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In this chapter we will discuss the organization of the ocular motor system and how visual information guides eye movements. This review will inclucde the function of the six extraocular muscles, the neuronal control systems, which keep the fovea (that part of the retina responsible for sharp vision) on the object of interest, the neuronal systems for saccadic eye movements (they shift the fovea rapidly to a visual target in the periphery of the visual field), and the neuronal systems, which control smooth pursuit (keeps the image of a moving target on the fovea), vergence (move the eyes in opposite direction so the image is still and stablizes the image when the object moves or when the head moves), vestibulo-ocular movements (these hold images still on the retina during brief movements and are under the control of the vestibular system), and optokinetic movements (these hold images during sustained head rotation and are driven by visual stimuli). We will review disorders of the neuromuscular junction and their effect on ocular muscles, as well as some of the myopathies which involve ocular muscles. We will first review the morphogenesis of the CNS to gain some understanding of the origin of the cranial nerves (CN) III, IV, and VI and the extraocular muscles.
In this article we will first discuss the embryology and anatomy of the visual pathway (the physiologic process by which we are able to see the world around us). It is believed that once you have an understanding of the embryology and anatomy of the visual pathway, you will have a better comprehension of why some of the injuries produce the changes in vision they do. You will note that there is some repetition in both the illustrations and written text. This was done with the intent of the reader gaining a clearer understanding of the intricacies of the visual pathway.
Cranial nerves (CN) are those nerves, which arise from the brainstem with the exception of CN I and II, the nuclei (site of origin) of which are located in the forebrain and thalamus respectively. The forebrain consists of the cerebrum, thalamus, hypothalamus and the limbic system. The CNs are not considered part of the central nervous system (CNS) but are part of the peripheral nervous system (PNS) with the exception of CN O, CN I (olfactory nerve) and CN II (optic nerve). This article is devoted to CNs O and I.
There are several types of skull fractures, which include linear skull fractures, depressed skull fractures, comminuted and multiple skull fractures, expressed skull fractures, contracoup fractures, ping pong fractures (pond fractures), birth fractures, infant skull fractures, diastatic fractures, and growing skull fractures. In this article we will explore the embryological development of the skull, as well as, how these fractures arise and manifest themselves.
Bleeding into the subarachnoid space is most commonly the result of trauma. It can be either as a thin-layered diffuse hemorrhage over the cerebral or cerebrellar hemispheres, assume a patchy distribution or present as a space occupying mass of blood referred to as a hematoma. In this article we will discuss the various causes of traumatic subarachnoid hemorrhage and their pathophysiology.
Subarachnoid hemorrhage (SAH) affects approximately 30,000 individuals per year in the United States, with an annual incidence of 1 per 10,000. In most populations primary non-traumatic SAH accounts for 5 to 9% of all strokes. SAH as the result of aneurysms is about 10 to 11 per 100,000 populations in Western Countries, with somewhat higher frequencies in the United States and Finland and among the Asian countries, Japan. It is lower in New Zealand and the other Scandinavian countries.
Subdural hemorrhage is bleeding below the dural membrane, whereas epidural hemorrhage, as previously discussed, is bleeding above the dura, i.e. between the dura and the overlying calvarium or base of the skull. The most common cause of a subdural hematoma is some form of direct impact trauma either as the result of an assault, fall or vehicular accident. Acute subdural hemorrhages can also result from sudden acceleration-deceleration of the head in which there is no injury to the head as would be the case in a whiplash-like injury such as occurs with a rear-end collision by a motor vehicle, blast injury or violent shaking during torture as reported by Pounder et al in 1997.
Intraspinal hemorrhage can occur in one of three extramedullary compartments: subarachnoid, subdural and epidural. Sometimes you will see these hemorrhages referred to as extra-axial bleeds, which means the hemorrhage is occurring outside of the brain or spinal cord tissue. Intra-axial bleeds are those hemorrhages, which occur within the substance of the brain or spinal cord. Such hemorrhages are also referred to as intraparenchymal or if they occur in the ventricles of the brain, intraventricular. Another infrequently used term to describe extramedullary hemorrhages involving the spinal cord is hematorrhachis. Hematomyelia is a term used to describe hemorrhages in the substance of the spinal cord (intra-axial). These latter two terms are primarily seen in the neuropathology literature. Click the link below to view the entire article.
