We report a case of an arachnoid cyst that was found incidentally during autopsy but was not the cause of death. A 25-year-old male soldier was found dead in the morning at a military barracks by a corporal of the same unit. A colleague stated that he had been breathing heavily at dawn. The deceased had no abnormal findings in a health check-up conducted a few months earlier and had no significant medical history, except for being prescribed to painkillers for a lumbar herniated intervertebral disc.

The autopsy was requested and conducted one day after death. He appeared to be in good general health and was 167 cm tall. The chest showed signs of cardiopulmonary resuscitation. The heart weighed 382 g and exhibited petechiae on its surface. The coronary arteries were patent, and no abnormalities were found in the heart structures, including the myocardium. A 2.3×1.7 cm unilocular cystic lesion was found near the interpeduncular fossa of the brain in the intracranial cavity. The cyst had a transparent, thin membrane and was filled with colorless serous fluid (Fig. 1). Microscopy showed that the cyst membrane was lined by a single layer of meningothelial cells, with an outer layer of collagenous connective tissue. The membrane was not ruptured or inflamed. No abnormal findings, such as inflammation or swelling, were observed in the surrounding structures where the cyst was attached (Fig. 2). These findings implied an intact arachnoid cyst. Since there was no evidence that the deceased had complained of any specific symptoms, it was concluded that the arachnoid cyst did not contribute to his death. No abnormalities were found in other organs, including the lungs, liver, kidneys, pancreas, spleen, and gastrointestinal tract. Additionally, toxicological and chemical tests showed no abnormal findings. As the autopsy revealed almost negative findings, the cause of death was determined to be sudden cardiac death.

An arachnoid cyst can be primary (congenital) or secondary. Congenital cysts occur because of the splitting of arachnoid membranes in utero, while secondary cysts result from trauma, infection, surgery, and genetic factors [1,2]. They account for approximately 0.3%-1.7% of intracranial mass lesions [1,3]. Approximately, 50%-65% of arachnoid cysts are located in the middle cranial fossa [1]. Galassi et al. [4] classified arachnoid cysts in the middle cranial fossa into three types. Type I cysts are small, mostly asymptomatic, and located in the anterior middle cranial fossa. Type II cysts extend to the Sylvian fissure, with some displacing the temporal lobe. Type III cysts are large and can affect the entire middle cranial fossa, displacing the frontal, temporal, and parietal lobes. Additionally, suprasellar cysts occur mainly near the third ventricle. Interhemispheric cysts are located in the parasagittal and midline regions, and the rare quadrigeminal cyst also exists [1]. The arachnoid cyst in our case was located near the interpeduncular fossa, which is situated inside the anterior middle cranial fossa, and can be classified as Type I according to Galassi classification.

Gross findings of an arachnoid cyst include a cystic lesion composed of a thin, transparent membrane filled with colorless cerebrospinal fluid (CSF). Microscopically, the cyst membrane is lined by a single layer of meningothelial cells, with an outer layer of collagenous connective tissue, as observed in our case [5]. However, other types of cysts can resemble arachnoid cysts. These cysts may consist solely of fibrous connective tissue or contain aberrant structures, such as choroid plexus, microvilli, and ciliated cells [2]. Additionally, the fluid inside these cysts may differ from CSF. These cysts have distinct pathophysiological origins compared to arachnoid cysts and should be differentiated through microscopic examination.

Most arachnoid cysts are asymptomatic lesions found incidentally during neuroimaging. However, some arachnoid cysts can cause symptoms, such as headaches, hydrocephalus, intracranial hypertension, dizziness, nausea, vomiting, ataxia, seizures, and hearing loss [1]. In pediatric patients, some cysts increase the head circumference, delay the development, and lead to psychiatric alterations [6]. Symptoms can vary depending on their location. Cysts located in the middle cranial fossa can cause sudden deterioration due to hemorrhage, which may occur within the cyst itself, or from rupture or tearing of bridging veins [3]. Cysts located in the Sylvian fissure can primarily cause unilateral headache in the temporal and supraorbital regions [7]. They can also cause various seizures, complications associated with cyst rupture, or increased intracranial pressure, including sudden deterioration [7]. Suprasellar and quadrigeminal cysts can cause obstructive hydrocephalus or brainstem compression, which can be fatal [8]. Cyst enlargement can result in the tearing of bridging veins with subsequent subdural hemorrhage, or intracystic hemorrhage with rapid expansion, leading to compression of adjacent structures, which can also prove fatal [9]. A study reported a case of a middle-aged man who died from nontraumatic hemorrhage because of venous sinus changes resulting from compression and bone defects caused by a large congenital arachnoid cyst [10]. In our case, the deceased had no history of any brain abnormalities. Additionally, he did not complain of any symptoms, including neurological symptoms like headache. The cyst was not large, was located in the interpeduncular fossa, and no pathological abnormalities in surrounding structures, including both cerebral peduncles, optic chiasm, oculomotor nerves, pons, and mamillary bodies were observed. Therefore, the arachnoid cyst found in our case was considered asymptomatic and not the cause of death.

In summary, the authors described an incidentally found arachnoid cyst in a young male. In this case, the arachnoid cyst did not cause symptoms that led to death, so after excluding other causes, the cause of death was determined to be sudden cardiac death. Most arachnoid cysts are asymptomatic and found incidentally through neuroimaging for other reasons. However, in rare cases, arachnoid cysts can cause neurological symptoms or life-threatening complications. Therefore, an arachnoid cyst found during an autopsy necessitates careful examination to determine whether it may be related to the cause and manner of death.