spasticity

Explanation of phases

Ditunno et al. proposed a four-phase model for spinal shock in 2004 as follows:

Phase 1 is characterized by a complete loss—or weakening—of all reflexes below the SCI. This phase lasts for a day. The neurons involved in various reflex arc

Reflex arc

A reflex arc is a neural pathway that controls an action reflex. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord...

s normally receive a basal level of excitatory stimulation from the brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

. After an SCI, these cells lose this input, and the neurons involved become hyperpolarized and therefore less responsive to stimuli.

Phase 2 occurs over the next two days, and is characterized by the return of some, but not all, reflexes below the SCI. The first reflexes to reappear are polysynaptic in nature, such as the bulbocavernosus reflex

Bulbocavernosus reflex

The bulbocavernosus reflex is a polysynaptic reflex that is useful in testing for spinal shock and gaining information about the state of spinal cord injuries . The test involves monitoring anal sphincter contraction in response to squeezing the glans penis or tugging on an indwelling Foley...

. Monosynaptic reflexes, such as the deep tendon reflexes, are not restored until Phase 3. Note that restoration of reflexes is not rostral to caudal as previously (and commonly) believed, but instead proceeds from polysynaptic to monosynaptic. The reason reflexes return is the hypersensitivity of reflex muscles following denervation – more receptors for neurotransmitters are expressed and are therefore easier to stimulate.

Phases 3 and 4 are characterized by hyperreflexia, or abnormally strong reflexes usually produced with minimal stimulation. Interneurons and lower motor neurons below the SCI begin sprouting, attempting to re-establish synapses. The first synapses to form are from shorter axons, usually from interneurons – this categorizes Phase 3. Phase 4 on the other hand, is soma

Soma (biology)

The soma , or perikaryon , or cyton, is the bulbous end of a neuron, containing the cell nucleus. The word "soma" comes from the Greek σῶμα, meaning "body"; the soma of a neuron is often called the "cell body"...

-mediated, and will take longer for the soma to transport various growth factors, including proteins, to the end of the axon.

Autonomic effects

In spinal cord injuries above T6, autonomic dysreflexia

Autonomic dysreflexia

Autonomic dysreflexia, "AD" also known as "autonomic hyperreflexia or Hyperreflexia, is a potentially life threatening condition which can be considered a medical emergency requiring immediate attention. AD occurs most often in spinal cord-injured individuals with spinal lesions above the T6 spinal...

 may occur, from the loss of autonomic

Autonomic nervous system

The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...

 innervation from the brain. Sacral parasympathetics (S2-S4) are lost, as are many sympathetic levels, depending on the level of the SCI. Cervical lesions cause total loss of sympathetic innervation and lead to vasovagal hypotension

Hypotension

In physiology and medicine, hypotension is abnormally low blood pressure, especially in the arteries of the systemic circulation. It is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the...

 and bradyarrythmias – which resolve in 3–6 weeks. Autonomic dysreflexia is permanent, and occurs from Phase 4 onwards. It is characterized by unchecked sympathetic stimulation below the SCI (from a loss of cranial regulation), leading to often extreme hypertension

Hypertension

Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...

, loss of bladder/bowel control, sweating

Sweating

Perspiration is the production of a fluid consisting primarily of water as well as various dissolved solids , that is excreted by the sweat glands in the skin of mammals...

, headaches, and other sympathetic effects.