What is Forensic Neuropathology? It is that aspect of neuropathology, which is the study of the brain, spinal cord, peripheral nerves and muscle disease, that focuses on the potential relationship of the manner of death and injuries to the brain and spinal cord. Typically the Forensic Neuropathologist is consulted in those forensic cases in which there is evidence or suspicion of traumatic injury to the brain or spinal cord, whether that be accidental or due to homicide and those cases in which the individual dies suddenly or under suspicious circumstances from a natural disease process, which may have affected the Central Nervous System (Brain and Spinal Cord).
Over the past several years I have had the opportunity to function both as a Forensic Pathologist and Forensic Neuropathologist. During this time I have examined thousands of brains, given lectures to medical students, pathology residents and neurosurgery residents, and have had many discussions with attorneys both in and out of the courtroom. During this process it has become clear there is a general lack of understanding of the fundamental concepts within forensic neuropathology among health care professionals, attorneys, and the public. It is with these thoughts in mind that this Blog on Forensic Neuropathology was established, the purpose of which is to give some understanding of these fundamental concepts.
In light of the unfortunate death a few months ago of Natasha Richardson due to an epidural hematoma I thought we could start with a discussion on this subject. We will discuss epidural hematomas (EDHs) of the head. EDHs also occur in the spine, which will be discussed in the next article. As it is generally understood, Natasha was taking a skiing lesson at a ski resort in Canada. During this lesson she fell striking her head. She was not wearing protective headgear. Initially she felt fine and declined to see a doctor. Approximately one hour later she complained of a headache, following which her condition rapidly deteriorated. She was taken to a local hospital and soon transferred to a hospital in Montreal. Natasha was then flown to Lenox Hospital in Manhattan where she died two days after her fall. An autopsy was performed at the New York City Medical Examiner’s Office where it was determined she had died as the result of an epidural hematoma (EDH). What is an EDH and why can it cause death so quickly and unexpectedly in some cases?
Epidural hemorrhage/hematoma is an accumulation of blood between the dura mater and the skull either involving the cranial vault or the spinal column. The dura mater is a tough connective tissue membrane, which is tightly adherent to the inner surface of the bones forming the cranial vault and spinal canal. Normally there is no actual space due to the tightness of the adherence of the dura mater to the overlying bones forming the cranial vault. What creates the space is bleeding from torn blood vessels coursing between the dura mater and the overlying bone. Often this bleeding is from torn arteries; hence the blood exits the defect in the artery under considerable pressure, which in turn can quickly separate the dura mater from the bone. If the bleeding is the result of tearing of an emissary vein or dural sinus, the pressure exerted by the blood exiting the vessel or sinus is not as great, consequently it takes longer for the EDH to evolve. This will be discussed in greater detail below. What must be understood is that epidural hemorrhages are neurosurgical emergencies, requiring immediate neurosurgical evaluation.
General Information
EDHs occur in approximately 2 to 3% of all head injuries. In those cases in which the person dies as a result of head injuries they are found in 5 to 15%. They are found in approximately 25% of all individuals with skull fractures. Although in adults EDHs are associated with skull fractures in 85 to 95% of cases, they can occur without them. This is especially true in children under the age of 10. EDHs are uncommon below the age of 2 and over the age of 55. This is primarily due to the tight adherence of the dura mater to the overlying bone in these age groups.
Clinical Presentation
The need for such immediate evaluation is often not appreciated, especially in light of the fact that approximately 10 to 30% of patients show no clinical symptoms immediately after trauma to the head. This lucid interval may last from a few minutes to several hours, the exact time of which is determined by the rate of bleeding from the torn artery, vein or dural sinus. Typically it takes approximately 50 to 75 ml of blood to accumulate before symptoms begin to occur. These symptoms may manifest themselves in the form of a headache, nausea, vomiting, seizures or focal neurologic deficits such as weakness and numbness. Some patients who develop an EDH are initially unconscious. In general such patients will have other types of brain damage such as contusions, lacerations, hemorrhages within the substance of the brain (intraparenchymal) or into the ventricles (intraventricular) in addition to having an EDH, which accounts for their immediate unconscious state. Typically these patients have a poor outcome. One of the things that must be remembered is some of these patients who are initially unconscious, may regain consciousness giving the appearance they are recovering only to suddenly become unconscious. The reason they suddenly become unconscious is their EDH has accumulated enough blood, usually 75 to 120 ml, to exert a mass effect. This mass effect will be discussed below.
Clinical Evaluation
These patients should be immediately examined and followed clinically looking for any signs of traumatic sequelae, which include the following: Hypertension and bradycardia (heart rate below 60 beats per minute), bradypnea (abnormal slow breathing for the age), decrease in level of consciousness, which is determined utilizing the Glasgow Coma Score (this will be discussed further below) and dilatation of either one or both pupils due to compression of the oculomotor nerve or nerves (anisocoria) all of which are manifestations of a increase in intracranial pressure (ICP); palpable skull fractures with or without scalp lacerations; clear fluid (cerebrospinal fluid) coming from the ears (otorrhea) or nose (rhinorhea); blood in the external ear canal and ecchymoses behind the external ear overlying the mastoid process (Battle’s sign) are both suggestive of a fracture of the petrous portion of the temporal bone; periorbital ecchymosis (raccoon eyes) is a sign of fracture of the base of the skull; facial nerve injury manifested by facial paralysis, with sagging of the muscles of the lower ipsilateral face, etc.; hemiparesis in which half of the body is weakened or hemiplegia in which half of the body is paralyzed due to compression of the cerebral peduncle, which are due to an increase in ICP with shifting of the brain (herniation); aphasia in which there is impairment of speech; ataxia in which there is a loss of ability to coordinate muscular movement manifested by unsteady movements and staggering walk and visual field defects. The visual field defects may be in the form of loss of vision above or below the horizontal plane, loss of vision at the sides of one or both eyes or loss of central vision.
Radiologic Studies
In order to properly assess an EDH it is absolutely essential that the clinical evaluation include radiologic studies, with unenhanced CT scan being the imaging study of choice. This will show the location and size of the EDH as well as its effect on the brain and the existence of other pathology. An important feature of EDHs is they rarely extend beyond suture lines due to the very firm attachment of the dura in these regions. Having said that, there is some evidence to suggest that in rare circumstances they can cross suture lines. It is also important in evaluating CT scans to note the presence of isodense or hypodense areas. Their presence is an indication of active bleeding; hence this particular EDH has the potential to expand. Air within an EDH suggest fracture through a sinus or the petrous portion of the temporal bone with involvement of the mastoid air sinuses.
Causes
In approximately 50 to 66% of cases EDHs are caused by tears in the middle meningeal artery or one of its branches in the temporparietal region usually adjacent to the squamosal portion of the temporal bone, which is immediately medial and slightly in front of your external ear. The underlying reason is that this is the thinnest part of your skull. As an example, one person committed suicide by driving a ballpoint pen through his skull in this region and he did it multiple times before he died. EDHs can also occur elsewhere including the frontal region due to tears in the anterior ethmoidal artery, in the occipital region due to tears in the transverse or sigmoid sinus, over the convexity of the calvarium due to tears in the superior sagittal sinus and at the base of the skull due to tears in the carotid artery before it enters the intracranial dura. As indicated most EDHs are the result of tears in arteries, hence the EDH evolves rapidly, often reaching their maximum volume in a matter of minutes. If however, the EDH is the result of tears in the middle meningeal veins, approximately 30%, tears in the emissary veins (diploic veins), approximately 10%, or tears in the venous sinuses, which usually occur in the parietal-occipital region or the posterior fossa, such hemorrhages are slow to reach their maximum volume often taking hours. It is these hemorrhages that can give rise to a clinical subacute or chronic course, which occurs in 13 to 14.3% of cases. Many cases of venous derived EDHs are the result either of depressed skull fractures or due to shearing injury from rotational or linear forces, in which parts of the brain rotate or undergo gyrational movement opposite to other parts of the brain. This is believed to be due to the movement of parts of the brain, which have different densities. Whereas EDHs of arterial origin require immediate neurosurgical intervention, some venous derived EDHs, especially those with markedly delayed clinical presentation can be treated nonsurgically. However, non-operative management is somewhat controversial. In one prospective study done by Knuckey et al, of 22 patients with small asymptomatic EDHs, 32% eventually required surgery. What must be remembered is that all EDHs, whether due to arterial or venous bleeding, can progress in volume after reaching their initial maximum size. These small to medium sized EDHs may increase in size by up to 50% until approximately 10-14 days following the injury, hence these patients need to be watched closely during this period of time or until the EDH stops expanding. This continued expansion occurs in approximately 9% of patients and is due to rebleeding into the hematoma.
Although EDHs typically occur on the same side of the calvarium, which sustained the fracture, in rare cases they can occur on the side opposite to the fracture. It is believed this is due to skull bending and deformation. This may also explain why in rare cases some adults will show bilateral EDHs with and without skull fractures in approximately 2-10%.
Pediatric Epidural Hematomas
Although not common, EDHs do occur in infants and children. What distinguishes their presentation from adults is they often are not associated with an overlying skull fracture. Even if a skull fracture is present, the EDH can be found at another location in the skull. There are two fundamental reasons why infants and children often do not show an association between an EDH and a skull fracture. First, the middle meningeal artery and its branches are located within the diploie of the plates of bone forming the calvarium rather than resting in a groove on the inner surface of the bones as occurs in adults. Secondly, the bones of the calvarium are very plastic, they tend to give when force is applied to them, bending rather than breaking as occurs in adults. Unfortunately, this same plasticity, which on the one hand protects the calvarium from fractures, can give rise to avulsions of the vessels with the formation of EDH with the consequences being no less lethal than in an adult. Another issue to keep in mind is the presentation of symptoms may be delayed by several hours. This delay is probably based on two factors. The very tight adherence of the dura mater to the skull and the torn vessel may be an emissary vein.
EDH occurring in the perinatal period (interval extending from the 28th week of gestation to 28th day after birth) can be the result of complications of delivery such as in a forcible breech extraction. Other signs of trauma such as forceps lacerations, scalp hemorrhages and cephalohematomas often accompany these EDHs. The documentation of such difficult deliveries with their attended traumatic induced lesions is extremely important in differentiating such complications of delivery from the trauma of abuse administered by the caretakers of the infant.
In the period of infancy (birth to 2years) EDHs are virtually pathogonomic of child abuse until proven otherwise. This takes into account that the average infant begins to walk between 13 and 15 months. Although such infants do fall, the kinetic energy involved in a simple fall is not great enough to induce an EDH unless you are talking about a fall down the stairs onto a hard surface such as concrete, and even then it is suspicious.
Pathophysiology
The underlying cause of death in patients with EDHs is the volume of blood that constitutes them as well as the rapidity in which this blood accumulates. In most fatal cases the volume of blood is between 75 to 120 ml, with some showing greater volumes. When epidurals reach this volume rapidly, as occurs in arterial bleeds, they exert what is referred to as mass effect, which in turn increases intracranial pressure (ICP).
Within the skull there is a normal ICP, which is usually between 0-10 mm Hg, which is found in the brain itself and cerebrospinal fluid (CSF). This ICP in turn exerts pressure on the blood vessels within the cranium. Any change in ICP is due to a change in volume of one of these components, the brain, CSF or the blood in the vessels. This pressure –volume relationship within the cranial vault is referred to as the Monro-Kellie hypothesis. In essence what it says is that in order to maintain normal ICP any increase in volume of one of these components, such as an EDH, must be compensated for by a decrease in the volume of the other components. Thus, the mass effect induced by the expanding EDH must give rise to a decrease in volume within the brain, the ventricles with its contained CSF and the volume of blood within the vessels in order to try and maintain normal ICP. These compensatory mechanisms can maintain an ICP up to 20-25 mm Hg, which is considered the upper range of normal, for any change in volume of less than 75 to 120 ml. However, once the volume of EDH exceeds the range of 75 to 120 ml, these compensatory mechanisms can no longer maintain even the upper limits of a normal ICP. Once a pressure of 25 mm Hg is reached any continued expansion of the EDH will lead to marked life threatening increases in ICP. At this point the continued expansion of the EDH will cause the underlying cerebral hemisphere to shift toward the opposite side. This shifting of the cerebral hemisphere is referred to as a midline shift and or subfalcine herniation. Since subfalcine herniation is the most common type and believed to be the precursor of all other types of herniation I will give a brief description of this form of herniation.
Subfalcine herniation is also referred to as cingulated herniation. It is due to displacement of the frontal lobe of the shifting cerebral hemisphere toward the opposite side. This results in its most medial part, that which is adjacent to the flax cerebri, the cingulated gyrus, to pass beneath the inferior border of the falx. In the process this can cause compression of the anterior cerebral artery, which in turn decreases blood supply to the frontal lobe. Such shifting is also associated with collapse of the shifting cerebral hemispheres ventricles with displacement of its contained CSF into the spinal canal. The compromise of blood supply of the anterior cerebral artery can lead to edema of the parenchyma supplied by that artery, which in turn causes an increase in volume of that portion of the brain due to the accumulation of edema fluid, and in turn is followed by further increase in ICP. Thus, a vicious cycle is created, which if not addressed quickly and adequately will lead to other severe forms of herniation leading to pressure on the brainstem through its downward displacement into the foramen magnum and its consequent compression by the cerebellar tonsils. Such pressure will ultimately lead to hemorrhages within the midbrain (Duret hemorrhages) with compromise of the reticular activating system and subsequent coma and compromise of the cardiac and respiratory control centers within the medulla, which leads to death.
Mortality-Morbidity
In general mortality-morbidity of patients with EDHs is directly related to the initial clinical presentation, the existence of other intracranial pathology, the age of the patient and the rapidity of appropriate medical care.
The overall mortality rate varies between 5-50%. In general higher mortality rates are associated with older individuals, EDHs occurring in the region of the squamosal portion of the temporal bone, and continued increase in size of the EDH. Higher mortality rates are also seen in those who show a rapidly progressive clinical course in which the patient goes from no or minimal symptoms to coma, slowly reactive, dilated or fixed pupil or pupils and a decrease in the Glasgo Coma Score, all of which indicate an increase in ICP with herniation.
Typically, patients with a Glasgo Coma Score of 8 or lower are considered in coma and are regarded as evidence of severe unconsciousness. Those with a score between 9 and 12 are considered to have a moderate degree of unconsciousness. Those who have a GCS of 13 or greater have a mild degree of unconsciousness, thus, the lower the GCS the greater the mortality rate.
Typically patients who initially are not in coma and receive immediate appropriate medical care will survive. Patients who present as being obtunded have a 9-10% mortality rate.
Obtunded means the patient shows compromise of their mental state, presenting themselves as being dull. Obtundation is determined by making a judgment as to whether the patient is cognizant of time, place and person. If the patient has any difficulty in answering these questions they are considered being obtunded.
Patients who have bilateral EDHs have a mortality rate between 15-20%. Those with posterior fossa hematomas have a rate of approximately 26%. What makes posterior fossa EDHs especially lethal is the rapidity with which the patient can deteriorate clinically. The patient can be totally lucid and talking and within a matter of a few minutes lapse into coma followed rapidly by cessation of respiratory and cardiac activity and death.
The existence of associated intracranial trauma such as contusions, lacerations, intraparenchymal and intraventricular hemorrhage will adversely affect the mortality rate.
The mortality rate for EDHs is also affected by age. Although EDHs are uncommon in those younger than 2 years and older than 60 years, when they occur under the age of 5 and older than 55 there is an increase in mortality. According to Lesstma patients under 20 years of age have a mortality rate of approximately 11%. Those between the ages of 20 and 40 have a rate between 18 and 40% and those over 40 have a rate that various between 25 and 29%.
Conclusion
In light of the fact that the interval of time between an injury to the head, giving rise to EDH and death can be less than 30 minutes to a few hours, it is absolutely essential the possibility of an evolving EDH be considered in all cases of head trauma no matter how minor. Even though a patient may be conscious and show no symptoms or signs of a traumatic injury to the brain, they should undergo a period of neurologic checks coupled with a CT scan of the head. Although some will regard this as overkill and not cost effective, such treatment will lead to early appropriate medical care and thus prevent needless deaths and or morbidity.
Forensic Medicine with Dr. Cox 